A look-up table protocol for calibrating standing SAW acoustofluidics

The acoustic radiation force (ARF) acting on particles measures the performance of microfluidic devices driven by standing surface acoustic waves (SSAWs). However, existing ARF calibration techniques rely on image post-processing or additional equipment. This work proposes a look-up table method to determine the ARF by examining the particle acoustophoresis mode in discrete phase-modulated SSAW fields, where the phase difference between the two counter-propagating SAWs is changed at fixed time intervals. Theoretical analysis indicates that particles in a straight channel migrate laterally either in the "locked" mode or the "drift" mode, while mode switching can be observed when the interval reaches a critical value highly dependent on the ARF amplitude. A look-up table can then be established for a given SSAW device. By observing the particle acoustophoresis modes at different phase-changing intervals, the ARF amplitude can be obtained from the easily determined critical interval. The procedure is demonstrated experimentally in an SSAW acoustofluidic device and compared with the particle tracking protocol to verify the former's effectiveness and demonstrate its operational simplicity. Inspired by the established theory, a method to improve the efficiency of particle acoustophoresis by optimizing the phase-modulating parameters is also proposed.