Sliding of a Droplet on a Micro-grooved Surface

We characterize non-wetting behavior of a micro-grooved surface by measuring the sliding angle of a water droplet on it. The rectangular micro-grooves are prepared using photo-lithography techniques on a silicon wafer by coating SU8 photo-resist. A high-speed camera is used to record the motion of the droplet, while the surface is tilted gradually to measure the sliding angle. We vary the pitch of the grooves and study its effect on the sliding angle of the droplet in two configurations: (a) droplet migration along the direction of grooves, and (b) droplet migration in the direction perpendicular to the grooves. In case (b), the advancing contact line attaches and receding contact line detaches with the surface as the droplet migrates downward. In contrast to this behavior, in case (a), no change in the relative position of the advancing and receding contact lines is observed. The sliding angle in case (a) is found to be smaller than that observed in case (b). In both cases, it is found that with the increase of the pitch, the sliding angle decreases due to the increase in the contact angle. A theoretical model for sliding angle is also developed and compared against our experimental results.