A method is presented for measurements of secondary acoustic radiation forces acting on solid particles and biological cells in a plain ultrasonic standing wave. The method allows for measurements of acoustic interaction forces between particles/cells located in arbitrary positions such as in between a pressure node and a pressure anti-node. By utilizing a model that considers both density- and compressibility-dependent effects, the observed particle-particle/cell interaction dynamics can be well understood. Two differently sized polystyrene micro-particles (4.8 µm and 25 µm, respectively), a silica particle with 20 µm diameter, red blood cells (RBCs), and MCF-7 cells were used in order to achieve pronounced interaction effects. The particulate was subjected to a 2-MHz ultrasonic standing wave in a microfluidic channel, such as commonly used for acoustophoresis. Observation of deflections in the particle/cell pathways shows that the acoustic interaction force is not negligible under this circumstance and has to be considered in accurate particle/cell manipulation applications. The effect is primarily pronounced when the distance between two particles is small, the sizes of the particles are different, and the acoustic properties of the particles are different relative to the media. As predicted by theory, we also observe that the interaction forces are affected by the angle between the inter-particle centerline and the axis of the standing wave propagation direction. Key words Microfluidic, Secondary acoustic force, Primary acoustic force, Particle-cell interactio
