Control of Droplet Spreading On Ultrasonically Vibrated Hydrophobic Surfaces*

The way a liquid wets a solid surface depends on chemistry, geometry, and local energy inputs. Low-frequency surface vibration causes wetting changes prompted by contact line oscillations. High-frequency (ultrasonic) surface vibration also causes liquid to wet or spread out on a surface, but governing mechanisms are relatively uncharacterized. This work investigates the wetting effects of high frequency vibration (> 20 kHz). Droplets (10 to 50 µL) on hydrophobic surfaces are imaged as they are vibrated with ultrasonic piezoelectric transducers. Spreading of droplets occurs abruptly when a threshold surface acceleration is exceeded. Droplet volume decreases the threshold acceleration, while frequency and initial contact angle impact the extent of spreading. Experimental results are compared to numerical simulations of vibrating droplets to obtain an accurate model. Wetting changes remain after cessation of vibration, however new droplets wet with the original contact angle except for some cases where vibration of liquid can affect the integrity of the coating. Spreading of droplets can be controlled by tuning vibration frequency and amplitude. This control has various industry applications where droplet manipulation is used.