Control of droplet movement on an inclined wall with sawtoothed wettability pattern by applying ultrasonic vibration

This study deals with the control of the movement of liquid droplets rolling down an inclined plate based on the differences in the wettability of the plate. We used a photoreactive polymer poly(7-methacryloyloxy coumarin) (PMC) whose molecular structure can be changed reversibly to realize different wettabilities by ultraviolet irradiation. We proposed employing sawtooth patterns at boundaries between areas with different contact angles to control the droplet trajectory. Furthermore, we experimentally observed that the droplet moves along a line inclined to the direction of gravity. The droplet behavior can be analyzed using a theoretical model based on the droplet dynamics wherein the surface tension acting on the contact line and the gravitational force are considered. The theoretical results suggest that inclination from the gravitational direction can be increased if the advancing contact angle is reduced. In the experiments conducted herein, ultrasonic vibration was applied to the inclined plate to reduce the contact angle hysteresis. The results showed that the advancing contact angle actually decreased and that the droplet trajectory was controlled to realize motion along a line with inclination angle almost twice of that realized without vibration.

A saw-tooth pattern of boundary between the areas with different contact angles was proposed to control the trajectory of a droplet rolling down on an inclined plate. As shown in the right figure, the droplet immediately diverges from the direction of gravity and moves in accordance with the target lines.