Rayleigh wave scattering from sessile droplets

Radiation of energy by large-amplitude leaky Rayleigh waves is regarded as one of the key physical mechanisms regulating the actuation and manipulation of droplets in surface acoustic wave (SAW) microfluidic devices. The interaction between a SAW and a droplet is highly complex and is presently the subject of extensive research. This paper investigates the existence of an additional interaction mechanism based on the propagation of quasi-Stoneley waves inside sessile droplets deposited on a solid substrate. In contrast with the leaky Rayleigh wave, the energy of the Stoneley wave is confined within a thin fluid layer in contact with the substrate. The hypothesis is confirmed by three-dimensional finite element simulations and ultrasonic scattering experiments measuring the reflection of Rayleigh waves from droplets of different diameters. Moreover, real-time monitoring of the droplet evaporation process reveals a clear correlation between the droplet contact angle and the spectral information of the reflected Rayleigh signal, thus paving the way for ultrasonic measurements of surface tension.