(435d) 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 5^th power of the SAW decay
rate for very localized acoustic pressures.