A study of the limits of microparticle collection over a broad frequency range

Abstract

Acoustic fields can be used to collect microparticles over a wide range of frequencies. Within the ultrasonic range acoustic radiation forces cause patterns of particles to form, typically along pressure nodes. At much lower frequencies (in the order of hundreds of Hertz), the collection mechanism is inertial in nature, a particle’s inability to follow the fluid motion causes drag forces to act on the particle which are non-zero when integrated across a time cycle due to gradients in the flow field. In both cases, however, the ability to collect particles is limited by acoustic streaming—the steady state fluid flow which results from gradients in the harmonic flow field. In this study, we examine the smallest particle size that can be collected as a function of frequency. For the low frequency vibration, an open fluid chamber is oscillated in the horizontal plane, these conditions are also applied to the ultrasonic case, and the acoustic energy is kept constant across frequencies. It is found that the minimum particle size can be collected at each end of the frequency scale.