Dynamics of water drop detachment from a superhydrophobic surface induced by an ultrasonic field

We present the dynamics of sessile water drops during their detachment from a superhydrophobic surface induced by ultrasound. The superhydrophobic surface not only serves as a reflector of the ultrasound emitted from the source but also reduces the adhesive force between the drop and the solid surface. The drop is subject to an acoustic radiation force in the ultrasonic field due to the nonlinear effect of the latter. By shifting the reflector upward to approach the first resonance distance, the sessile drop is first elongated in the vertical direction, with its contact line and contact angle decreasing, and finally detaches from the superhydrophobic surface when the acoustic radiation force overcomes the sum of the gravitational and adhesive forces. The acoustic radiation pressure and acoustic radiation force are calculated by solving the acoustic field with the finite element method. The results indicate that the distribution of acoustic radiation pressure provides the upward force to make the drop detach. After its detachment from the reflector, the drop undergoes vertical vibration accompanied by shape oscillations. Oscillations of a water drop that is pinned on the reflector are also demonstrated.