Acoustophoretic buffer exchange in microfluidics: quantifying radiation-driven migration, streaming, and flow effects

Acoustofluidic buffer exchange enables continuous, contact-free replacement of the fluid surrounding suspended particles and cells, offering a gentle alternative to conventional centrifugation-based processing. Its performance is governed by the interplay between acoustic radiation-driven particle migration and streaming-induced fluid transport. Here, we investigate buffer exchange in a silicon-glass bulk acoustic wave microchannel actuated by a piezoelectric transducer, combining experimental characterization, numerical modeling, and biological validation. Particle transport was examined across a range of actuation voltages and flow rates to evaluate the competing effects of particle migration, interface displacement, and hydrodynamic residence time. No-flow calibration experiments confirmed the expected quadratic dependence of migration velocity on acoustic actuation, establishing a radiation-dominated reference independent of flow. Under continuous operation, however, finite residence time and acoustically induced interface motion limited achievable buffer exchange. Metrics relating particle migration to interface velocity and residence time enabled identification of operating regimes associated with efficient particle transfer and varying degrees of buffer preservation. Numerical simulations supported the experimental observations and clarified the distinct contributions of acoustic radiation forces and acoustic streaming. Biological validation using U2OS and INS-1 pancreatic β-cells demonstrated effective buffer exchange, high cell viability (>98%), and improved recovery compared with centrifugation-based processing. These findings provide practical design guidelines for balancing particle transfer, buffer retention, and biological compatibility in continuous acoustofluidic cell-processing systems.