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.