Driving liquid droplets on microstructured gradient surface by mechanical vibration

Dynamic transportation of water droplet on microstructured hydrophobic silicon substrate with a contact angle gradient is studied in this article. We propose a new type of substrate designed with microridges on a silicon wafer fabricated by photolithograph and subsequent coating with octadecyltrichlorosilane (OTS). When horizontal vibration is applied on the substrate, the water droplet can move to the direction with larger solid–liquid contact area fraction. It is found that the dynamic contact angle of the water droplet varied with the vibration direction and the speed of the substrate. The contact angle difference at the left and the right edge of the water droplet on the vibrated surface is obviously magnified compared to the contact angle difference of the droplet on the static surfaces, resulting in the increasing driving force. When the vibration amplitude of the exciter source (20 Hz) increases from 0.14 to 0.43 mm, the average velocity of 10 μL water droplet increases from 10 to 23 mm/s. The internal flow pattern of the water droplet moving on the microstructured hydrophobic surfaces is also obtained using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques. Both rolling and slipping motions are observed for the water droplet during its movement in the vibrated substrate.