Acoustic cavitation using superhydrophobic surfaces*

Both surface roughness and hydrophobicity have been shown to promote cavitation, by trapping small bubbles (or cavitation nuclei) or lowering the energy barrier required to nucleate a bubble. With the aim of enhancing cavitation, we experimentally examine the acoustic cavitation dynamic of superhydrophobic surfaces consisting of hydrophobic micro-structures. Acoustic waves generated by a High-Intensity Focused Ultrasound transducer are focalized onto the probed surfaces. Two high-speed cameras are used to simultaneously record the cavitation dynamic from the side and bottom views. Microstructures with a low packing fraction do not exhibit enhanced cavitation behavior, compared to flat surfaces. Interestingly, substrates with a more compacted microstructure can trap an air layer (i.e., in the Cassie-Baxter wetting state) and act as an acoustic mirror due to the trapped air-water interface. As the acoustic intensity is increased, the acoustic radiation forces overcame the capillary forces and push water into the microstructures. We further study the influence of acoustic power on the wetting state and transition, and cavitation dynamics using microstructures of various roughness.