Surface acoustic wave (SAW) - induced particle rotation and aggregation in microdroplet

In this paper, we present different dynamical behaviors of micro particles in a droplet in the surface acoustic wave (SAW) field generated by using an annular interdigital transducer (AIDT). It was found that the micro particles can be forced into a ring type pattern, rotational or aggregating motions under the different experimental conditions. The propagation of SAWs results in a corresponding deformation of the substrate and the degree of deformation reflects the magnitude of the force. The simulation results show that the contour of the displacement in some area was similar to a ring, which indicates a standing acoustic wave field is formed throughout the droplet and the particles are hence forced to a ring type pattern. In the case that the acoustic radiation force acting on the micro droplet placed in the center of the device is nonsymmetrical, the particles in the droplet will be subject to an unbalanced force and driven into a rotational motion. The simulation results indicate that the anisotropic elastic properties of the LiNbO₃ substrate and the nonsymmetrical SAW radiation force on the micro particles are the key factors leading to these phenomena.