Visualization of Hydrodynamic Particle Interactions: Validation of a Numerical Model

An experimental study is presented to investigate hydrodynamic interactions acoustically induced between particles in monodiperse aerosols. In particular, hydrodynamic mechanisms of viscous origin dominating these interactions are analyzed for several orientations of the particle center-lines with respect to the acoustic axis. The measurements were carried out in standing wave field with a homogeneous and well determined acoustic velocity field. A CCD-camera with a microscope lens attachment was used to visualize the displacements of 7.9 μm glass beads in a frequency of 3 kHz under Oseen flow conditions. A number of 46 attraction processes between pairs of particles were visualized and quantified in the experiments. Maximal attractions were found for particles aligned along the acoustic field direction, requiring the shortest times to collide. These observations are in agreement with other previous experiments [1] carried out to verify the hydrodynamic interaction known as the "acoustic wake effect". In the present paper the dependence of this interaction on the initial conditions of the particles is analyzed. Attractions become weaker when the angle formed by the particle center-line and the acoustic axis increases. Attraction patterns including collisions have been found and quantified for angles of the particle center-line up to 45° off the acoustic axis. The experimental data were positively compared with the results of a numerical study developed by the authors in a recent paper [2] in such a way that the numerical model has been now validated.