Aggregation of a dense suspension of particles in a microwell using surface acoustic wave microcentrifugation

Abstract

We investigate the aggregation of a dense suspension of particles (volume fraction, $\varphi \sim 0.1$) in a PDMS microwell by employing surface acoustic wave (SAW) microcentrifugation. In spite of acoustic attenuation at the LiNbO₃–PDMS interface, a significant portion of the energy (> 80%) is available for driving fluid actuation, and, in particular, microcentrifugation in the microwell via acoustic streaming. Rapid particle aggregation can then be affected in the microcentrifugation flow, arising as a consequence of the interplay between the hydrodynamic pressure gradient force $F_{\text{p}}$ responsible for the migration of particles to the center of the microwell and shear-induced diffusion force $F_{\text{SID}}$ that opposes their aggregation. Herein, we experimentally investigated the combined effect of the particle size $a$ and sample concentration $c$ on these microcentrifugation flows. The experimental results show that particles of smaller size and lower sample concentration (such that $F_{\text{p}} > F_{\text{SID}}$) are concentrated efficiently into an equilibrium spot, whose diameter scales with the initial particle volume fraction as $d_{\text{cs}} \sim \varphi^{0.3}$. In contrast, we found that as the local particle volume fraction at the center of the microwell approaches $\varphi \sim 0.1$ such that $F_{\text{SID}} \ge F_{\text{p}}$, the particle aggregation fails. Additionally, we also investigate the effects of the well diameter, and the height, lateral positioning of microwell and the liquid volume on the microcentrifugation.