Water Droplets Driven by the Giant Surface Potential of Organic Films

Water Droplets Driven by the Giant Surface Potential of Organic Films | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Water Droplets Driven by the Giant Surface Potential of Organic Films Tsuyoshi Tsujioka, Hiroyuki Kawashima, Kenji Koike, Naoki Matsumoto, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5613353/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Controlling droplet motion on solid surfaces presents significant challenges in chemistry, physics, and materials science, especially for potential applications in "lab-on-a-chip" technologies. Traditional methods, such as using electric fields produced by electrodes, often inadequately manage complex droplet motion. Here we show a novel approach utilizing giant surface potentials (GSPs) generated through customizable photopatterning during vacuum deposition. Water droplets move spontaneously due to the potential difference between the GSP surface and the surface where the droplet comes into contact, which has a reduced potential. Our study not only enhances droplet control but also improves the photostability of the surface potential. We identify the optimal conditions for GSP generation, achieved under vacuum-deposition conditions conducive to maximum enthalpy relaxation, and demonstrate its potential for creating customizable, complex droplet flow channels. These advances offer significant promise for microfluidic devices, medical diagnostics, chemical synthesis, and energy harvesting applications. Physical sciences/Materials science Physical sciences/Chemistry/Materials chemistry giant surface potential organic film vacuum deposition water-droplet movement glass transition temperature enthalpy relaxation Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Liquid droplets moving over solid surfaces present a fascinating and significant challenge with promising applications in many fundamental and applied areas in chemistry, physics, and materials science. In applied fields, droplets on solid surfaces play a crucial role in "lab-on-a-chip" technologies. Such applications range from manipulating tiny quantities of materials to medical diagnostics, food technology, chemical synthesis, and the detection of chemical and biological analytes 1–3 . A key factor driving such a broad range of applications is the ability to precisely control the transport of liquid droplets on the surface 4,5 . Although several principles exist for driving liquid droplets on solid surfaces, such as using ultrasonic waves or surface tension gradients 6,7 , most methods apply an electric field to the droplets through external electrodes or create a charge distribution on the surface 8–12 . However, these techniques often fail to achieve complex droplet motion, and methods involving electric fields require electrodes to be pre-embedded on the surface. It has long been recognized that a surface potential is generated when polar molecular materials are vacuum-deposited. Seki et al. first reported the significant surface potential resulting from the room-temperature deposition of tris(8-hydroxyquinolinato)aluminum (Alq3) 13,14 , a material widely used for light emission and electron transport. Tsekouras et al. observed that when various alcohols are deposited on a substrate cooled to extremely low temperatures, the deposited molecules spontaneously orient themselves, creating a negative potential on the free surface. This potential can reverse to a positive potential depending on the substrate temperature 15,16 . In organic electronics, materials exhibiting negative surface potentials as a result of well-designed molecular structures have also been documented 17,18 . Such spontaneously generated surface potentials have significant interest as they influence the carrier injection characteristics of organic light-emitting devices 19–25 . In the burgeoning field of energy harvesting, attempts have utilized giant surface potential (GSP) films in devices that generate electricity from vibrations and accelerations occurring in daily life 18,26 . Consequently, GSPs have become a major focus in the realm of organic devices and materials. In this paper, we introduce innovative droplet-driven channels through customizable photopatterning techniques utilizing GSP. To this end, we delve into its intricate formation mechanism during vacuum deposition, describe the precise conditions required for achieving photostability, and demonstrate the spontaneous and complex motion of droplets. Spontaneous water-droplet moving on GSP surfaces As a unique feature of organic surfaces, we previously unveiled selective metal deposition based on modulation during metal vapor deposition on organic surfaces 27–31 . During this investigation, since we measured water-droplet contact angles to assess the surface free energy of fluorinated organic films, we encountered a striking phenomenon: the spontaneous movement of water droplets across the surface. Further analysis revealed that this phenomenon stems from the generation of GSP, uncovering a key factor behind the observed behavior. Figure 1 a and a supplementary movie (Smovie1) vividly demonstrate the spontaneous movement of a water upon deposition onto the surface of a fluorinated organic film (1-N,1-N,3-N,3-N-tetrakis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl]adamantane-1,3-diamine, PTAA) formed through vacuum deposition. The two water droplets initially moved slowly, gradually approached each other before stopping, and halting without merging. The arrows in the figure trace the droplet trajectories. Unlike most high-speed water droplet movement techniques, which rely on electric fields to roll spherical droplets across superhydrophobic surfaces 9–12 , the spontaneous movement observed on the PTAA surface involved neither a superhydrophobic surface nor rolling droplets. This observation points to a novel mechanism driving the droplet motion, setting it apart from conventional methods. Figure 1 b captures the striking fluorescence patterns created by the trails of moving droplets. After the droplets traversed the surface, we delicately sprinkled fluorescent Alq3 molecules onto their path. When exposed to UV light, the trails lit up, revealing intricate and captivating fluorescent patterns that highlighted the subtle interactions between the droplets and the surface. The fluorescence data show that the areas to which water droplets moved display weaker fluorescence than the untouched regions. Interestingly, the strongest fluorescence is observed at the boundary between the affected and unaffected areas, suggesting unique electric field distributions in these regions. Figure 1 c illustrates the driving mechanism behind the movement of water droplets on the GSP surface. Moisture adheres to the GSP surface, reducing the surface potential due to the inflow of charges from the outside and surface adsorption. This creates a potential difference between regions with reduced and unchanged surface potential. Even the slightest dynamical perturbation induces an imbalance in the left and right electric fields, attracting the droplets and causing them to move spontaneously. Indeed, the surface potential of PTAA significantly dropped to about one-third of its initial value: from − 150 to -50 V after being exposed to water. Figures 1 d and 1 e offer further examples of the driving force behind droplet movement (Smovie2). In Fig. 1 d, water droplets, which are moving on a PTAA film deposited on a Si substrate, exhibit rapid movement, and their trajectories reflect off the edges of both the substrate and the PTAA film. This occurs because the surface potential is zero in areas without PTAA film (Fig. S1 a). As a result, no attractive force is present at the outer edge, causing the droplet to be drawn back towards the inner regions where the surface potential exists. In the right panel of Fig. 1 d, several water droplets on the surface interact and move in a manner resembling “slow billiard balls.” This movement can be explained by a principle similar to the one described in Fig. 1 c (see also Fig. S1 b). Based on this water-droplet driving principle, a PTAA-GSP-pattern, formed on a Si substrate, enables the rapid movement of water droplets along the designed path. This principle allows for controlled water-droplet motion (Fig. 1 e and Smovie3). Horseshoe-shaped and zigzag-shaped PTAA patterns were created on a Si substrate using vacuum deposition with shadow masks. When a water droplet was placed at the edge of the pattern, it efficiently followed the curved or zigzag PTAA path, achieving speeds up to 15 cm/s. Optimal GSP conditions for spontaneous droplet movement After confirming that the GSP of the vacuum-deposited PTAA film enables the spontaneous movement of water droplets, we initiated a foundational study into the GSP properties of various organic materials and explored the possibility of water-droplet driving. This exploration aimed to assess their potential for driving water droplets on a variety of surfaces. Several challenges are associated with the generation mechanisms and applications of GSP. One key issue is understanding the origin of spontaneous molecular-orientation polarization induced by vapor deposition. Although the equilibrium between the thin supercooled liquid layer formed during deposition and the underlying glassy state is believed to play a crucial role 18,32,33 , the optimal conditions and film structure remain elusive for maximizing GSP. Another significant challenge is the tendency for GSPs to vanish upon exposure to light 13,14,34 , a situation which presents a substantial obstacle in achieving photostability for devices. First, we determined the GSP slope of PTAA, various diarylethenes (DAEs) and spiropyran (SPP) (Fig. S2). DAEs, known for their excellent photochromic properties, exhibit remarkable thermal stability in both isomeric states and exceptional fatigue resistance during isomerization reactions 35,36 . They are also gaining attention as potential materials for organic electronics 37–39 . The molecular orientation of the deposited film was derived from the measured GSP slope, the molecular permanent dipole moment (PDM), the molecular volume, and the dielectric constant: GSP slope \(\:\equiv\:\frac{Surface\:potential}{d}=\frac{1}{{\epsilon\:}_{r}{\epsilon\:}_{0}}\frac{p⟨\text{cos}\theta\:⟩}{{V}_{m}}\) , where p represents PDM, V m denotes the molar volume, ε₀ and εᵣ correspond to the vacuum and relative permittivity, and d indicates the GSP layer’s thickness. The correlation between and Tg was investigated (Figs. S3-S7 and Tables S1 and S2). Figure 2 a demonstrates a distinct correlation between Tg and . Notably, materials with Tg near room temperature, such as DAE6, DAE7, DAE8, and DAE9, underwent the thermal randomization of molecular orientation during film formation via vapor deposition at room temperature, resulting in almost negligible surface potential. The dependence of ⟨cosθ⟩ on Tg indicates that the former varies with both substrate temperature T sub during deposition and the material’s Tg. This suggests the presence of an optimal T sub that maximizes the GSP slope. To explore this further, we investigated how the GSP slope varies with substrate temperature for three representative GSP materials: SPP, PTAA, and DAE2, all of which have different levels of Tg and GSP slopes. Although we anticipated that molecular orientation would be thermally disturbed and the GSP slope would decrease when the substrate temperature approaches Tg, our findings revealed an intriguing trend. Surprisingly, the GSP slope decreased even when the substrate temperature was significantly lower than T g . For the three materials studied, we plotted a graph with substrate temperature T sub normalized by T g on the horizontal axis and the GSP slope on the vertical axis (Fig. 2 b, top panel). The maximum GSP slope occurred at a substrate temperature approximately 0.8–0.85 times T g . For instance, PTAA, with a T g of 343 K, displayed a maximum GSP slope at a substrate temperature of 287 K, which is close to room temperature (see Fig. S8a for the actual T sub dependence graphs for each material). Ediger et al. reported that an organic film formed by vacuum deposition reaches an enthalpy-relaxed state, where the maximum enthalpy relaxation occurs at substrate temperature T sub of 0.8 to 0.85 times T g 40,41 . Building on their work, our results suggest that the substrate temperature at which a maximum GSP slope is achieved corresponds to a temperature where the most significant enthalpy relaxation occurs. To further investigate this correlation, we examined the dependence of enthalpy relaxation on T sub in vacuum-deposited films using differential scanning calorimetry (Fig. S8b). Our findings revealed that the temperature for maximum enthalpy relaxation aligns with the substrate temperature at which the maximum GSP slope is observed (Fig. 2 b, bottom panel). Next, we explore the origin of the molecular orientation responsible for GSP. Initially, the substrate's influence on molecular orientation appears most significant in thin films, with this effect clearly depending on film thickness, and is considered to be negligible in films with a thickness of 1 µm 42 . A second suggested factor is the minimization of surface free energy during deposition 18 . However, our findings challenge this hypothesis. To infer the surface free energy, we observed the water contact angles before and after annealing above Tg. Following annealing, the contact angle either increased (indicating a decrease in surface energy, as in PTAA), decreased (DAE2), or remained nearly the same (SPP), as shown in Fig. S9. These results point to a more complex interplay of factors. As shown in Fig. 2 b, the substrate temperature at which maximum enthalpy relaxation occurs aligns with the temperature where GSP is maximized. It implies that surface molecular migration during enthalpy relaxation, along with molecular interactions in the glassy state below Tg , are keys to achieve the molecular orientation responsible for GSP. We therefore analyzed molecular packing in the film at a substrate temperature corresponding to the maximum value (B) of the GSP slope and enthalpy relaxation, as well as at temperatures (A) and (C) where the GSP slope decreases. Figure 2 c presents atomic force microscopy (AFM) images of the DAE2 surface formed at different substrate temperatures. The AFM analysis scanned a 2-µm square area, followed by scanning the same region within a larger 10-µm area. For samples (B) and (C), the mark from the initial 2-µm scan near the center is barely visible, indicating minimal surface disturbance. In contrast, this mark is clearly visible for sample (A), suggesting that sample (A)’ surface was more affected by the cantilever during the initial scan, altering its surface morphology. This effect arises because a smaller scanning area increases the density of the cantilever taps in the dynamic mode. These findings suggest that the molecular packing at substrate temperatures corresponding to samples B and C results in surfaces that are relatively robust and resistant to deformation. In contrast, the weaker packing at sample A leads to a more fragile surface, which is more susceptible to changes under mechanical stress. The robust molecular packing observed in samples B likely contributes to the enhanced molecular orientation and GSP observed at this temperature, highlighting the critical role of substrate temperature in controlling both molecular packing and surface morphology. On the other hand, when DAE2 was evaporated onto a grating substrate at varying levels of T sub , AFM observations revealed that, under similar conditions, the grating grooves in sample (C) were more partially filled compared to sample (B) (Fig. 2 d). This suggests that, under condition (C), the grooves tend to fill due to the presence of a thin supercooled liquid layer on the surface, a phenomenon consistent with previous studies 43,44 . Based on these results, we propose three models to explain how the film surface behavior is related to the generation of GSPs, depending on T sub , and Tg and the enthalpy relaxation/recovery during vacuum-deposition (Fig. 2 e): Model (A) : Molecules are rapidly frozen from the vapor phase directly into a glassy state, bypassing the liquid phase. Such rapid freezing restricts molecular migration, leading to the formation of numerous internal voids and a rough, brittle surface. Molecular orientation is suppressed, and GSP does not emerge. Model (B) : Molecules migrate effectively across the surface, compacting into the underlying glassy layer to form a smooth, dense film. This migration enhances intermolecular interactions, promoting molecular orientation and resulting in the development of GSP. Model (C) : At high substrate temperatures (but still below Tg), a thin layer of supercooled liquid forms between the migrating layer and the glassy surface. This liquid layer disrupts molecular orientation and enhances surface smoothness through surface tension, thereby inhibiting GSP formation. Surface conditions for droplet moving Initially, water droplets exhibited spontaneous movement only on certain surfaces. For example, the droplet remained stationary on the DAE2 film, which had a surface potential of approximately − 100 V, but moved on the PTAA film, which had the same surface potential. To understand the cause of this difference, we measured the advancing and receding contact angles ( q A : water flows in, θ R : it flows out, from the syringe nozzle at 5 µL/s) of a water droplet on the DAE2 and PTAA surfaces. Contact angle hysteresis (defined by D q = q A - q R ) indicates the surface slipperiness of the droplet, with a smaller Δ θ suggesting increased slipperiness at the droplet’s edge. Figure 3 a shows the contact angles for the two surfaces PTAA and DAE2. The contact angle hysteresis D q was 30° for DAE2 and only 6° for PTAA, indicating that water droplets move more easily on the PTAA surface. Factors influencing contact angle hysteresis include surface roughness (Ra) and the interaction between the water molecules and the surface molecules 45 . Therefore, we used AFM to compare the surface roughness of the PTAA/glass and the DAE2/glass. AFM analysis revealed that PTAA had a smooth surface with an Ra of 0.9 nm, whereas DAE2 exhibited a much larger Ra of 5.7 nm (Fig. 3 b). Such surface smoothness is attributed to the active molecular migration of molecules during deposition, which is facilitated by the low Tg of PTAA. We further investigated the relationship among the surface potential, the water droplet motion, and the Ra across various DAEs. The water droplets moved spontaneously on PTAA and some DAEs. Figure 3 c presents conditions for water droplet movement, with a scatter plot correlating the surface potential with Ra (filled symbols: droplets do not move; open symbols: droplets can move). Our analysis suggests that for droplet motion to occur, a surface potential of approximately 100 V and an Ra of around 1 nm or less are required. Additionally, the difference in droplet movement speed on the PTAA films on Si versus glass substrates can be attributed to surface roughness. Si substrates are atomically flat, whereas glass surfaces exhibit inherent nanometer-scale irregularities, which influence the roughness of the PTAA films. As the film thickness increases, both surface potential and Ra tend to rise (Fig. S10). PTAA films thicker than 3 µm showed increased surface roughness, resulting in a lack of droplet movement. As expected, the polarity of the surface potential (positive or negative) did not influence the droplet movement. When depositing a GSP material with a Tg between 50 and 70°C onto a Si substrate at room temperature, resulting in a film thickness of 1 to 2 µm, we achieved conditions conductive to water droplet movement. The DAEs that exhibited spontaneous water-droplet movement on a Si substrate include DAE4, DAE11, DAE5, and DAE10 (Smovie4). We aimed to control the movement of the water droplets on the photogenerated GSP patterns. First, we deposited DAE4 (known to exhibit prominent GSP even on Si) onto a Si substrate (Fig. S11, left panels). While DAE4 achieved a surface roughness comparable to that of PTAA, the contact angle hysteresis for the water droplets on DAE4 was larger than on PTAA (Fig. S11, right panels), leading to a lower likelihood of droplet movement. Among the materials tested, DAE4, DAE11, DAE10, and DAE5 exhibited water-droplet movement. Meanwhile their mobility was generally lower than that of PTAA. The higher mobility of water on PTAA is attributed to the reduced chemical affinity of its fluorine atoms to water molecules, also to its lower Ra value. We tried to compare the details between DAE4 and PTAA at the molecular level, adopting the electrostatic potential of the molecular electron density by a COSMO-RS method 46,47 . A conductor’s screening-charge density profile (s-profile), which reduces the 3D charge distribution to a 2D histogram, characterizes the electrostatic polarity and the charge distribution of the molecule of interest (see Fig. S11 and related description). The representative s-profiles for DAE4, PTAA and water are shown in Fig. 3 d. The entire s-profile areas can be roughly divided into three regions: the polar (electron rich) region (right side), the nonpolar region (middle area), and the polar (hole rich) region (left side) from the negative to the positive s-range. As shown in Fig. 3 d, the s-profile of the DAE4 molecule has a relatively broad distribution on both sides (hydrophilic region), closer to the s-profile of the water molecule. Since a large σ value indicates an affinity between the surface organic molecules and water molecules, the DAE4 with a higher polarization degree resulted in higher water affinity than that of PTAA. This is consistent with the fact that the less affinity with water the more active droplet movement is. Photo-controllability of GSP: What factors enable stability? Examining the light stability or the instability of the surface potential is essential for such applications as vibration power-generation devices or droplet movement along photo-patterns. The loss of GSPs upon light irradiation is frequently attributed to carrier diffusion within the film 48 . Our studies on the surface potential stability on light exposure reveal a spectrum of behaviors, ranging from materials exhibiting exceptional photostability to those prone to degradation. Figure 4 a illustrates the decrease in surface potential during UV irradiation that is used to assess the stability of PTAA, DAE2, DAE4, DAE5, and Alq3 as a reference sample. UV light with a wavelength of 254 nm was employed, since all of these materials exhibit optical absorption at this wavelength. DAE2 and Alq3 showed a rapid decrease in surface potential to near-zero levels within a few seconds. Such rapid decrease in surface potential upon UV irradiation was also noted for DAE1 and DAE3. In contrast, PTAA and DAE5 remained stable even after 300 seconds of irradiation. DAE4 exhibited a gradual decline over several tens of seconds. A similar trend was observed with UV light of a 365 nm wavelength, a wavelength at which PTAA does not absorb (Figs. S13 and S14). The gradual decrease in surface potential is attributed to conventional dipole neutralization resulting from photocarrier diffusion within the film 48 . In contrast, the rapid decrease occurs too quickly to be explained solely by carrier diffusion. To investigate its cause, we examined the effects of UV irradiation on bilayer samples of DAE2 and PTAA. We first investigated the rapid decrease in surface potential induced by UV irradiation on the DAE2/PTAA bilayer film. Since the PTAA film does not absorb UV light at λ = 365 nm, the GSP should remain largely unaffected by UV exposure. Figure 4 b illustrates the changes in the surface potential of the DAE2/PTAA bilayer film during repeated UV irradiation and subsequent dark storage. After UV irradiation, the surface potential dropped rapidly to below 240 V, which is lower than the contribution level of the PTAA layer. When stored in the dark, the surface potential gradually recovered, suggesting that the carriers generated by UV irradiation shield the contribution of the PTAA layer's surface potential (Fig. 4 b, inset). UV irradiation generates both positive and negative carriers in the surface layer, and some positive carriers neutralize the negative surface potential, and excess negative charges are expelled from the film. Once UV irradiation stops and the film is placed in darkness, the excess positive charges within it gradually dissipate, leading to a slow increase in surface potential (Fig. 4 b. right panel). A similar gradual recovery of surface potential in the dark was also observed for the DAE2 monolayer (Fig. S15). To illustrate the crucial role of the DAE2 free surface in the outflow of excess carriers, we conducted a comparative experiment using an inverted PTAA/DAE2 bilayer device subject to UV light (Fig. 4 c). In this setup, UV light passes through the upper PTAA layer and is absorbed by the DAE2 layer; the DAE2 free surface does not exist. The surface potential initially dropped sharply to a level corresponding to the PTAA layer and remains stable, with no recovery even in the dark. This result suggests that in this configuration, there is no charge movement through the free surface, confirming that charges cannot flow in and out through the surface. These results enforce our conclusion that the rapid decrease in the surface potential under UV irradiation is due to the ejection of excess carriers through the surface and the shielding effect of the residual surface carriers against internal polarization. The significant role of photocarrier conduction in the decrease of GSP suggests a strong relationship between the material's electrical conductivity or insulating properties and its photostability. We explored this relationship by comparing the electrical resistivities of the materials. Figure 4 d displays the electrical resistivity of each deposited film. Photostable GSP materials, such as DAE5 and PTAA, exhibit resistivities two to three orders of magnitude higher than that of the unstable DAE2. Such higher resistivity is likely due to the tertiary butyl modification in DAE5 or the perfluorination in PTAA (Fig. S2), both of which prevent carriers from dissociating from the molecules. This variation in electrical resistivity is consistent with the observed trends in UV radiation stability. These results suggest that the photostability observed in DAE5 and PTAA is due to high insulating properties. In other words, these materials have a very low probability of generating carriers upon light absorption. Even if carriers are generated, they do not diffuse within the film. This variation in carrier conductivity among materials also affects the observation of GSP when the material is deposited onto a conductive substrate (Fig. S16 and Table S3). Table 1 summarizes the factors determining the phenomenon of the current study: the GSP slope, Ra, Tg, affinity with H 2 O molecules, and the UV light controllability of the surface potential. As the interaction between water molecules and the surface molecules intensifies, the adhesion force anchoring the water droplet to the surface increases. From the perspective of intermolecular interaction energy, moving a water droplet across a surface with an electric gradient demands a driving force substantial enough to overcome the resistive force. The stronger the affinity between the water molecules and the surface—reflected in a smaller ΔG value (as shown in Table 1 )—the greater the resistive force, thereby diminishing the effective driving force. This table highlights also a challenging task: it is a future difficult subject to achieve a single material, without layering multiple materials, which is capable to control water droplet movement on a light-irradiated pattern. (See also the conditions shown in Fig. 3 c; Ra < 1nm and |surface potential|~100V.) As a final demonstration, based on these results, we made a bilayer structure consisting of a PTAA layer on top, which facilitates water-droplet movement. To create a light-guided path, we put a DAE4 layer at the bottom, for the purpose to designing structure that reduce GSP upon UV exposure and disturb water movement. As shown in Fig. 4 e, the DAE4 and PTAA layers were deposited onto a Si substrate. Initially, the surface potential of the film was set to -100V. The sample was then subjected to UV irradiation (365 nm, 330 µW/cm²) for 60 seconds through a photomask with a right-angle crank-shaped light-shielding pattern, resulting in a patterned reduction in the surface potential (Fig. 4 e, left panel). Consequently, the surface potential in the UV-irradiated regions decreased to about − 30V, reflecting the contribution of the PTAA layer. The photo-irradiated pattern was visually confirmed, as part of the DAE4 underwent isomerization. When water was introduced to the edge of this pattern, the droplet moved smoothly along the right-angle pattern (Fig. 4 e, right panel and Smovie5). We examined the spontaneous movement of various liquid droplets other than water. Saturated saltwater demonstrated behavior akin to that of pure water without any notable differences. In contrast, methylene iodide and ethylene glycol showed minimal movement (Fig. S17). We concluded that the rapid movement of water droplets driven by GSP is due to water's exceptionally high dielectric constant and low viscosity. Conclusion and perspective We reported significant advancements in generating GSP, optimizing photostability, and discovering spontaneous water-droplet movement as a novel feature. We elucidated the conditions for maximizing GSP generation and enhancing photostability and identified optimal substrate temperatures as 0.8 - 0.85 times Tg, which aligns with enthalpy relaxation conditions. The enhanced photostability of surface potential was achieved with highly insulating molecules, paving the way for improved durability in GSP-based applications. The discovery of spontaneous high-speed water-droplet movement on GSP surfaces represents a groundbreaking advancement with a wide range of potential applications. The proposed method for driving water droplets using GSP enables the creation of customizable flow paths controlled by light and even supports high-speed movement along right-angle paths. This capability to maneuver water droplets on GSP surfaces opens up a new horizon of possibilities in microfluidic devices and "lab-on-a-chip" technologies: including applications in materials processing, energy harvesting, advanced manufacturing, and the detection of chemical and biological analytes. Materials and methods GSP materials PTAA was newly synthesized at Tosoh corp. All DAEs were synthesized at Kobe Natural Products Chemical Co., Ltd. SPP (product number D5626) was purchased from Tokyo Chemical Industry Co., Ltd. GSP film preparation The glass and Si substrates were ultrasonically cleaned with acetone for 15 minutes in a normal indoor environment, followed by UV ozone treatment. GSP film samples were formed by a conventional vacuum deposition under a pressure of 1x10 − 3 Pa, with a deposition rate of 10–20 nm/s unless otherwise specified. Characterization Differential scanning calorimetry was performed using a Rigaku Thermos Plus DSC8230 at a scan rate of 10 K/min under still air. The contact angle of a water droplet was measured using a homemade device. The electrostatic induction method was used to measure the surface potential. See Supporting Information for the surface potential measurement device. AFM observations were performed using a Shimadzu SPM-9600. Abbreviations (PTAA) 1-N,1-N,3-N,3-N-tetrakis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl]adamantane-1,3-diamine (DAE1) 4-[4-[2-[2,4-dimethyl-5-[4-(N-phenylanilino)phenyl]thiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopenten-1-yl]-3,5-dimethylthiophen-2-yl]-N,N-diphenylaniline (DAE2) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-6-(N-phenylanilino)-1-benzothiophen-3-yl]cyclopenten-1-yl]-2-methyl-N,N-diphenyl-1-benzothiophen-6-amine (DAE3) 4-[3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-6-[4-(N-phenylanilino)phenyl]-1-benzothiophen-3-yl]cyclopenten-1-yl]-2-methyl-1-benzothiophen-6-yl]-N,N-diphenylaniline (DAE4) 3-[3,3,4,4,5,5-hexafluoro-2-[5-[2-(4-methoxyphenyl)ethynyl]-2,4-dimethylthiophen-3-yl]cyclopenten-1-yl]-5-[2-(4-methoxyphenyl)ethynyl]-2,4-dimethylthiophene (DAE5) 2-(4-tert-butylphenyl)-4-[2-[5-(4-tert-butylphenyl)-2,4-dimethylthiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopenten-1-yl]-3,5-dimethylthiophene (DAE6) 3-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-1-benzothiophen-3-yl)cyclopenten-1-yl]-2-methyl-1-benzothiophene (DAE7) 3,4-bis(2,4,5-trimethylthiophen-3-yl)furan-2,5-dione (DAE8) [5-[4-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-5-(5-trimethylsilylthiophen-2-yl)thiophen-3-yl]cyclopenten-1-yl]-5-methylthiophen-2-yl]thiophen-2-yl]-trimethylsilane (DAE9) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methoxy-5-(4-methoxyphenyl)thiophen-3-yl]cyclopenten-1-yl]-2-methoxy-5-(4-methoxyphenyl)thiophene (DAE10) 3,4-bis(2-methyl-1-benzothiophen-3-yl)furan-2,5-dione (DAE11) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-5-[(E)-2-phenylethenyl]thiophen-3-yl]cyclopenten-1-yl]-2-methyl-5-[(E)-2-phenylethenyl]thiophene (SPP) 1',3'-Dihydro-8-methoxy-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-[2 H ]indole] (Alq3) Tris(8-hydroxyquinolinato)aluminum Declarations Author contributions TT designed this study and performed all experiments. JS and SN performed the molecular science calculations of the GSP materials. HK, KK, and NM synthesized PTAA. TT, JS, and SN discussed and co-wrote the manuscript. All authors given approval to the final version of the manuscript. Acknowledgements This work was partially supported by JSPS KAKENHI Grant number 21K05214. We acknowledge KAKETSUKEN. Conflicts of interest, The giant surface potential material PTAA used in this paper has been applied for as a Japanese patent “Electrostatic Materials and Electret” (Application No. JP2023-189522) in 2023 by Tosoh Corporation and Osaka Kyoiku University. Availability of data and material, The datasets generated during the current study are available from the corresponding author (TT) on reasonable request. References P. Pattanayak, S.K. Singh, M. Gulati, S. Vishwas, B. Kapoor, D.K. Chellappan, K. Anand, G. Gupta, N.K. Jha, P.K. Gupta, P. Prasher, K. Dua, H. Dureja, D. Kumar, V. 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Supplementary Files SupplementaryinformationSubmitver.docx Supplementary information Smovie1.mp4 Spontaneous movement of water droplets on a PTAA film surface Smovie2.mp4 Reflection of water droplets at the edge of the PTAA film and Smovie3.mp4 Fast and complex motion of water droplets along PTAA patterns Smovie4.mp4 Water droplet movement on various GSP material surfaces Table1.docx Smovie5.mp4 Controlling the movement of water droplets using light irradiation patterns Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5613353","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":415375691,"identity":"c5599a7b-f02e-4cf0-8a14-116caf81eab1","order_by":0,"name":"Tsuyoshi Tsujioka","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-6434-8052","institution":"Osaka Kyoiku Univ.","correspondingAuthor":true,"prefix":"","firstName":"Tsuyoshi","middleName":"","lastName":"Tsujioka","suffix":""},{"id":415375692,"identity":"cb92614d-2d37-497f-9b9b-379a856be34b","order_by":1,"name":"Hiroyuki Kawashima","email":"","orcid":"","institution":"Tosoh Corp.","correspondingAuthor":false,"prefix":"","firstName":"Hiroyuki","middleName":"","lastName":"Kawashima","suffix":""},{"id":415375693,"identity":"7651465e-1fff-470e-8c0f-83de23a0869b","order_by":2,"name":"Kenji Koike","email":"","orcid":"","institution":"Tosoh Corp.","correspondingAuthor":false,"prefix":"","firstName":"Kenji","middleName":"","lastName":"Koike","suffix":""},{"id":415375694,"identity":"d3b4db21-876f-49ae-9ef7-b1ded0a9f4b9","order_by":3,"name":"Naoki Matsumoto","email":"","orcid":"","institution":"Tosoh Corp.","correspondingAuthor":false,"prefix":"","firstName":"Naoki","middleName":"","lastName":"Matsumoto","suffix":""},{"id":415375695,"identity":"731e63c3-fd67-42eb-89cc-576b974353e9","order_by":4,"name":"Junwei Shen","email":"","orcid":"","institution":"Kumamoto Univ.","correspondingAuthor":false,"prefix":"","firstName":"Junwei","middleName":"","lastName":"Shen","suffix":""},{"id":415375696,"identity":"3a3b4cf0-e336-442c-8e6c-32c88c939c69","order_by":5,"name":"Shinichiro Nakamura","email":"","orcid":"","institution":"Kumamoto Univ.","correspondingAuthor":false,"prefix":"","firstName":"Shinichiro","middleName":"","lastName":"Nakamura","suffix":""}],"badges":[],"createdAt":"2024-12-10 05:40:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5613353/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5613353/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":76267660,"identity":"335861a7-b6ca-476a-aa09-236904cdd2b8","added_by":"auto","created_at":"2025-02-14 07:59:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":254004,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Spontaneous movement of water droplets on PTAA surface. (scale bar: 10 mm, Smovie1.mp4) (b) Observation of water-droplet trajectory using Alq3 fluorescence. (c) Schematic illustration of water-droplet driving force. (d) “Reflection” of droplet at edge of film and “repulsion” between droplets.（surface potential=-100～-200 V, scale bar: 10 mm, Smovie2.mp4） (e) Water droplet driving using PTAA patterns (surface potential=-100~-200 V, scale bar: 10 mm, Smovie3.mp4)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/d0da5ff540ef0bd628bacc52.png"},{"id":76266846,"identity":"aea0ae39-e09f-4de8-bddc-df1bab25b5ed","added_by":"auto","created_at":"2025-02-14 07:51:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":455697,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Correlation between Tg and molecular orientation \u0026lt;cosθ\u0026gt; of vacuum deposited organic films. (b) Tsub dependence of GSP slope and DH. (c) AFM images of DAE2 films prepared at three temperatures. After first scan of a 2-mm square, a 10-mm square was subsequently scanned. DAE2 (thickness: 150 nm)/Si (measured 30 minutes after deposition. Cantilever spring constant: 42 nN/nm in dynamic mode). (d) Tsub dependence of DAE2 morphology on grating substrate. (e) Three models for film formation in vacuum-deposition depending on Tsub.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/6acec96fd85225345a642459.png"},{"id":76266839,"identity":"fecae9cf-637d-4b04-8148-75506c8f478f","added_by":"auto","created_at":"2025-02-14 07:51:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":563661,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Advancing and receding contact angles (\u003cem\u003eq\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e and q\u003csub\u003eR\u003c/sub\u003e) on PTAA and DAE2 surfaces. Inflow and outflow rates from micro syringe were 5 mL/s. (b) Surface roughness (Ra) of PTAA and DE-2A films. (c) Conditions for water droplets to move. (Filled marks: droplets don’t move, open marks: can move.) (d) Charge density distribution for PTAA and DAE4 vs water (top figure) and relation between Interaction energies ΔG, contact angle hysteresis D\u003cem\u003eq\u003c/em\u003e , Ra, and Tg (bottom).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/c6e83d88a974d21a66147649.png"},{"id":76266850,"identity":"89eb0548-7902-45fa-9eb8-f11246da3828","added_by":"auto","created_at":"2025-02-14 07:51:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":90899,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Residual ratio of surface potential against UV irradiation (l=254 nm). The initial potential is in brackets. Surface potential change due to UV irradiation (l=365 nm, 330 mW/cm2) on DAE2/PTAA double-layer. (c) Surface potential change due to UV irradiation (l=365 nm, 330 mW/cm2) on PTAA/DAE2 double-layer. (d) Electrical resistivity of PTAA, DAE5, DAE4 and DAE2. (e) Water-droplet movement control by photopatterning using PTAA/DAE4/Si. (scale bar : 10 mm)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/177ce42ca845bdb6c527cdab.png"},{"id":78189872,"identity":"04d48682-b2fb-4e6a-bc48-6a836cad6069","added_by":"auto","created_at":"2025-03-10 19:46:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2447570,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/3bc70e58-62e5-4cf6-8ebf-80c009134bef.pdf"},{"id":76266851,"identity":"3d287c15-2873-4059-abc3-d972fbecd628","added_by":"auto","created_at":"2025-02-14 07:51:30","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5213794,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary information\u003c/p\u003e","description":"","filename":"SupplementaryinformationSubmitver.docx","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/2ce94551eae57ecbd3aae594.docx"},{"id":76266854,"identity":"a166b2b0-72c1-47ac-b500-13df41c9b93b","added_by":"auto","created_at":"2025-02-14 07:51:31","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":12624472,"visible":true,"origin":"","legend":"\u003cp\u003eSpontaneous movement of water droplets on a PTAA film surface\u003c/p\u003e","description":"","filename":"Smovie1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/2753bd4185cb69142651f0bd.mp4"},{"id":76267662,"identity":"9f52cf09-ac11-477e-b870-4c6d80534a66","added_by":"auto","created_at":"2025-02-14 07:59:31","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":20564553,"visible":true,"origin":"","legend":"\u003cp\u003eReflection of water droplets at the edge of the PTAA film and\u003c/p\u003e","description":"","filename":"Smovie2.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/4b7cb50dd5011dcaeba2325b.mp4"},{"id":76268655,"identity":"cf4b99ea-9934-448b-ac8d-6ed85e762617","added_by":"auto","created_at":"2025-02-14 08:15:31","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":4995157,"visible":true,"origin":"","legend":"\u003cp\u003eFast and complex motion of water droplets along PTAA patterns\u003c/p\u003e","description":"","filename":"Smovie3.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/26f97f8d93fe3db31775b14e.mp4"},{"id":76266870,"identity":"347a2b2e-3e78-4f92-b8f3-194447768976","added_by":"auto","created_at":"2025-02-14 07:51:31","extension":"mp4","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":20929886,"visible":true,"origin":"","legend":"\u003cp\u003eWater droplet movement on various GSP material surfaces\u003c/p\u003e","description":"","filename":"Smovie4.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/c7da02a92fd7f7e99e8df023.mp4"},{"id":76266842,"identity":"a37e726b-6af7-4777-8adb-09f2bb8dbf7b","added_by":"auto","created_at":"2025-02-14 07:51:30","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":50578,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Table1.docx","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/3dac008337c375abaa611458.docx"},{"id":76266860,"identity":"5051a4c3-f5b5-4ac3-a8c2-b325983808a0","added_by":"auto","created_at":"2025-02-14 07:51:31","extension":"mp4","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":4911454,"visible":true,"origin":"","legend":"\u003cp\u003eControlling the movement of water droplets using light irradiation patterns\u003c/p\u003e","description":"","filename":"Smovie5.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5613353/v1/7965ef10eac924d6264baf11.mp4"}],"financialInterests":"\u003cb\u003eYes\u003c/b\u003e there is potential Competing Interest.\nThe giant surface potential material PTAA used in this paper has been applied for as a Japanese patent “Electrostatic Materials and Electret” (Application No. JP2023-189522) in 2023 by Tosoh Corporation and Osaka Kyoiku University.","formattedTitle":"Water Droplets Driven by the Giant Surface Potential of Organic Films","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLiquid droplets moving over solid surfaces present a fascinating and significant challenge with promising applications in many fundamental and applied areas in chemistry, physics, and materials science. In applied fields, droplets on solid surfaces play a crucial role in \u0026quot;lab-on-a-chip\u0026quot; technologies. Such applications range from manipulating tiny quantities of materials to medical diagnostics, food technology, chemical synthesis, and the detection of chemical and biological analytes \u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. A key factor driving such a broad range of applications is the ability to precisely control the transport of liquid droplets on the surface \u003csup\u003e4,5\u003c/sup\u003e. Although several principles exist for driving liquid droplets on solid surfaces, such as using ultrasonic waves or surface tension gradients \u003csup\u003e6,7\u003c/sup\u003e, most methods apply an electric field to the droplets through external electrodes or create a charge distribution on the surface \u003csup\u003e8\u0026ndash;12\u003c/sup\u003e. However, these techniques often fail to achieve complex droplet motion, and methods involving electric fields require electrodes to be pre-embedded on the surface.\u003c/p\u003e\n\u003cp\u003eIt has long been recognized that a surface potential is generated when polar molecular materials are vacuum-deposited. Seki et al. first reported the significant surface potential resulting from the room-temperature deposition of tris(8-hydroxyquinolinato)aluminum (Alq3) \u003csup\u003e13,14\u003c/sup\u003e, a material widely used for light emission and electron transport. Tsekouras et al. observed that when various alcohols are deposited on a substrate cooled to extremely low temperatures, the deposited molecules spontaneously orient themselves, creating a negative potential on the free surface. This potential can reverse to a positive potential depending on the substrate temperature \u003csup\u003e15,16\u003c/sup\u003e. In organic electronics, materials exhibiting negative surface potentials as a result of well-designed molecular structures have also been documented \u003csup\u003e17,18\u003c/sup\u003e. Such spontaneously generated surface potentials have significant interest as they influence the carrier injection characteristics of organic light-emitting devices \u003csup\u003e19\u0026ndash;25\u003c/sup\u003e. In the burgeoning field of energy harvesting, attempts have utilized giant surface potential (GSP) films in devices that generate electricity from vibrations and accelerations occurring in daily life \u003csup\u003e18,26\u003c/sup\u003e. Consequently, GSPs have become a major focus in the realm of organic devices and materials.\u003c/p\u003e\n\u003cp\u003eIn this paper, we introduce innovative droplet-driven channels through customizable photopatterning techniques utilizing GSP. To this end, we delve into its intricate formation mechanism during vacuum deposition, describe the precise conditions required for achieving photostability, and demonstrate the spontaneous and complex motion of droplets.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSpontaneous water-droplet moving on GSP surfaces\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAs a unique feature of organic surfaces, we previously unveiled selective metal deposition based on modulation during metal vapor deposition on organic surfaces \u003csup\u003e27\u0026ndash;31\u003c/sup\u003e. During this investigation, since we measured water-droplet contact angles to assess the surface free energy of fluorinated organic films, we encountered a striking phenomenon: the spontaneous movement of water droplets across the surface. Further analysis revealed that this phenomenon stems from the generation of GSP, uncovering a key factor behind the observed behavior.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea and a supplementary movie (Smovie1) vividly demonstrate the spontaneous movement of a water upon deposition onto the surface of a fluorinated organic film (1-N,1-N,3-N,3-N-tetrakis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl]adamantane-1,3-diamine, PTAA) formed through vacuum deposition. The two water droplets initially moved slowly, gradually approached each other before stopping, and halting without merging. The arrows in the figure trace the droplet trajectories. Unlike most high-speed water droplet movement techniques, which rely on electric fields to roll spherical droplets across superhydrophobic surfaces \u003csup\u003e9\u0026ndash;12\u003c/sup\u003e, the spontaneous movement observed on the PTAA surface involved neither a superhydrophobic surface nor rolling droplets. This observation points to a novel mechanism driving the droplet motion, setting it apart from conventional methods.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb captures the striking fluorescence patterns created by the trails of moving droplets. After the droplets traversed the surface, we delicately sprinkled fluorescent Alq3 molecules onto their path. When exposed to UV light, the trails lit up, revealing intricate and captivating fluorescent patterns that highlighted the subtle interactions between the droplets and the surface. The fluorescence data show that the areas to which water droplets moved display weaker fluorescence than the untouched regions. Interestingly, the strongest fluorescence is observed at the boundary between the affected and unaffected areas, suggesting unique electric field distributions in these regions.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec illustrates the driving mechanism behind the movement of water droplets on the GSP surface. Moisture adheres to the GSP surface, reducing the surface potential due to the inflow of charges from the outside and surface adsorption. This creates a potential difference between regions with reduced and unchanged surface potential. Even the slightest dynamical perturbation induces an imbalance in the left and right electric fields, attracting the droplets and causing them to move spontaneously. Indeed, the surface potential of PTAA significantly dropped to about one-third of its initial value: from \u0026minus;\u0026thinsp;150 to -50 V after being exposed to water.\u003c/p\u003e\n\u003cp\u003eFigures \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed and \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee offer further examples of the driving force behind droplet movement (Smovie2). In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed, water droplets, which are moving on a PTAA film deposited on a Si substrate, exhibit rapid movement, and their trajectories reflect off the edges of both the substrate and the PTAA film. This occurs because the surface potential is zero in areas without PTAA film (Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003ea). As a result, no attractive force is present at the outer edge, causing the droplet to be drawn back towards the inner regions where the surface potential exists. In the right panel of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed, several water droplets on the surface interact and move in a manner resembling \u0026ldquo;slow billiard balls.\u0026rdquo; This movement can be explained by a principle similar to the one described in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec (see also Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003eb).\u003c/p\u003e\n\u003cp\u003eBased on this water-droplet driving principle, a PTAA-GSP-pattern, formed on a Si substrate, enables the rapid movement of water droplets along the designed path. This principle allows for controlled water-droplet motion (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee and Smovie3). Horseshoe-shaped and zigzag-shaped PTAA patterns were created on a Si substrate using vacuum deposition with shadow masks. When a water droplet was placed at the edge of the pattern, it efficiently followed the curved or zigzag PTAA path, achieving speeds up to 15 cm/s.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOptimal GSP conditions for spontaneous droplet movement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAfter confirming that the GSP of the vacuum-deposited PTAA film enables the spontaneous movement of water droplets, we initiated a foundational study into the GSP properties of various organic materials and explored the possibility of water-droplet driving. This exploration aimed to assess their potential for driving water droplets on a variety of surfaces. Several challenges are associated with the generation mechanisms and applications of GSP. One key issue is understanding the origin of spontaneous molecular-orientation polarization induced by vapor deposition. Although the equilibrium between the thin supercooled liquid layer formed during deposition and the underlying glassy state is believed to play a crucial role \u003csup\u003e18,32,33\u003c/sup\u003e, the optimal conditions and film structure remain elusive for maximizing GSP. Another significant challenge is the tendency for GSPs to vanish upon exposure to light \u003csup\u003e13,14,34\u003c/sup\u003e, a situation which presents a substantial obstacle in achieving photostability for devices.\u003c/p\u003e\n\u003cp\u003eFirst, we determined the GSP slope of PTAA, various diarylethenes (DAEs) and spiropyran (SPP) (Fig. S2). DAEs, known for their excellent photochromic properties, exhibit remarkable thermal stability in both isomeric states and exceptional fatigue resistance during isomerization reactions \u003csup\u003e35,36\u003c/sup\u003e. They are also gaining attention as potential materials for organic electronics \u003csup\u003e37\u0026ndash;39\u003c/sup\u003e. The molecular orientation\u0026thinsp;\u0026lt;\u0026thinsp;cos\u0026theta;\u0026thinsp;\u0026gt;\u0026thinsp;of the deposited film was derived from the measured GSP slope, the molecular permanent dipole moment (PDM), the molecular volume, and the dielectric constant:\u003c/p\u003e\n\u003cp\u003eGSP slope \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\equiv\\:\\frac{Surface\\:potential}{d}=\\frac{1}{{\\epsilon\\:}_{r}{\\epsilon\\:}_{0}}\\frac{p\u0026lang;\\text{cos}\\theta\\:\u0026rang;}{{V}_{m}}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n\u003cp\u003ewhere \u003cem\u003ep\u003c/em\u003e represents PDM, \u003cem\u003eV\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e denotes the molar volume, \u0026epsilon;₀ and \u0026epsilon;ᵣ correspond to the vacuum and relative permittivity, and \u003cem\u003ed\u003c/em\u003e indicates the GSP layer\u0026rsquo;s thickness. The correlation between \u0026lt;\u0026thinsp;cos\u0026theta;\u0026thinsp;\u0026gt;\u0026thinsp;and Tg was investigated (Figs. S3-S7 and Tables S1 and S2).\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea demonstrates a distinct correlation between Tg and \u0026lt;\u0026thinsp;cos\u0026theta;\u0026thinsp;\u0026gt;\u0026thinsp;. Notably, materials with Tg near room temperature, such as DAE6, DAE7, DAE8, and DAE9, underwent the thermal randomization of molecular orientation during film formation via vapor deposition at room temperature, resulting in almost negligible surface potential.\u003c/p\u003e\n\u003cp\u003eThe dependence of \u0026lang;cos\u0026theta;\u0026rang; on Tg indicates that the former varies with both substrate temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e during deposition and the material\u0026rsquo;s Tg. This suggests the presence of an optimal \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e that maximizes the GSP slope. To explore this further, we investigated how the GSP slope varies with substrate temperature for three representative GSP materials: SPP, PTAA, and DAE2, all of which have different levels of Tg and GSP slopes.\u003c/p\u003e\n\u003cp\u003eAlthough we anticipated that molecular orientation would be thermally disturbed and the GSP slope would decrease when the substrate temperature approaches Tg, our findings revealed an intriguing trend. Surprisingly, the GSP slope decreased even when the substrate temperature was significantly lower than \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e. For the three materials studied, we plotted a graph with substrate temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e normalized by \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e on the horizontal axis and the GSP slope on the vertical axis (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb, top panel). The maximum GSP slope occurred at a substrate temperature approximately 0.8\u0026ndash;0.85 times \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e. For instance, PTAA, with a \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e of 343 K, displayed a maximum GSP slope at a substrate temperature of 287 K, which is close to room temperature (see Fig. S8a for the actual \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e dependence graphs for each material).\u003c/p\u003e\n\u003cp\u003eEdiger et al. reported that an organic film formed by vacuum deposition reaches an enthalpy-relaxed state, where the maximum enthalpy relaxation occurs at substrate temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e of 0.8 to 0.85 times \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e \u003csup\u003e40,41\u003c/sup\u003e. Building on their work, our results suggest that the substrate temperature at which a maximum GSP slope is achieved corresponds to a temperature where the most significant enthalpy relaxation occurs. To further investigate this correlation, we examined the dependence of enthalpy relaxation on \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003esub\u003c/em\u003e\u003c/sub\u003e in vacuum-deposited films using differential scanning calorimetry (Fig. S8b). Our findings revealed that the temperature for maximum enthalpy relaxation aligns with the substrate temperature at which the maximum GSP slope is observed (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb, bottom panel).\u003c/p\u003e\n\u003cp\u003eNext, we explore the origin of the molecular orientation responsible for GSP. Initially, the substrate\u0026apos;s influence on molecular orientation appears most significant in thin films, with this effect clearly depending on film thickness, and is considered to be negligible in films with a thickness of 1 \u0026micro;m \u003csup\u003e42\u003c/sup\u003e. A second suggested factor is the minimization of surface free energy during deposition \u003csup\u003e18\u003c/sup\u003e. However, our findings challenge this hypothesis. To infer the surface free energy, we observed the water contact angles before and after annealing above Tg. Following annealing, the contact angle either increased (indicating a decrease in surface energy, as in PTAA), decreased (DAE2), or remained nearly the same (SPP), as shown in Fig. S9. These results point to a more complex interplay of factors. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb, the substrate temperature at which maximum enthalpy relaxation occurs aligns with the temperature where GSP is maximized. It implies that surface molecular migration during enthalpy relaxation, along with molecular interactions in the glassy state below \u003cem\u003eTg\u003c/em\u003e, are keys to achieve the molecular orientation responsible for GSP.\u003c/p\u003e\n\u003cp\u003eWe therefore analyzed molecular packing in the film at a substrate temperature corresponding to the maximum value (B) of the GSP slope and enthalpy relaxation, as well as at temperatures (A) and (C) where the GSP slope decreases. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec presents atomic force microscopy (AFM) images of the DAE2 surface formed at different substrate temperatures. The AFM analysis scanned a 2-\u0026micro;m square area, followed by scanning the same region within a larger 10-\u0026micro;m area. For samples (B) and (C), the mark from the initial 2-\u0026micro;m scan near the center is barely visible, indicating minimal surface disturbance. In contrast, this mark is clearly visible for sample (A), suggesting that sample (A)\u0026rsquo; surface was more affected by the cantilever during the initial scan, altering its surface morphology. This effect arises because a smaller scanning area increases the density of the cantilever taps in the dynamic mode. These findings suggest that the molecular packing at substrate temperatures corresponding to samples B and C results in surfaces that are relatively robust and resistant to deformation. In contrast, the weaker packing at sample A leads to a more fragile surface, which is more susceptible to changes under mechanical stress. The robust molecular packing observed in samples B likely contributes to the enhanced molecular orientation and GSP observed at this temperature, highlighting the critical role of substrate temperature in controlling both molecular packing and surface morphology.\u003c/p\u003e\n\u003cp\u003eOn the other hand, when DAE2 was evaporated onto a grating substrate at varying levels of \u003cem\u003eT\u003c/em\u003e\u003csub\u003esub\u003c/sub\u003e, AFM observations revealed that, under similar conditions, the grating grooves in sample (C) were more partially filled compared to sample (B) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed). This suggests that, under condition (C), the grooves tend to fill due to the presence of a thin supercooled liquid layer on the surface, a phenomenon consistent with previous studies \u003csup\u003e43,44\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eBased on these results, we propose three models to explain how the film surface behavior is related to the generation of GSPs, depending on \u003cem\u003eT\u003c/em\u003e\u003csub\u003esub\u003c/sub\u003e, and Tg and the enthalpy relaxation/recovery during vacuum-deposition (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee):\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eModel (A)\u003c/strong\u003e: Molecules are rapidly frozen from the vapor phase directly into a glassy state, bypassing the liquid phase. Such rapid freezing restricts molecular migration,\u0026nbsp;leading to the formation of numerous internal voids and a rough, brittle surface. Molecular orientation is suppressed, and GSP does not emerge.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eModel (B)\u003c/strong\u003e: Molecules migrate effectively across the surface, compacting into the underlying glassy layer to form a smooth, dense film. This migration enhances intermolecular interactions, promoting molecular orientation and resulting in the development of GSP.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eModel (C)\u003c/strong\u003e: At high substrate temperatures (but still below Tg), a thin layer of supercooled liquid forms between the migrating layer and the glassy surface. This liquid layer disrupts molecular orientation and enhances surface smoothness through surface tension, thereby inhibiting GSP formation.\u003c/p\u003e\n\u003cp\u003eSurface conditions for droplet moving\u003c/p\u003e\n\u003cp\u003eInitially, water droplets exhibited spontaneous movement only on certain surfaces. For example, the droplet remained stationary on the DAE2 film, which had a surface potential of approximately \u0026minus;\u0026thinsp;100 V, but moved on the PTAA film, which had the same surface potential. To understand the cause of this difference, we measured the advancing and receding contact angles (\u003cem\u003eq\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e: water flows in, \u003cem\u003e\u0026theta;\u003c/em\u003e\u003csub\u003eR\u003c/sub\u003e: it flows out, from the syringe nozzle at 5 \u0026micro;L/s) of a water droplet on the DAE2 and PTAA surfaces. Contact angle hysteresis (defined by D\u003cem\u003eq\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003eq\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e - \u003cem\u003eq\u003c/em\u003e\u003csub\u003eR\u003c/sub\u003e) indicates the surface slipperiness of the droplet, with a smaller \u0026Delta;\u003cem\u003e\u0026theta;\u003c/em\u003e suggesting increased slipperiness at the droplet\u0026rsquo;s edge.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the contact angles for the two surfaces PTAA and DAE2. The contact angle hysteresis D\u003cem\u003eq\u003c/em\u003e was 30\u0026deg; for DAE2 and only 6\u0026deg; for PTAA, indicating that water droplets move more easily on the PTAA surface. Factors influencing contact angle hysteresis include surface roughness (Ra) and the interaction between the water molecules and the surface molecules \u003csup\u003e45\u003c/sup\u003e. Therefore, we used AFM to compare the surface roughness of the PTAA/glass and the DAE2/glass. AFM analysis revealed that PTAA had a smooth surface with an Ra of 0.9 nm, whereas DAE2 exhibited a much larger Ra of 5.7 nm (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb). Such surface smoothness is attributed to the active molecular migration of molecules during deposition, which is facilitated by the low Tg of PTAA.\u003c/p\u003e\n\u003cp\u003eWe further investigated the relationship among the surface potential, the water droplet motion, and the Ra across various DAEs. The water droplets moved spontaneously on PTAA and some DAEs. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec presents conditions for water droplet movement, with a scatter plot correlating the surface potential with Ra (filled symbols: droplets do not move; open symbols: droplets can move). Our analysis suggests that for droplet motion to occur, a surface potential of approximately 100 V and an Ra of around 1 nm or less are required.\u003c/p\u003e\n\u003cp\u003eAdditionally, the difference in droplet movement speed on the PTAA films on Si versus glass substrates can be attributed to surface roughness. Si substrates are atomically flat, whereas glass surfaces exhibit inherent nanometer-scale irregularities, which influence the roughness of the PTAA films. As the film thickness increases, both surface potential and Ra tend to rise (Fig. S10). PTAA films thicker than 3 \u0026micro;m showed increased surface roughness, resulting in a lack of droplet movement. As expected, the polarity of the surface potential (positive or negative) did not influence the droplet movement.\u003c/p\u003e\n\u003cp\u003eWhen depositing a GSP material with a Tg between 50 and 70\u0026deg;C onto a Si substrate at room temperature, resulting in a film thickness of 1 to 2 \u0026micro;m, we achieved conditions conductive to water droplet movement. The DAEs that exhibited spontaneous water-droplet movement on a Si substrate include DAE4, DAE11, DAE5, and DAE10 (Smovie4).\u003c/p\u003e\n\u003cp\u003eWe aimed to control the movement of the water droplets on the photogenerated GSP patterns. First, we deposited DAE4 (known to exhibit prominent GSP even on Si) onto a Si substrate (Fig. S11, left panels). While DAE4 achieved a surface roughness comparable to that of PTAA, the contact angle hysteresis for the water droplets on DAE4 was larger than on PTAA (Fig. S11, right panels), leading to a lower likelihood of droplet movement.\u003c/p\u003e\n\u003cp\u003eAmong the materials tested, DAE4, DAE11, DAE10, and DAE5 exhibited water-droplet movement. Meanwhile their mobility was generally lower than that of PTAA. The higher mobility of water on PTAA is attributed to the reduced chemical affinity of its fluorine atoms to water molecules, also to its lower Ra value.\u003c/p\u003e\n\u003cp\u003eWe tried to compare the details between DAE4 and PTAA at the molecular level, adopting the electrostatic potential of the molecular electron density by a COSMO-RS method \u003csup\u003e46,47\u003c/sup\u003e. A conductor\u0026rsquo;s screening-charge density profile (s-profile), which reduces the 3D charge distribution to a 2D histogram, characterizes the electrostatic polarity and the charge distribution of the molecule of interest (see Fig. S11 and related description). The representative s-profiles for DAE4, PTAA and water are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed. The entire s-profile areas can be roughly divided into three regions: the polar (electron rich) region (right side), the nonpolar region (middle area), and the polar (hole rich) region (left side) from the negative to the positive s-range. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed, the s-profile of the DAE4 molecule has a relatively broad distribution on both sides (hydrophilic region), closer to the s-profile of the water molecule. Since a large \u0026sigma; value indicates an affinity between the surface organic molecules and water molecules, the DAE4 with a higher polarization degree resulted in higher water affinity than that of PTAA. This is consistent with the fact that the less affinity with water the more active droplet movement is.\u003c/p\u003e\n\u003cp\u003ePhoto-controllability of GSP: What factors enable stability?\u003c/p\u003e\n\u003cp\u003eExamining the light stability or the instability of the surface potential is essential for such applications as vibration power-generation devices or droplet movement along photo-patterns. The loss of GSPs upon light irradiation is frequently attributed to carrier diffusion within the film \u003csup\u003e48\u003c/sup\u003e. Our studies on the surface potential stability on light exposure reveal a spectrum of behaviors, ranging from materials exhibiting exceptional photostability to those prone to degradation.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea illustrates the decrease in surface potential during UV irradiation that is used to assess the stability of PTAA, DAE2, DAE4, DAE5, and Alq3 as a reference sample. UV light with a wavelength of 254 nm was employed, since all of these materials exhibit optical absorption at this wavelength. DAE2 and Alq3 showed a rapid decrease in surface potential to near-zero levels within a few seconds. Such rapid decrease in surface potential upon UV irradiation was also noted for DAE1 and DAE3. In contrast, PTAA and DAE5 remained stable even after 300 seconds of irradiation. DAE4 exhibited a gradual decline over several tens of seconds. A similar trend was observed with UV light of a 365 nm wavelength, a wavelength at which PTAA does not absorb (Figs. S13 and S14).\u003c/p\u003e\n\u003cp\u003eThe gradual decrease in surface potential is attributed to conventional dipole neutralization resulting from photocarrier diffusion within the film \u003csup\u003e48\u003c/sup\u003e. In contrast, the rapid decrease occurs too quickly to be explained solely by carrier diffusion. To investigate its cause, we examined the effects of UV irradiation on bilayer samples of DAE2 and PTAA.\u003c/p\u003e\n\u003cp\u003eWe first investigated the rapid decrease in surface potential induced by UV irradiation on the DAE2/PTAA bilayer film. Since the PTAA film does not absorb UV light at \u0026lambda;\u0026thinsp;=\u0026thinsp;365 nm, the GSP should remain largely unaffected by UV exposure. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb illustrates the changes in the surface potential of the DAE2/PTAA bilayer film during repeated UV irradiation and subsequent dark storage. After UV irradiation, the surface potential dropped rapidly to below 240 V, which is lower than the contribution level of the PTAA layer. When stored in the dark, the surface potential gradually recovered, suggesting that the carriers generated by UV irradiation shield the contribution of the PTAA layer\u0026apos;s surface potential (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, inset). UV irradiation generates both positive and negative carriers in the surface layer, and some positive carriers neutralize the negative surface potential, and excess negative charges are expelled from the film. Once UV irradiation stops and the film is placed in darkness, the excess positive charges within it gradually dissipate, leading to a slow increase in surface potential (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb. right panel). A similar gradual recovery of surface potential in the dark was also observed for the DAE2 monolayer (Fig. S15).\u003c/p\u003e\n\u003cp\u003eTo illustrate the crucial role of the DAE2 free surface in the outflow of excess carriers, we conducted a comparative experiment using an inverted PTAA/DAE2 bilayer device subject to UV light (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). In this setup, UV light passes through the upper PTAA layer and is absorbed by the DAE2 layer; the DAE2 free surface does not exist. The surface potential initially dropped sharply to a level corresponding to the PTAA layer and remains stable, with no recovery even in the dark. This result suggests that in this configuration, there is no charge movement through the free surface, confirming that charges cannot flow in and out through the surface. These results enforce our conclusion that the rapid decrease in the surface potential under UV irradiation is due to the ejection of excess carriers through the surface and the shielding effect of the residual surface carriers against internal polarization.\u003c/p\u003e\n\u003cp\u003eThe significant role of photocarrier conduction in the decrease of GSP suggests a strong relationship between the material\u0026apos;s electrical conductivity or insulating properties and its photostability. We explored this relationship by comparing the electrical resistivities of the materials. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed displays the electrical resistivity of each deposited film. Photostable GSP materials, such as DAE5 and PTAA, exhibit resistivities two to three orders of magnitude higher than that of the unstable DAE2. Such higher resistivity is likely due to the tertiary butyl modification in DAE5 or the perfluorination in PTAA (Fig. S2), both of which prevent carriers from dissociating from the molecules. This variation in electrical resistivity is consistent with the observed trends in UV radiation stability.\u003c/p\u003e\n\u003cp\u003eThese results suggest that the photostability observed in DAE5 and PTAA is due to high insulating properties. In other words, these materials have a very low probability of generating carriers upon light absorption. Even if carriers are generated, they do not diffuse within the film. This variation in carrier conductivity among materials also affects the observation of GSP when the material is deposited onto a conductive substrate (Fig. S16 and Table S3).\u003c/p\u003e\n\u003cp\u003eTable\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the factors determining the phenomenon of the current study: the GSP slope, Ra, Tg, affinity with H\u003csub\u003e2\u003c/sub\u003eO molecules, and the UV light controllability of the surface potential. As the interaction between water molecules and the surface molecules intensifies, the adhesion force anchoring the water droplet to the surface increases. From the perspective of intermolecular interaction energy, moving a water droplet across a surface with an electric gradient demands a driving force substantial enough to overcome the resistive force. The stronger the affinity between the water molecules and the surface\u0026mdash;reflected in a smaller \u0026Delta;G value (as shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e)\u0026mdash;the greater the resistive force, thereby diminishing the effective driving force.\u003c/p\u003e\n\u003cp\u003eThis table highlights also a challenging task: it is a future difficult subject to achieve a single material, without layering multiple materials, which is capable to control water droplet movement on a light-irradiated pattern. (See also the conditions shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec; Ra\u0026thinsp;\u0026lt;\u0026thinsp;1nm and |surface potential|~100V.)\u003c/p\u003e\n\u003cp\u003eAs a final demonstration, based on these results, we made a bilayer structure consisting of a PTAA layer on top, which facilitates water-droplet movement. To create a light-guided path, we put a DAE4 layer at the bottom, for the purpose to designing structure that reduce GSP upon UV exposure and disturb water movement. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee, the DAE4 and PTAA layers were deposited onto a Si substrate. Initially, the surface potential of the film was set to -100V. The sample was then subjected to UV irradiation (365 nm, 330 \u0026micro;W/cm\u0026sup2;) for 60 seconds through a photomask with a right-angle crank-shaped light-shielding pattern, resulting in a patterned reduction in the surface potential (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee, left panel). Consequently, the surface potential in the UV-irradiated regions decreased to about \u0026minus;\u0026thinsp;30V, reflecting the contribution of the PTAA layer. The photo-irradiated pattern was visually confirmed, as part of the DAE4 underwent isomerization. When water was introduced to the edge of this pattern, the droplet moved smoothly along the right-angle pattern (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee, right panel and Smovie5).\u003c/p\u003e\n\u003cp\u003eWe examined the spontaneous movement of various liquid droplets other than water. Saturated saltwater demonstrated behavior akin to that of pure water without any notable differences. In contrast, methylene iodide and ethylene glycol showed minimal movement (Fig. S17). We concluded that the rapid movement of water droplets driven by GSP is due to water\u0026apos;s exceptionally high dielectric constant and low viscosity.\u003c/p\u003e"},{"header":"Conclusion and perspective","content":"\u003cp\u003eWe reported significant advancements in generating GSP, optimizing photostability, and discovering spontaneous water-droplet movement as a novel feature. We elucidated the conditions for maximizing GSP generation and enhancing photostability and identified optimal substrate temperatures as 0.8 - 0.85 times Tg, which aligns with enthalpy relaxation conditions. The enhanced photostability of surface potential was achieved with highly insulating molecules, paving the way for improved durability in GSP-based applications.\u003c/p\u003e\n\u003cp\u003eThe discovery of spontaneous high-speed water-droplet movement on GSP surfaces represents a groundbreaking advancement with a wide range of potential applications. The proposed method for driving water droplets using GSP enables the creation of customizable flow paths controlled by light and even supports high-speed movement along right-angle paths. This capability to maneuver water droplets on GSP surfaces opens up a new horizon of possibilities in microfluidic devices and \u0026quot;lab-on-a-chip\u0026quot; technologies: including applications in materials processing, energy harvesting, advanced manufacturing, and the detection of chemical and biological analytes.\u003c/p\u003e"},{"header":"Materials and methods","content":"\n\u003ch3\u003eGSP materials\u003c/h3\u003e\n\u003cp\u003ePTAA was newly synthesized at Tosoh corp. All DAEs were synthesized at Kobe Natural Products Chemical Co., Ltd. SPP (product number D5626) was purchased from Tokyo Chemical Industry Co., Ltd.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eGSP film preparation\u003c/h2\u003e \u003cp\u003eThe glass and Si substrates were ultrasonically cleaned with acetone for 15 minutes in a normal indoor environment, followed by UV ozone treatment. GSP film samples were formed by a conventional vacuum deposition under a pressure of 1x10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e Pa, with a deposition rate of 10\u0026ndash;20 nm/s unless otherwise specified.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCharacterization\u003c/h3\u003e\n\u003cp\u003eDifferential scanning calorimetry was performed using a Rigaku Thermos Plus DSC8230 at a scan rate of 10 K/min under still air. The contact angle of a water droplet was measured using a homemade device. The electrostatic induction method was used to measure the surface potential. See Supporting Information for the surface potential measurement device. AFM observations were performed using a Shimadzu SPM-9600.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003e(PTAA) 1-N,1-N,3-N,3-N-tetrakis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl]adamantane-1,3-diamine\u003c/p\u003e\n\u003cp\u003e(DAE1) 4-[4-[2-[2,4-dimethyl-5-[4-(N-phenylanilino)phenyl]thiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopenten-1-yl]-3,5-dimethylthiophen-2-yl]-N,N-diphenylaniline\u003c/p\u003e\n\u003cp\u003e(DAE2) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-6-(N-phenylanilino)-1-benzothiophen-3-yl]cyclopenten-1-yl]-2-methyl-N,N-diphenyl-1-benzothiophen-6-amine\u003c/p\u003e\n\u003cp\u003e(DAE3) 4-[3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-6-[4-(N-phenylanilino)phenyl]-1-benzothiophen-3-yl]cyclopenten-1-yl]-2-methyl-1-benzothiophen-6-yl]-N,N-diphenylaniline\u003c/p\u003e\n\u003cp\u003e(DAE4) 3-[3,3,4,4,5,5-hexafluoro-2-[5-[2-(4-methoxyphenyl)ethynyl]-2,4-dimethylthiophen-3-yl]cyclopenten-1-yl]-5-[2-(4-methoxyphenyl)ethynyl]-2,4-dimethylthiophene\u003c/p\u003e\n\u003cp\u003e(DAE5) 2-(4-tert-butylphenyl)-4-[2-[5-(4-tert-butylphenyl)-2,4-dimethylthiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopenten-1-yl]-3,5-dimethylthiophene\u003c/p\u003e\n\u003cp\u003e(DAE6) 3-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-1-benzothiophen-3-yl)cyclopenten-1-yl]-2-methyl-1-benzothiophene\u003c/p\u003e\n\u003cp\u003e(DAE7) 3,4-bis(2,4,5-trimethylthiophen-3-yl)furan-2,5-dione\u003c/p\u003e\n\u003cp\u003e(DAE8) [5-[4-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-5-(5-trimethylsilylthiophen-2-yl)thiophen-3-yl]cyclopenten-1-yl]-5-methylthiophen-2-yl]thiophen-2-yl]-trimethylsilane\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;(DAE9) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methoxy-5-(4-methoxyphenyl)thiophen-3-yl]cyclopenten-1-yl]-2-methoxy-5-(4-methoxyphenyl)thiophene\u003c/p\u003e\n\u003cp\u003e(DAE10) 3,4-bis(2-methyl-1-benzothiophen-3-yl)furan-2,5-dione\u003c/p\u003e\n\u003cp\u003e(DAE11) 3-[3,3,4,4,5,5-hexafluoro-2-[2-methyl-5-[(E)-2-phenylethenyl]thiophen-3-yl]cyclopenten-1-yl]-2-methyl-5-[(E)-2-phenylethenyl]thiophene\u003c/p\u003e\n\u003cp\u003e(SPP) 1\u0026apos;,3\u0026apos;-Dihydro-8-methoxy-1\u0026apos;,3\u0026apos;,3\u0026apos;-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2\u0026apos;-[2\u003cem\u003eH\u003c/em\u003e]indole]\u003c/p\u003e\n\u003cp\u003e(Alq3) Tris(8-hydroxyquinolinato)aluminum\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTT designed this study and performed all experiments. JS and SN performed the molecular science calculations of the GSP materials. HK, KK, and NM synthesized PTAA. TT, JS, and SN discussed and co-wrote the manuscript. All authors given approval to the final version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was partially supported by JSPS KAKENHI Grant number 21K05214. We acknowledge KAKETSUKEN.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of interest,\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe giant surface potential material PTAA used in this paper has been applied for as a Japanese patent \u0026ldquo;Electrostatic Materials and Electret\u0026rdquo; (Application No. JP2023-189522) in 2023 by Tosoh Corporation and Osaka Kyoiku University.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material,\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated during the current study are available from the corresponding author (TT) on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eP. Pattanayak, S.K. Singh, M. Gulati, S. Vishwas, B. Kapoor, D.K. Chellappan, K. Anand, G. Gupta, N.K. Jha, P.K. Gupta, P. Prasher, K. Dua, H. Dureja, D. Kumar, V. 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C\u003c/em\u003e, \u003cstrong\u003e115\u003c/strong\u003e, 2356 (2011).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"giant surface potential, organic film, vacuum deposition, water-droplet movement, glass transition temperature, enthalpy relaxation","lastPublishedDoi":"10.21203/rs.3.rs-5613353/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5613353/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Controlling droplet motion on solid surfaces presents significant challenges in chemistry, physics, and materials science, especially for potential applications in \"lab-on-a-chip\" technologies. Traditional methods, such as using electric fields produced by electrodes, often inadequately manage complex droplet motion. Here we show a novel approach utilizing giant surface potentials (GSPs) generated through customizable photopatterning during vacuum deposition. Water droplets move spontaneously due to the potential difference between the GSP surface and the surface where the droplet comes into contact, which has a reduced potential. Our study not only enhances droplet control but also improves the photostability of the surface potential. We identify the optimal conditions for GSP generation, achieved under vacuum-deposition conditions conducive to maximum enthalpy relaxation, and demonstrate its potential for creating customizable, complex droplet flow channels. These advances offer significant promise for microfluidic devices, medical diagnostics, chemical synthesis, and energy harvesting applications.","manuscriptTitle":"Water Droplets Driven by the Giant Surface Potential of Organic Films","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-14 07:51:24","doi":"10.21203/rs.3.rs-5613353/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9dcb9fcb-2341-47e3-94dc-6c887015e427","owner":[],"postedDate":"February 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":44293658,"name":"Physical sciences/Materials science"},{"id":44293659,"name":"Physical sciences/Chemistry/Materials chemistry"}],"tags":[],"updatedAt":"2026-01-28T10:30:32+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-14 07:51:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5613353","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5613353","identity":"rs-5613353","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}