257397 Sculpting and Atomizing Pinned Drops with Localized Acoustic Pressures of Surface Acoustic Waves: Exponentially Small Contact Angles
Surface acoustic wave microelectromechanical (MEMS) devices are piezoelectric crystals with an interdigitated transducer on the surface of the crystal. Upon the application of a high frequency electrical signal, a surface acoustic wave (SAW) propagates along the surface of the crystal as a Rayleigh wave. Recently, SAW devices have been coupled with small amounts of liquid (~ 0.1 mm3) in microfluidic devices. When a SAW wave interacts with a liquid film, it generates acoustic streaming enabling the manipulation of the liquid film, and, at sufficient amplitude, aerosolizes the film. Due to viscous dissipation, the SAW wave diffracting from the solid substrate into the liquid drop produces an exponentially decaying time-averaged pressure in the drop. In this work, we show that if the drop is pinned against a bounding wall such as the filter paper used in this study, the localized acoustic pressure generates a sequence of surface droplets at the contact line, whose dimensions decay in the same manner as the acoustic pressure. The undulating interfacial profile near the contact line also inherits this exponential decay, such that the averaged contact angle is exponentially small. The size distribution of surface drops is collapsed under the exponential scaling that depends only on the SAW decay rate and amplitude. This paper also presents a numerical and experimental study of the streaming and aerosolization of liquids via surface acoustic waves. A high-speed imaging system is used to characterize the fluid's free surface and aerosolization as a function of power and liquid properties. Several regimes, including a stable liquid film, initial droplet formation, thin film aerosolization, and cavitation are identified and explored. A two-dimensional numerical model based on the Laplace-Young equation is developed to model the profile of the liquid film. It is further shown that the threshold power for atomization scales as 5th power of the SAW decay rate for very localized acoustic pressures.