Experimental and numerical studies of droplet self-assembly behaviors on hydrophilic/hydrophobic patterned surface

Abstract

Self-assembled and well-defined fluid patterns of ink droplet deposition are building blocks for printed electronics and next generation of OLED displays. Using surface energy differences created by hydrophobic/hydrophilic patterned (Alt-Wet) surface to propel/retain liquid during the droplet deposition has renewed interests for high resolution printing. In this study, the droplet deposition and split dynamics on the patterned surface have been visualized experimentally. The numerical simulations have revealed that the contact angle contrast (ΔΘ) of the Alt-Wet surface can be as small as 25° to successfully split a droplet by the hydrophobic stripe, providing large enough Reynolds number (Re) for kinetic energy against droplet internal viscous bonding energy holding droplet together. In addition, the droplet splitting, satelliting, and splashing modes are highly influenced by the Alt-Wet surface pattern geometry that governs the stability of liquid diffusion and pressure distribution during the droplet deposition. The derived energy conservation model has theoretically confirmed the experimental and numerical findings that the droplet deposition dynamics are the function of fluid properties and surface wetting characteristics when given Alt-Wet unit. This systematic study on droplet deposition and self-assemble dynamics can provide general design guidance for high resolution ink patterning for printed flexible electronics and micro-fluidics applications.