Frequency-dependent streaming flows from acoustically actuated bubbles and sharp edges

Acoustic streaming is used to produce fluid flows for applications such as mixing in microfluidics and propulsion in microrobotics. Typically, acoustically responsive structures, such as a bubbles and sharp edges, generate streaming by vibrating when excited with an acoustic field. To date, most work has focused on building microscale systems with a single type of acoustically responsive structure actuated at its primary resonance frequency. In this work, we develop a joint computational/experimental framework to predict the frequency-dependent streaming flows produced in systems with more than one type of acoustically responsive structure, each with its own resonance behavior.

Experimentally, we fabricated bubbles and sharp edges with two-photon lithography, used piezoelectric transducers to acoustically excite the structures, visualized the flows using confocal microscopy, and measured the flow fields with microparticle image velocimetry. We quantitatively compared these results with numerical predictions from a perturbation theory model of streaming. We then used eigenfrequency analyses of the bubble and sharp edge vibrations to interpret the frequency response of the fluid flows. Using these measurements, we predicted the frequency dependence of the fluid flows produced by a microparticle that has both a bubble and a sharp edge. This work provides a framework to predict and control the streaming flows produced in systems containing multiple acoustically responsive structures.