Driving Droplet by Scale Effect on Microstructured Hydrophobic Surfaces

A new type of water droplet transportation mechanism on a microstructured hydrophobic surface is proposed and investigated experimentally and theoretically: a water droplet could be driven by scale effect under disturbance and vibration, which is different from the traditional contact angle-gradient-based method. A scale-gradient microstructured hydrophobic surface is fabricated in which the area fraction is kept constant, but the scales of the micropillars are monotonically changed. When additional water or horizontal vibration is applied, the original water droplet could move unidirectionally in the direction from the small scale to the large scale. A new model with line tension energy developed very recently could be used to explain these phenomena. When compared with the traditional contact angle-gradient smooth surface, it is also found that dynamic contact angle decreases with increasing the scale of the micropillars along the moving direction under disturbance. These new findings will deepen our understanding of the relationship between topology and dynamic wetting properties, and could be very helpful in designing liquid droplet transportation devices in microfluidic systems.