The influence of entrainment on acoustically induced interactions between aerosol particles—an experimental study

An experimental study of particle interactions acoustically induced in monodisperse aerosols is presented in this paper. It is mainly devoted to analyse the influence of the acoustic entrainment experienced by the single particles on their attraction processes and, in particular, on the hydrodynamic mechanisms that govern them. Glass micro-spheres immersed in air as very dilute aerosols were subjected to homogeneous plane standing waves at frequencies ranging from 200 Hz up to 5 kHz. At these variable acoustic conditions the particle experiences different acoustic entrainment coefficients, varying from qp=0.86 down to 0.05 and covering almost its complete range of variability. A collection of experimental particle interactions were filmed, from which the particle behaviours were analysed. Our results experimentally confirm for the first time the presence of the acoustic wake effect (AWE) as a dominant mechanism of attraction at every entrainment ratio. In addition, the mutual radiation pressure is observed as a repulsion mechanism acting on nearby particles (separated at few diameter distances). It produces a disturbing effect on the particle attractions due to the AWE. Its influence becomes negligible for entrainment coefficients below qp=0.1.