As a basis for biometric and chemical analysis, issues of how to dilute or concentrate substances such as particles or cells to specific concentrations have long been of interest to researchers. In this study, travelling surface acoustic wave (TSAW)-based devices with three frequencies (99.1, 48.8, 20.4 MHz) have been used to capture the suspended Polystyrene (PS) microspheres of various sizes (5, 20, 40 μm) in sessile droplets, which are controlled by acoustic field-induced fluid vortex (acoustic vortex) and aggregate into clusters or rings with particles. These phenomena can be explained by the interaction of three forces, which are drag force caused by ASF, ARF caused by Leaky-SAW and varying centrifugal force. Eventually, a novel approach of free transition between the particle ring and cluster was approached via modulating the acoustic amplitude of TSAW. By this method, multilayer particles agglomerate with 20 μm wrapped around 40 μm and 20 μm wrapped around 5 μm can be obtained, which provides the possibility to dilute or concentrate the particles to a specific concentration.
Keywords: analytical methods; biochemical analysis; droplet microfluidics; travelling SAW.