Spreading of droplets impacting on ultrasonically oscillating substrates

Controlling droplet spread upon impact on a substrate is important in many industrial processes, including spray cooling, additive manufacturing, inkjet printing, and spray coating. Advances in acoustofluidics have paved the way for using ultrasonic vibrations as an effective way to manipulate fluids. To understand the effects on spreading dynamics, we present a study of droplet impact on an ultrasonically oscillating substrate. The substrate was a fabricated lithium niobate piezoelectric device operating in thickness mode at a frequency close to 7MHz. The oscillation amplitude was modulated by changing the voltage provided to the device. The morphology and spreading dynamics at various amplitude and impact Weber numbers were recorded using a high-speed camera. We will show that the maximum spreading diameter changes non-monotonically where it first decreases and then increases with the increasing amplitude of the substrate oscillation. We will also demonstrate that the droplet shows vertical and horizontal stretching for impact on an oscillating substrate, while only radial stretching is observed for impact on the static substrate. Subsequently, we will show that these changes in spreading dynamics and droplet morphology are a consequence of additional forces (streaming forces and acoustic radiation pressure) arising from non-linearities in the fluid-acoustic interactions along with the droplet inertia and surface tension.