Emergence of exotic microscale morphologies at the gas–liquid interface of bubbles in polymeric liquids

Emergence of exotic microscale morphologies at the gas–liquid interface of bubbles in polymeric liquids | 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 Research Article Emergence of exotic microscale morphologies at the gas–liquid interface of bubbles in polymeric liquids Mitsuhiro Ohta, Shohei Yamamoto, Shugo Fujimoto, Norihiko Tokui, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9220639/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 5 You are reading this latest preprint version Abstract This study reports the discovery of novel interfacial phenomena at the gas–liquid interfaces of rising bubbles in polymeric liquids. These phenomena are characterized by the formation of unique and exotic microstructures that extend from the bottom of the bubble. Experimental observations indicate that the morphology of these microstructures depends strongly on both the viscoelastic properties of the polymeric liquid and the bubble equivalent diameter. Significantly, the stability and complexity of these structures defy classical explanations based purely on surface tension, which would typically favor interface minimization. Instead, our findings highlight the critical role of localized molecular interactions between the gas phase and the polymeric liquid. This study provides new insights into the microscale coupling of polymer dynamics and interfacial physics, suggesting that local molecular interaction can dominate over bulk rheological properties in complex fluid systems. bubble rise motion viscoelastic properties interfacial microstructures Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Hydrophobically modified alkali-soluble emulsion (HASE) polymers are commercial associative thickeners widely utilized in water-borne coating formulations as alternatives to conventional non-associative thickeners. Aqueous HASE solutions exhibit pronounced non-Newtonian, shear-thinning, and viscoelastic behavior. Previous studies by Soto et al. [ 1 ] and Ohta et al. [ 2 , 3 ] on the motion of single bubbles rising freely in HASE solutions revealed extraordinary bubble morphologies. These include extremely long trailing tails, multi-branched threadlike structures, elongated skirts, and deformed bubbles with rotating thread-shaped tails, which deviate significantly from the typical profiles observed in other viscoelastic fluids [ 4 – 7 ]. Such distinctive morphologies are attributed to the unique viscoelastic nature of HASE solutions. Beyond these macroscale features, Soto et al. [ 1 ] also reported unprecedented “fishbone-type” interfacial microstructures. Building on these findings, the present study investigates the dynamic motion of single bubbles in highly viscous HASE solutions, with a particular focus on the emergence of structurally complex microscale interfacial phenomena. Experiment and materials Primal™ TT-935 (Dow Chemical Co.) was employed as the hydrophobically modified alkali-soluble emulsion (HASE) polymer. Two HASE solutions (1.3 and 2.1 wt.%) were prepared by adjusting the pH to 9.0 using a sodium hydroxide solution, followed by dilution with ion-exchanged water. Our experimental apparatus shown in Fig. 1 is a rectangular column with a height of 1.1 m and 0.15 m X 0.15 m cross-section made from acrylic resin. At the bottom of the column, a dumping cup and an inverted funnel with a syringe nozzle were installed. After collecting the desired bubble volume injected from the syringe inside the dumping cup, the bubble was released via rotation, which allowed controlling the bubble size. The released bubble began to freely rise through an inverted funnel placed just above the dumping cup. The released bubbles rose freely through the inverted funnel. Bubble morphologies were captured using a high-speed video camera (HAS-D3, Ditect Corp.) at either 2000 frames/s (800 × 600 pixels) or 1000 frames/s (1280 × 1024 pixels), with an LED backlight for illumination. Quantitative data were subsequently extracted from the recorded images using a PC. Physical properties The rheological properties of the 1.3 and 2.1 wt.% HASE solutions were measured using a rheometer (RS-600, Thermo Fisher Scientific K.K.) at a controlled temperature of T S = 293 K. Figure 2 presents the apparent viscosity ( h L ) and the first normal stress difference ( N 1 ) as functions of the shear rate ( γ ). In this figure, open circles and solid squares represent the h L and N 1 profiles, respectively. Data for the 2.1 wt.% HASE solution are shown in red, while those for the 1.3 wt.% solution are in blue. As illustrated in Figure 2, both HASE solutions exhibit shear-thinning and viscoelastic properties. The liquid density ( r L ) was measured using a densimeter (DA-110, Kyoto Electronics Manufacturing Co., Ltd). When measuring the surface tension ( s ) of viscoelastic liquids using conventional techniques with large solid-liquid contact interfaces, such as the Du Noüy ring method, the value of s , including the harmful effects of the elastic property, can be obtained. The measured value of s is influenced by the elastic effect when the liquid film of the viscoelastic liquid tears from the ring. Accordingly, s was estimated using the drop weight method with a correction factor proposed by Lee et al. [8] to minimize the negative elastic effects of viscoelastic solutions on the surface tension. Indeed, Lee et al. [9] demonstrated that for viscoelastic liquids, the drop weight method with the correction of Lee et al. [8] higher accuracy than the Du Noüy ring method. The relaxation time ( l ) was estimated from the h L and N 1 profiles following the method of Leider and Bird [10]. The measured physical properties, including r L , s and l , are summarized in Table 1. Here, m 0 denotes the zero-shear viscosity, which corresponds to the constant apparent viscosity observed in the low-shear-rate regime. For both solutions, the viscosity begins to decrease in the high-shear-rate regime. The estimated relaxation times are l = 2.5 X 10 –4 s and l = 6.5 X 10 –3 s, respectively. These values suggest that the viscoelastic properties of both HASE solutions are relatively weak from a macroscopic physical perspective. Table 1 Physical properties of the HASE solutions. Solution ρ L [kg/m 3 ] µ 0 [Pa·s] σ [N/m] l [s] 2.1 wt.% 1008.9 1.6 X 10 1 4.8 X 10 –2 6.5 X 10 –3 1.3 wt.% 1007.3 7.4 X 10 –1 4.7 X 10 –2 2.5 X 10 –4 Results and discussions Bubble Shapes Figures 3 and 4 illustrate the global bubble morphologies observed in 1.3 wt.% and 2.1 wt.% HASE solutions. These results demonstrate that bubble shapes are highly sensitive to both polymer concentration and bubble size. Notably, the observed features deviate significantly from those reported in our previous studies (Ohta et al. [ 2 , 3 ]), with complex microstructures emerging at the trailing edges. In the 1.3 wt.% solution (Fig. 3 ), a conventional cusped shape is observed at a sphere volume-equivalent diameter of d = 4.9 mm. As d increases to 11.5 mm, a slender, elongated trailing tail forms from the bottom of the bubble. This structure further evolves into a long bifurcating tail at d = 13.9 mm. However, at d = 15.6 mm, the morphology undergoes a distinct transition: the tail adopts a thin, planar, membrane-like configuration that appears transparent in the images due to its extreme thinness. For larger bubbles ( d = 19.7 and 22.5 mm), while the membrane-like planar structure persists, it develops intricate fishbone-like branching microstructures. Figure 4 presents bubble morphologies consistent with those reported in our previous studies (Ohta et al. [ 2 , 3 ]). Small bubbles exhibit a cusped profile with a distinct trailing tail at d = 9.1 and 11.5 mm. As the bubble size increases, this tail bifurcates into multiple elongated threadlike structures. For larger bubbles ( d = 15.6, 19.7, and 24.8 mm), several of these elongated filaments are observed. Notably, these structures are thicker at the lateral edges than in the central region; the latter is sufficiently thin to appear nearly transparent in the photographs. Consequently, these central regions may be nearly imperceptible under standard observation. Detailed inspection reveals intricate microstructures formed between the primary lateral tails. These observations from Figs. 3 and 4 indicate that the unique viscoelasticity of the HASE solutions becomes increasingly pronounced as the bubble diameter and its terminal rise velocity increase. Interfacial microstructures Figures 5 and 6 present snapshots of the microstructures observed in the 1.3 wt.% and 2.1 wt.% HASE solutions. For the 1.3 wt.% solution, as shown in Fig. 3 , microscale fishbone-like branching structures emerged at bubble diameters of d = 19.7 and 22.5 mm. Figure 5 provides magnified images of these structures across five sequential segments, from the upper to the lower portions of the membrane-like trailing tail. Two distinct morphologies were identified: Type 1, characterized by bilateral branching from both the left and right sides (see Multimedia view), and Type 2, characterized by unilateral branching. While Soto et al. [ 1 ] previously reported fishbone-like microstructures at the distal tips of elongated tails, the structures observed in Fig. 5 are qualitatively distinct. Crucially, these branching morphologies defy explanation via classical continuum fluid dynamics. If surface tension were the dominant governing force, these intricate interfaces would be expected to relax into spherical droplets or smooth out to minimize surface energy. Instead, these fishbone-like structures maintain their complex, non-equilibrium geometries for a sustained period. Figure 6 also presents four snapshots for d = 14.5 mm. The three images on the left capture the upper, middle, and lower segments of the elongated filaments, while the right-most image provides a magnified view of the bubble base (see Multimedia view). In this case, approximately five elongated, serrated (jagged) filaments are formed between the thicker, primary bifurcated tails. Furthermore, complex branching structures are either generated from or fragmented from these serrated tails across a wide spatial range. Notably, these intricate features elude macro-scale observation, as they are not discernible in the global bubble profile shown in Fig. 4 . High-magnification inspection of the bubble base (right-most snapshot) reveals that these microscale branching structures exhibit a self-similar pattern, reminiscent of fractal geometries. Similar to the observations at d = 12.4 mm, it is evident that the unique viscoelasticity of the HASE solution exerts a more dominant influence on the gas–liquid interface than surface tension. Nevertheless, localized exceptions exist; a closer examination reveals occasional microbubbles downstream, where certain branching structures have retracted or curled inward due to the localized dominance of surface tension. Figure 6 also presents four snapshots for d = 14.5mm. The three images on the left capture the upper, middle, and lower segments of the elongated filaments, while the right-most image provides a magnified view of the bubble base (see Multimedia view). In this case, approximately five elongated, serrated (jagged) filaments are formed between the thicker, primary bifurcated tails. Furthermore, complex branching structures are either generated from or fragmented from these serrated tails across a wide spatial range. Notably, these intricate features elude macro-scale observation, as they are not discernible in the global bubble profile shown in Fig. 4 . High-magnification inspection of the bubble base (right-most snapshot) reveals that these microscale branching structures exhibit a self-similar pattern, reminiscent of fractal geometries. Echoing the behavior observed at d = 12.4 mm, it is evident that the unique viscoelasticity of the HASE solution exerts a more dominant influence on the gas–liquid interface than surface tension. Nevertheless, localized exceptions exist; a closer examination reveals occasional microbubbles downstream, where certain branching structures have retracted or curled inward due to the localized dominance of surface tension. For larger bubbles ( d = 19.7 mm), the prominent microscale branching structures observed at d = 14.5 mm were notably absent. Instead, the morphology was characterized by more than 20 slender, threadlike trailing tails originating from the bubble base. As shown in the left snapshot, the lateral (outer) filaments are significantly thicker than the internal ones. These internal filaments do not extend directly downward; rather, their lengths progressively decrease from the center toward the periphery. Detailed inspection of the magnified views reveals that while microscale branching still occurs at the filament interfaces, the patterns differ significantly from those observed under other conditions, manifesting here as relatively short, discrete branches. The causes of the observed microscale interfacial phenomena remain difficult to isolate due to both experimental and theoretical constraints. The freely rising bubbles, particularly at high terminal velocities, complicate the direct, in-situ measurement of interfacial interactions within the HASE system. Simultaneously, current viscoelastic theoretical frameworks are insufficient for capturing intricate interfacial phenomena at the microscale. Consequently, elucidating the coupling between polymer dynamics and gas–liquid interfaces at the microscale represents a critical objective for future investigations. Bubble rise velocity Table 2 summarizes the bubble rise velocity ( V ) as a function of the bubble diameter ( d ) for both HASE solutions. Due to its higher viscosity, the V values for the 2.1 wt.% solution are consistently lower than those for the 1.3 wt.% solution. Notably, a discontinuous jump in the rise velocity is observed for both concentrations between d = 4.9 and 5.1 mm. Beyond this threshold ( d ≥ 5.1 mm), V increases monotonically with bubble size. This velocity jump is closely associated with a morphological transition: while bubbles at d = 4.9 mm exhibit a simple cusped shape, those at d = 5.1 mm develop an elongated trailing tail. This correlation between the velocity jump and tail formation is consistent with the findings reported by Soto et al. [ 1 ] . Table 2 Bubble rise velocities d [mm] V [m/s] (1.3 wt.% HASE solution) V [m/s] (2.1 wt.% HASE solution) 4.9 1.5 X 10 –2 8.0 X 10 –4 5.1 3.0 X 10 –2 1.4 X 10 –3 6.4 6.5 X 10 –2 2.7 X 10 –3 9.1 1.5 X 10 –1 8.1 X 10 –3 11.5 2.1 X 10 –1 1.3 X 10 –2 15.6 2.5 X 10 –1 2.9 X 10 –2 19.7 2.7 X 10 –1 5.2 X 10 –2 22.5 2.9 X 10 –1 7.0 X 10 –2 24.8 3.0 X 10 –1 8.7 X 10 –2 Conclusions This study unveiled novel interfacial phenomena at the gas-liquid interface of rising bubbles in HASE solutions, previously undocumented in studies of polymeric liquids. These observations are characterized by unique and exotic microstructures, including intricate "fishbone-type" branching and membrane-like trailing tails that originate from the bottom of the bubble. We propose that these distinctive morphologies are attributed to localized molecular interactions between the gas phase and the HASE solution. Unlike macroscopic viscoelastic effects, the emergence of these microstructures appears to be driven by the specific nature of HASE molecules at the interface. These findings suggest that at the microscale, the molecular architecture of the viscoelastic liquids exerts a more profound influence on bubble dynamics than the bulk rheological properties alone, opening a new avenue for exploring interfacial polymer physics. Declarations Author contributions M.O initiated and planned the project. M.O., S.Y., S.F., and N.T. performed experiments. S.I. measured rheological properties and provided expert opinions on phenomena. All authors contributed extensively to the interpretation of the experimental results. The paper was written by M.O., and all authors edited the paper. Funding This work was partially supported by JSPS KAKENHI Grant Nos. JP 15K13872 and JP 19K04194. Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Declaration of interest The authors report no conflict of interest. References Soto E, Goujon C, Zenit R, Manero O (2006) A study of velocity discontinuity for single air bubbles rising in an associative polymer. Phys Fluids 18:121510 Ohta M, Kobayashi N, Shigekane Y, Yoshida Y, Iwata S (2015) The dynamic motion of single bubbles with unique shapes rising freely in hydrophobically modified alkali-soluble emulsion polymer solutions. J Rheol 59:303–316 Ohta M, Tokui N, Fujimoto S, Iwata S (2018) A consideration about threadlike shapes that emerge from the gas-liquid interface of single rising bubbles in a highly viscous viscoelastic liquid. Jpn J Multiph Flow 32:345–351 (in Japanese) Astarita G, Apuzzo G (1965) Motion of gas bubbles in non-Newtonian liquids. AIChE J 11:815–820 De Kee D, Chhabra RP (1988) A photographic study of shapes of bubbles and coalescence in non-Newtonian polymer solutions. Rheol Acta 27:656–660 Hassager O (1979) Negative wake behind bubbles in non-Newtonian liquids, Nature (1979):402–403 Herrera-Velarde JR, Zenit R, Chehata D, Mena B (2003) The flow of non-Newtonian fluids around bubbles and its connection to the jump discontinuity. J Non-Newtonian Fluid Mech 111:199–209 Lee B-B, Ravindra P, Chan E-S (2008) A critical review: surface and interfacial tension measurement by the drop weight method. Chem Eng Commun 195:889–924 Lee B-B, Chan E-S, Ravindra P, Khan TA (2012) Surface tension of viscous biopolymer solutions measured using the du Nouy ring method and the drop weight methods. Polym Bull 69:471–489 Leider PJ, Bird RB (1974) Squeezing flow between parallel disks. I. Theoretical Analysis Ind. Eng Chem Fundamen 13:336–341 Additional Declarations No competing interests reported. Supplementary Files SupplementaryMovie.zip Supplementary movies Supplementary movies are available at https://doi.org******* Cite Share Download PDF Status: Under Revision Version 1 posted Reviewers agreed at journal 05 May, 2026 Reviewers invited by journal 30 Apr, 2026 Editor assigned by journal 31 Mar, 2026 Submission checks completed at journal 31 Mar, 2026 First submitted to journal 25 Mar, 2026 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-9220639","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":634860435,"identity":"879e4984-e23a-4412-b62b-f839307cbed5","order_by":0,"name":"Mitsuhiro Ohta","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9UlEQVRIiWNgGAWjYHACNiC2YWDgYWBgZjBAEcWj5QBDmgTJWg5DtRADdBuYjz3+UHO+jp/njPHngoJt8gwSCYwffjDw5eHSYnaALd3gwLHbEpK9PWbSMwxuGzZIJDBL9jCwFePUcv+NmcQBttsSBud5zJh5DG4z7r+RwCANdG5iA05beIBa/p0DaTH+DNRiD7LlN0EtB9sOSBic7TGQBmpJBGphI2ALW5rE2b5kyZk9x8pAWpIbeB62WfYY4PHLAeZjEhXf7Pj5eZI3f+b5c9u2gT358I0fFcdwhhg2wAh0ksGxBFK0gEEN6VpGwSgYBaNguAIA+BtPuAu8OmwAAAAASUVORK5CYII=","orcid":"","institution":"Tokushima University","correspondingAuthor":true,"prefix":"","firstName":"Mitsuhiro","middleName":"","lastName":"Ohta","suffix":""},{"id":634860436,"identity":"9f402256-2d1d-4c4a-8cdd-d80d8731ae19","order_by":1,"name":"Shohei Yamamoto","email":"","orcid":"","institution":"Tokushima University","correspondingAuthor":false,"prefix":"","firstName":"Shohei","middleName":"","lastName":"Yamamoto","suffix":""},{"id":634860437,"identity":"3eed1468-b046-46c5-bcb1-95c9ac9c9583","order_by":2,"name":"Shugo Fujimoto","email":"","orcid":"","institution":"Tokushima University","correspondingAuthor":false,"prefix":"","firstName":"Shugo","middleName":"","lastName":"Fujimoto","suffix":""},{"id":634860438,"identity":"04fadd2f-c461-4e32-aa45-8091dda6f04e","order_by":3,"name":"Norihiko Tokui","email":"","orcid":"","institution":"Tokushima University","correspondingAuthor":false,"prefix":"","firstName":"Norihiko","middleName":"","lastName":"Tokui","suffix":""},{"id":634860439,"identity":"fb67652c-125b-4c72-bb67-bad96d53a447","order_by":4,"name":"Shuichi Iwata","email":"","orcid":"","institution":"Nagoya Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Shuichi","middleName":"","lastName":"Iwata","suffix":""}],"badges":[],"createdAt":"2026-03-25 08:55:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9220639/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9220639/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108814823,"identity":"76c50dd3-2a39-448f-bf3e-813190aaaa71","added_by":"auto","created_at":"2026-05-08 16:20:37","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":97910,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the experimental apparatus. The acrylic resin test section has a height of 1.1 m and a cross-section of 0.15 m X 0.15 m.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/2a23b7220d68ff383f63fd7d.png"},{"id":108812269,"identity":"4638e6f5-f93f-4ed1-8ce4-6bd8f50fe4e6","added_by":"auto","created_at":"2026-05-08 16:10:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":115965,"visible":true,"origin":"","legend":"\u003cp\u003eApparent viscosity (\u003cem\u003eh\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e) and first normal stress difference (\u003cem\u003eN\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e) as a function of the shear rate (\u003cstrong\u003eγ\u003c/strong\u003e).\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/d9756b3fcab97d26bf243c77.png"},{"id":108812267,"identity":"ff73db50-b9c8-498e-a884-31c8b276aa89","added_by":"auto","created_at":"2026-05-08 16:10:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":250344,"visible":true,"origin":"","legend":"\u003cp\u003eGlobal shapes of single bubbles rising in the 1.3 wt% HASE solution. Representative bubble morphologies are shown for diameters of \u003cem\u003ed\u003c/em\u003e= 4.9, 11.5, 13.9, 15.6, 19.7, and 22.5 mm.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/98f1edfac182016b6e7ede82.png"},{"id":108812273,"identity":"7b8fae42-8962-4462-9da3-d36f6597b3d0","added_by":"auto","created_at":"2026-05-08 16:10:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":260948,"visible":true,"origin":"","legend":"\u003cp\u003eGlobal shapes of single bubbles rising in the 2.1 wt% HASE solution. Representative bubble morphologies are shown for diameters of \u003cem\u003ed\u003c/em\u003e= 4.9, 9.1, 11.5, 12.4, 14.5, 15.6, 19.7, and 24.8 mm.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/ebb23601ca3c4889d86ff8dd.png"},{"id":108812266,"identity":"027efc8d-2341-429e-b485-54fed2b5fd33","added_by":"auto","created_at":"2026-05-08 16:10:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":289215,"visible":true,"origin":"","legend":"\u003cp\u003eMicroscopic observations of fishbone-like branching structures emerging from the bottom of bubbles (\u003cem\u003ed\u003c/em\u003e = 19.7 and 22.5 mm) in the 1.3 wt% HASE solution. (a) Type 1: Branching structures forming from both the left and right sides (Multimedia view: https://doi.org/......). (b) Type 2: Branching structures forming from a single side (Multimedia view: https://doi.org/.......).\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/c48366bc891b9bad7184ccad.png"},{"id":108812275,"identity":"77b66630-4e37-45fd-89aa-66b510774bd4","added_by":"auto","created_at":"2026-05-08 16:10:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":450865,"visible":true,"origin":"","legend":"\u003cp\u003eMagnified views of the trailing tails at the rear of bubbles in the 2.1 wt% HASE solution. (a) Two close-up snapshots for \u003cem\u003ed\u003c/em\u003e = 11.5 mm (Multimedia view: https://doi.org/.......). (b, c) For \u003cem\u003ed\u003c/em\u003e = 12.4 mm: (b) three snapshots on the left show the upper, middle, and lower sections of the elongated bifurcating tail (Multimedia view: https://doi.org/.......), and (c) the snapshot on the right provides a magnified view of the bubble base. (d, e) For \u003cem\u003ed\u003c/em\u003e = 14.5 mm: (d) three snapshots on the left show the top-to-bottom sections of the elongated threadlike tail (Multimedia view: https://doi.org/.......), and (e) the snapshot on the right shows a magnified view of the bubble base (Multimedia view: https://doi.org/.......). (f, g) For \u003cem\u003ed\u003c/em\u003e= 19.7 mm: (f) the left snapshot shows thin, elongated threadlike tails, (g) while the two snapshots on the right provide magnified views of these structures.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/1fa075126bdbc1077be03350.png"},{"id":108819192,"identity":"32c23279-1822-4d06-9188-1c5a05eb57e4","added_by":"auto","created_at":"2026-05-08 16:36:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1696487,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/857f8ab3-74b8-4420-8076-e129b03e9491.pdf"},{"id":108812262,"identity":"7e7d8bf0-c5e2-413f-8455-477e02fc736b","added_by":"auto","created_at":"2026-05-08 16:10:06","extension":"zip","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":23798193,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSupplementary movies\u003c/strong\u003e Supplementary movies are available at https://doi.org*******\u003c/p\u003e","description":"","filename":"SupplementaryMovie.zip","url":"https://assets-eu.researchsquare.com/files/rs-9220639/v1/0f8b9862885f1d2f6e47ac28.zip"}],"financialInterests":"No competing interests reported.","formattedTitle":"Emergence of exotic microscale morphologies at the gas–liquid interface of bubbles in polymeric liquids","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHydrophobically modified alkali-soluble emulsion (HASE) polymers are commercial associative thickeners widely utilized in water-borne coating formulations as alternatives to conventional non-associative thickeners. Aqueous HASE solutions exhibit pronounced non-Newtonian, shear-thinning, and viscoelastic behavior. Previous studies by Soto et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] and Ohta et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] on the motion of single bubbles rising freely in HASE solutions revealed extraordinary bubble morphologies. These include extremely long trailing tails, multi-branched threadlike structures, elongated skirts, and deformed bubbles with rotating thread-shaped tails, which deviate significantly from the typical profiles observed in other viscoelastic fluids [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSuch distinctive morphologies are attributed to the unique viscoelastic nature of HASE solutions. Beyond these macroscale features, Soto et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] also reported unprecedented \u0026ldquo;fishbone-type\u0026rdquo; interfacial microstructures. Building on these findings, the present study investigates the dynamic motion of single bubbles in highly viscous HASE solutions, with a particular focus on the emergence of structurally complex microscale interfacial phenomena.\u003c/p\u003e"},{"header":"Experiment and materials","content":"\u003cp\u003ePrimal\u0026trade; TT-935 (Dow Chemical Co.) was employed as the hydrophobically modified alkali-soluble emulsion (HASE) polymer. Two HASE solutions (1.3 and 2.1 wt.%) were prepared by adjusting the pH to 9.0 using a sodium hydroxide solution, followed by dilution with ion-exchanged water. Our experimental apparatus shown in Fig. \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e is a rectangular column with a height of 1.1 m and 0.15 m X 0.15 m cross-section made from acrylic resin. At the bottom of the column, a dumping cup and an inverted funnel with a syringe nozzle were installed. After collecting the desired bubble volume injected from the syringe inside the dumping cup, the bubble was released via rotation, which allowed controlling the bubble size. The released bubble began to freely rise through an inverted funnel placed just above the dumping cup. The released bubbles rose freely through the inverted funnel. Bubble morphologies were captured using a high-speed video camera (HAS-D3, Ditect Corp.) at either 2000 frames/s (800 \u0026times; 600 pixels) or 1000 frames/s (1280 \u0026times; 1024 pixels), with an LED backlight for illumination. Quantitative data were subsequently extracted from the recorded images using a PC.\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003ePhysical properties\u003c/h2\u003e\n \u003cp\u003eThe rheological properties of the 1.3 and 2.1 wt.% HASE solutions were measured using a rheometer (RS-600, Thermo Fisher Scientific K.K.) at a controlled temperature of \u003cem\u003eT\u003c/em\u003e\u003csub\u003eS\u003c/sub\u003e = 293 K.\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eFigure 2 presents the apparent viscosity (\u003cem\u003eh\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e) and the first normal stress difference (\u003cem\u003eN\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e) as functions of the shear rate (\u003cstrong\u003e\u0026gamma;\u003c/strong\u003e). In this figure, open circles and solid squares represent the \u003cem\u003eh\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e and \u003cem\u003eN\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e profiles, respectively. Data for the 2.1 wt.% HASE solution are shown in red, while those for the 1.3 wt.% solution are in blue. As illustrated in Figure 2, both HASE solutions exhibit shear-thinning and viscoelastic properties. The liquid density (\u003cem\u003er\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e) was measured using a densimeter (DA-110, Kyoto Electronics Manufacturing Co., Ltd). When measuring the surface tension (\u003cem\u003es\u003c/em\u003e) of viscoelastic liquids using conventional techniques with large solid-liquid contact interfaces, such as the Du No\u0026uuml;y ring method, the value of \u003cem\u003es\u003c/em\u003e, including the harmful effects of the elastic property, can be obtained. The measured value of \u003cem\u003es\u003c/em\u003e is influenced by the elastic effect when the liquid film of the viscoelastic liquid tears from the ring. Accordingly, \u003cem\u003es\u003c/em\u003e was estimated using the drop weight method with a correction factor proposed by Lee \u003cem\u003eet al.\u003c/em\u003e [8] to minimize the negative elastic effects of viscoelastic solutions on the surface tension. Indeed, Lee et al. [9] demonstrated that for viscoelastic liquids, the drop weight method with the correction of Lee \u003cem\u003eet al.\u003c/em\u003e [8] higher accuracy than the Du No\u0026uuml;y ring method. The relaxation time (\u003cem\u003el\u003c/em\u003e) was estimated from the \u003cem\u003eh\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e and \u003cem\u003eN\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e profiles following the method of Leider and Bird [10]. The measured physical properties, including \u003cem\u003er\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e, \u003cem\u003es\u003c/em\u003e and \u003cem\u003el\u003c/em\u003e, are summarized in Table 1. Here, \u003cem\u003em\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e denotes the zero-shear viscosity, which corresponds to the constant apparent viscosity observed in the low-shear-rate regime. For both solutions, the viscosity begins to decrease in the high-shear-rate regime. The estimated relaxation times are l = 2.5 X 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e s and l = 6.5 X 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e s, respectively. These values suggest that the viscoelastic properties of both HASE solutions are relatively weak from a macroscopic physical perspective.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePhysical properties of the HASE solutions.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSolution\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026rho;\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e [kg/m\u003csup\u003e3\u003c/sup\u003e]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e [Pa\u0026middot;s]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026sigma;\u003c/em\u003e [N/m]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e\u003cem\u003el\u003c/em\u003e [s]\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e2.1 wt.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e1008.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1.6 X 10\u003csup\u003e1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e4.8 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e6.5 X 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e1.3 wt.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e1007.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e7.4 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e4.7 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e2.5 X 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"Results and discussions","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eBubble Shapes\u003c/h2\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrate the global bubble morphologies observed in 1.3 wt.% and 2.1 wt.% HASE solutions. These results demonstrate that bubble shapes are highly sensitive to both polymer concentration and bubble size. Notably, the observed features deviate significantly from those reported in our previous studies (Ohta et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]), with complex microstructures emerging at the trailing edges. In the 1.3 wt.% solution (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e), a conventional cusped shape is observed at a sphere volume-equivalent diameter of \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.9 mm. As \u003cem\u003ed\u003c/em\u003e increases to 11.5 mm, a slender, elongated trailing tail forms from the bottom of the bubble. This structure further evolves into a long bifurcating tail at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;13.9 mm. However, at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;15.6 mm, the morphology undergoes a distinct transition: the tail adopts a thin, planar, membrane-like configuration that appears transparent in the images due to its extreme thinness. For larger bubbles (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;19.7 and 22.5 mm), while the membrane-like planar structure persists, it develops intricate fishbone-like branching microstructures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents bubble morphologies consistent with those reported in our previous studies (Ohta et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]). Small bubbles exhibit a cusped profile with a distinct trailing tail at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;9.1 and 11.5 mm. As the bubble size increases, this tail bifurcates into multiple elongated threadlike structures. For larger bubbles (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;15.6, 19.7, and 24.8 mm), several of these elongated filaments are observed. Notably, these structures are thicker at the lateral edges than in the central region; the latter is sufficiently thin to appear nearly transparent in the photographs. Consequently, these central regions may be nearly imperceptible under standard observation. Detailed inspection reveals intricate microstructures formed between the primary lateral tails. These observations from Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e indicate that the unique viscoelasticity of the HASE solutions becomes increasingly pronounced as the bubble diameter and its terminal rise velocity increase.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eInterfacial microstructures\u003c/h3\u003e\n\u003cp\u003eFigures \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e present snapshots of the microstructures observed in the 1.3 wt.% and 2.1 wt.% HASE solutions. For the 1.3 wt.% solution, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e, microscale fishbone-like branching structures emerged at bubble diameters of \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;19.7 and 22.5 mm. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e provides magnified images of these structures across five sequential segments, from the upper to the lower portions of the membrane-like trailing tail. Two distinct morphologies were identified: Type 1, characterized by bilateral branching from both the left and right sides (see Multimedia view), and Type 2, characterized by unilateral branching. While Soto et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] previously reported fishbone-like microstructures at the distal tips of elongated tails, the structures observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e are qualitatively distinct. Crucially, these branching morphologies defy explanation via classical continuum fluid dynamics. If surface tension were the dominant governing force, these intricate interfaces would be expected to relax into spherical droplets or smooth out to minimize surface energy. Instead, these fishbone-like structures maintain their complex, non-equilibrium geometries for a sustained period.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e also presents four snapshots for \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;14.5 mm. The three images on the left capture the upper, middle, and lower segments of the elongated filaments, while the right-most image provides a magnified view of the bubble base (see Multimedia view). In this case, approximately five elongated, serrated (jagged) filaments are formed between the thicker, primary bifurcated tails. Furthermore, complex branching structures are either generated from or fragmented from these serrated tails across a wide spatial range. Notably, these intricate features elude macro-scale observation, as they are not discernible in the global bubble profile shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e. High-magnification inspection of the bubble base (right-most snapshot) reveals that these microscale branching structures exhibit a self-similar pattern, reminiscent of fractal geometries. Similar to the observations at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;12.4 mm, it is evident that the unique viscoelasticity of the HASE solution exerts a more dominant influence on the gas\u0026ndash;liquid interface than surface tension. Nevertheless, localized exceptions exist; a closer examination reveals occasional microbubbles downstream, where certain branching structures have retracted or curled inward due to the localized dominance of surface tension.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e also presents four snapshots for \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;14.5mm. The three images on the left capture the upper, middle, and lower segments of the elongated filaments, while the right-most image provides a magnified view of the bubble base (see Multimedia view). In this case, approximately five elongated, serrated (jagged) filaments are formed between the thicker, primary bifurcated tails. Furthermore, complex branching structures are either generated from or fragmented from these serrated tails across a wide spatial range. Notably, these intricate features elude macro-scale observation, as they are not discernible in the global bubble profile shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e. High-magnification inspection of the bubble base (right-most snapshot) reveals that these microscale branching structures exhibit a self-similar pattern, reminiscent of fractal geometries. Echoing the behavior observed at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;12.4 mm, it is evident that the unique viscoelasticity of the HASE solution exerts a more dominant influence on the gas\u0026ndash;liquid interface than surface tension. Nevertheless, localized exceptions exist; a closer examination reveals occasional microbubbles downstream, where certain branching structures have retracted or curled inward due to the localized dominance of surface tension.\u003c/p\u003e \u003cp\u003eFor larger bubbles (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;19.7 mm), the prominent microscale branching structures observed at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;14.5 mm were notably absent. Instead, the morphology was characterized by more than 20 slender, threadlike trailing tails originating from the bubble base. As shown in the left snapshot, the lateral (outer) filaments are significantly thicker than the internal ones. These internal filaments do not extend directly downward; rather, their lengths progressively decrease from the center toward the periphery. Detailed inspection of the magnified views reveals that while microscale branching still occurs at the filament interfaces, the patterns differ significantly from those observed under other conditions, manifesting here as relatively short, discrete branches.\u003c/p\u003e \u003cp\u003eThe causes of the observed microscale interfacial phenomena remain difficult to isolate due to both experimental and theoretical constraints. The freely rising bubbles, particularly at high terminal velocities, complicate the direct, in-situ measurement of interfacial interactions within the HASE system. Simultaneously, current viscoelastic theoretical frameworks are insufficient for capturing intricate interfacial phenomena at the microscale. Consequently, elucidating the coupling between polymer dynamics and gas\u0026ndash;liquid interfaces at the microscale represents a critical objective for future investigations.\u003c/p\u003e\n\u003ch3\u003eBubble rise velocity\u003c/h3\u003e\n\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e summarizes the bubble rise velocity (\u003cem\u003eV\u003c/em\u003e) as a function of the bubble diameter (\u003cem\u003ed\u003c/em\u003e) for both HASE solutions. Due to its higher viscosity, the \u003cem\u003eV\u003c/em\u003e values for the 2.1 wt.% solution are consistently lower than those for the 1.3 wt.% solution. Notably, a discontinuous jump in the rise velocity is observed for both concentrations between \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.9 and 5.1 mm. Beyond this threshold (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;5.1 mm), \u003cem\u003eV\u003c/em\u003e increases monotonically with bubble size. This velocity jump is closely associated with a morphological transition: while bubbles at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.9 mm exhibit a simple cusped shape, those at \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;5.1 mm develop an elongated trailing tail. This correlation between the velocity jump and tail formation is consistent with the findings reported by Soto et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBubble rise velocities\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003ed\u003c/em\u003e [mm]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eV\u003c/em\u003e [m/s] (1.3 wt.% HASE solution)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eV\u003c/em\u003e [m/s] (2.1 wt.% HASE solution)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.0 X 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.0 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.4 X 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.5 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.7 X 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.1 X 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.1 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.3 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.5 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.9 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e19.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.7 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.2 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e22.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.9 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.0 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e24.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.0 X 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.7 X 10\u003csup\u003e\u0026ndash;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study unveiled novel interfacial phenomena at the gas-liquid interface of rising bubbles in HASE solutions, previously undocumented in studies of polymeric liquids. These observations are characterized by unique and exotic microstructures, including intricate \u0026quot;fishbone-type\u0026quot; branching and membrane-like trailing tails that originate from the bottom of the bubble. We propose that these distinctive morphologies are attributed to localized molecular interactions between the gas phase and the HASE solution. Unlike macroscopic viscoelastic effects, the emergence of these microstructures appears to be driven by the specific nature of HASE molecules at the interface. These findings suggest that at the microscale, the molecular architecture of the viscoelastic liquids exerts a more profound influence on bubble dynamics than the bulk rheological properties alone, opening a new avenue for exploring interfacial polymer physics.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e M.O initiated and planned the project. M.O., S.Y., S.F., and N.T. performed experiments. S.I. measured rheological properties and provided expert opinions on phenomena. All authors contributed extensively to the interpretation of the experimental results. The paper was written by M.O., and all authors edited the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e This work was partially supported by JSPS KAKENHI Grant Nos. JP 15K13872 and JP 19K04194.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e The data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of interest\u003c/strong\u003e The authors report no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSoto E, Goujon C, Zenit R, Manero O (2006) A study of velocity discontinuity for single air bubbles rising in an associative polymer. Phys Fluids 18:121510\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOhta M, Kobayashi N, Shigekane Y, Yoshida Y, Iwata S (2015) The dynamic motion of single bubbles with unique shapes rising freely in hydrophobically modified alkali-soluble emulsion polymer solutions. J Rheol 59:303\u0026ndash;316\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOhta M, Tokui N, Fujimoto S, Iwata S (2018) A consideration about threadlike shapes that emerge from the gas-liquid interface of single rising bubbles in a highly viscous viscoelastic liquid. Jpn J Multiph Flow 32:345\u0026ndash;351 (in Japanese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAstarita G, Apuzzo G (1965) Motion of gas bubbles in non-Newtonian liquids. AIChE J 11:815\u0026ndash;820\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDe Kee D, Chhabra RP (1988) A photographic study of shapes of bubbles and coalescence in non-Newtonian polymer solutions. Rheol Acta 27:656\u0026ndash;660\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHassager O (1979) Negative wake behind bubbles in non-Newtonian liquids, Nature (1979):402\u0026ndash;403\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHerrera-Velarde JR, Zenit R, Chehata D, Mena B (2003) The flow of non-Newtonian fluids around bubbles and its connection to the jump discontinuity. J Non-Newtonian Fluid Mech 111:199\u0026ndash;209\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLee B-B, Ravindra P, Chan E-S (2008) A critical review: surface and interfacial tension measurement by the drop weight method. Chem Eng Commun 195:889\u0026ndash;924\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLee B-B, Chan E-S, Ravindra P, Khan TA (2012) Surface tension of viscous biopolymer solutions measured using the du Nouy ring method and the drop weight methods. Polym Bull 69:471\u0026ndash;489\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLeider PJ, Bird RB (1974) Squeezing flow between parallel disks. I. Theoretical Analysis Ind. Eng Chem Fundamen 13:336\u0026ndash;341\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"bubble rise motion, viscoelastic properties, interfacial microstructures","lastPublishedDoi":"10.21203/rs.3.rs-9220639/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9220639/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study reports the discovery of novel interfacial phenomena at the gas\u0026ndash;liquid interfaces of rising bubbles in polymeric liquids. These phenomena are characterized by the formation of unique and exotic microstructures that extend from the bottom of the bubble. Experimental observations indicate that the morphology of these microstructures depends strongly on both the viscoelastic properties of the polymeric liquid and the bubble equivalent diameter. Significantly, the stability and complexity of these structures defy classical explanations based purely on surface tension, which would typically favor interface minimization. Instead, our findings highlight the critical role of localized molecular interactions between the gas phase and the polymeric liquid. This study provides new insights into the microscale coupling of polymer dynamics and interfacial physics, suggesting that local molecular interaction can dominate over bulk rheological properties in complex fluid systems.\u003c/p\u003e","manuscriptTitle":"Emergence of exotic microscale morphologies at the gas–liquid interface of bubbles in polymeric liquids","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-08 15:29:37","doi":"10.21203/rs.3.rs-9220639/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"324160913361655586380004259294786925849","date":"2026-05-05T13:42:49+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-30T08:48:10+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-31T09:21:16+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-31T09:20:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2026-03-25T08:48:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1e9ab96a-8251-4854-9b36-30c5b01db7e6","owner":[],"postedDate":"May 8th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"324160913361655586380004259294786925849","date":"2026-05-05T13:42:49+00:00","index":12,"fulltext":""},{"type":"reviewersInvited","content":"5","date":"2026-04-30T08:48:10+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-05-18T04:54:55+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-08 15:29:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9220639","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9220639","identity":"rs-9220639","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}