We investigate the aggregation of a dense suspension of particles (volume fraction, φ∼ 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 Fp responsible for the migration of particles to the center of the microwell and shear-induced diffusion force FSID 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 Fp> FSID) are concentrated efficiently into an equilibrium spot, whose diameter scales with the initial particle volume fraction as dcs∼ φ0.3. In contrast, we found that as the local particle volume fraction at the center of the microwell approaches φ∼ 0.1 such that FSID≥ Fp, 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. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.