Kinematics of Surface Acoustic Wave Driven Liquid Droplets

This paper reports the experimental characterization of a liquid droplet driven by surface acoustic wave (SAW). The SAW device was fabricated on a single-sided polished Y-cut 128° rotated lithium niobate (LiNbO₃) substrate. The kinematics and deformation of the droplet was investigated at different driving voltages and droplet volumes. The kinematics of the droplet is characterized by four regimes: initial stationary state, acceleration and strong deformation, deceleration and steady motion with constant velocity. The maximum velocity of the droplet is proportional to the square of the applied voltage and does not change significantly with its volume. Bellow a critical volume, the steady velocity increases with the applied voltage. Above this volume, the steady velocity decreases with the applied voltage. In general, a larger droplet volume results in a higher steady velocity. The results from the investigation reported here can be used for optimizing the driving scheme of SAW-driven droplet-based microfuidics.