Acoustic radiation forces at liquid interfaces impact the performance of acoustophoresis

Acoustophoresis is a method well suited for cell and microbead separation or concentration for downstream analysis in microfluidic settings. One of the main limitations that acoustophoresis share with other microfluidic techniques is that the separation efficiency is poor for particle-rich suspensions. We report that flow laminated liquids can be relocated in a microchannel when exposed to a resonant acoustic field. Differences in acoustic impedance between two liquids cause migration of the high-impedance liquid towards an acoustic pressure node. In a set of experiments we charted this phenomenon and show herein that it can be used to either relocate liquids with respect to each other, or to stabilize the interface between them. This resulted in decreased medium carry-over when transferring microbeads (4% by volume) between suspending liquids using acoustophoresis. Furthermore we demonstrate that acoustic relocation of liquids occurs for impedance differences as low as 0.1%.