Experimental study on the characteristics of capillary surface waves on a liquid film on an ultrasonically vibrated substrate

In this paper, we report experimental results on the propagation speed, wavelength, and frequency of capillary surface waves induced by imposing ultrasonic vibration on the substrate of a few millimeter thick water film. It is common knowledge that imposing a low frequency vertical vibration (similar to 1-10 (2) Hz) on a liquid body or layer results in bulk oscillation of the liquid and creation of standing surface waves with a frequency comparable to the excitation frequency (harmonic and subharmonic Faraday waves). Our experiments showed that the response of a liquid layer excited by horizontal and vertical ultrasonic frequency (20-68 kHz) is quite different, hypothetically because of a huge difference and mismatch between the characteristic times of vibration (small) and momentum diffusion (large) at high excitation frequencies. We observed that an excitation frequency of the order of 10(4) Hz results in the formation of traveling waves with frequencies of the order of 10(2) Hz, i.e. much smaller than the excitation frequency. In addition, we measured the speed of the Marangoni flow on the surface of a liquid film. We found that imposing ultrasonic vibration did not affect the speed of the Marangoni flow, corroborating that the ultrasonic vibration-induced traveling waves do not cause a bulk translational motion, and thus the flow on the surface is potential, whereas adjacent to the substrate a boundary layer forms. The studied problem is not only an intriguing fluid dynamics problem, it also has an existing application in the fabrication of thin coatings and solid films under ultrasonic excitation for emerging technologies, such as printed electronics and photovoltaics.