Experimental study of inter-particle acoustic forces

Abstract

When particles are under an acoustic field, they can undergo various acoustic forces. By finely tuning the transducer to emit at the resonance frequency, particles will quickly levitate and once in equilibrium, particles may remain in levitation or aggregate if the frequency is carefully adjusted. A resonator has been filled with a suspension composed of water and latex particles, bubbles or cells of different sizes and at different concentrations. Ultrasounds have been generated by means of a 3 MHz nominal frequency transducer. When ultrasounds are switched on, particles move towards the pressure nodes of the acoustic wave due to the acoustic radiation force. Moreover, a secondary force appears, the attraction or repulsion of particles to each other, and it occurs due to the secondary radiated frequencies of each pulsating particle, this is the so-called secondary Bjerknes Force. We present a new method for experimentally determine this force through different examples of particle aggregation at different frequencies, concentrations and sizes. From these examples, velocities, trajectories and forces have been extracted leading to a better understanding and characterization of the forces involved in the aggregation process. It is also known that the equilibrium position of the levitating aggregates is where the gravity force and the acoustic force balance. This levitation technique can be used to simulate microgravity conditions on-ground, without need of a microgravity platform. Furthermore, ultrasound levitation provides a way to obtain artificial gravity in space applications where a precise control of the position of a body is needed.