A Review of Physics of Droplet Impact on Various Solid Surfaces Ranging from Hydrophilic to Superhydrophobic and from Rigid to Flexible and its Current Advancements in Interfacial Science

Droplet impact dynamics is ubiquitous in nature, interfacial phenomena, and the underlying of numerous technological applications: bio-printing, tissue engineering, pharmaceutical, fight against COVID-19 pandemic, smart biomaterials, and flexible electronics. Over the last decade, expeditious advancement of novel functional interfacial surfaces, high-speed visualization, nanoscience, nanotechnology, machine learning, computational power, as well as the connection of flow physics with interfacial science have contributed enhancing the understanding of relevant complex physical phenomena. Droplet, upon impacting onto substrates, can deposit, spread, bounce, and splash. Features of droplet impact physics and surface wettability necessitate elaborate solid-liquid interactions. Given the significance of droplet impact physics for healthcare and electronics, it is recommended for scientific community to direct researches to profound understanding of such complex physics. Therefore, this review initially focuses on liquid-solid interfacial science. Secondly, droplet impact physics on numerous solid surfaces were discussed. Substrates with various wettability and physical features were considered: hydrophilic, hydrophobic, superhydrophobic, smooth, rough, and flexible elastic surfaces. Furthermore, numerous advancements of droplet impact on solid surfaces related to advanced technologies and challenges including printed electronics, smart biomaterials, tissue engineering, machine learning, and COVID-19 pandemic were reviewed. Finally, this review outlines future perspectives and research directions in complex droplet impact physics.