Microfluidic circulatory flows induced by resonant vibration of diaphragms

Abstract

Piezoelectric elements coupled to thin diaphragms in contact with a fluid chamber are useful to induce fluid motion. These devices are typically operated at the first resonant mode of the coupled piezoelectric/diaphragm composite with low electrical power excitation to achieve large microscale mode shape displacement. Experimental studies of hydrodynamics within the diaphragm coupled chamber at the first and the higher resonant modes have revealed the expected bulk fluid motion associated with the mode shapes, and a coupled set of two complex flow phenomena that drive particle migration to modal nodes and direct bulk flow circulation within the chamber. The phenomena are believed to be acoustic in origin with a focusing force that drives particles to the nodal lines of the mode shapes, and a net directional flow believed to be attributable to acoustic streaming. A diaphragm driven acoustic chamber 6 mm in diameter, 30 μm in depth with 0.2 mm thick lead-zirconate-titanate (PZT) actuator was fabricated for an experimental survey. A quantitative flow field characterization was performed with particle image velocimetry (PIV) using a laser scanning confocal microscope with 2-μm resolution. Complicated in-plane velocity profiles were found near the interface of circulations, where acoustic intensity was considered large. One dominant contributor to observed particle focusing has been associated with flow stagnation regions within the flow chamber. Acoustic streaming regions were successfully identified near the interface between circulation partitions, coincident with particle focusing. Possible applications of the flow phenomena presented include active mixing, pumping, and particle focusing.