Fluid rheology modulates particle enrichment kinetics in acoustic-oscillatory microfluidics

Efficient particle enrichment underpins microfluidic clinical diagnostics, yet many biologically relevant samples are non-Newtonian and their behaviour in acoustic-oscillatory systems remains incompletely understood. Here, we compare particle-enrichment kinetics in Newtonian glycerol-water (GW), viscoelastic poly(ethylene oxide) (PEO), and shear-thinning xanthan gum (XG) solutions using a common acoustic-oscillatory microfluidic platform. The enrichment rate followed the order PEO > water > XG > GW under representative conditions. At f_ac = 1 MHz, V_pp = 17 V, and f_flow = 12 Hz, 1.4 wt% PEO reduced the enrichment time by approximately 60% relative to water. Increased hydrodynamic resistance accounts for the slower response in GW, whereas shear thinning lowers the apparent viscosity of XG as the deformation rate increases. The faster response in PEO suggests an additional viscoelastic contribution. This contribution may assist migration toward the central node at 1 MHz but oppose migration locally between the centreline and the two off-centre nodes at 2 MHz. The voltage-dependent enrichment rate was described empirically by 1/t_e = aVbpp. Phase-resolved simulations based on the measured rheology and channel geometry showed fluid-dependent velocity, shear-rate, and apparent-viscosity fields. Fluid rheology must therefore be considered when designing acoustic-oscillatory enrichment systems for complex samples.