Effect of the sound speed mismatch between fluid and channel on the particle alignment in a standing surface acoustic wave device

Standing surface acoustic wave (SSAW) combined with a microfluidic channel has been emerging as a device to manipulate biological and chemical particles for separation, mixing, and enrichment. Particles are trapped at pressure minima in the interference pattern determined by the acoustic properties of the fluid and the channel wall. To predict particle motion, it is essential to analyze the pressure distribution formed in the fluid. Here, we investigated the effect of sound speeds of fluid and channel wall on the pattern of aligned particles. We developed the numerical full model capable of analyzing wave propagation in each component of a SSAW device and tracing particles. The computational simulation with varied sound speeds of fluid and channel wall demonstrated how the particle alignment pattern is altered by the mismatch between their sound speeds. The computational results were compared with the experimental ones obtained for fluids with varied sound speeds. We revealed the difference in the propagation path of acoustic wave between fluid and channel wall plays a critical role in determining the particle movement. Our study provides an intuition on how particles subjected to SSAW migrate, which is beneficial in designing an acoustofluidic device to manipulate particles in a desired manner.