Interparticle attraction along the direction of the pressure gradient in an acoustic standing wave.

Scattering of an acoustic wave by particles gives rise to microstreaming, as well as to acoustic radiation and interaction forces on the particles. We numerically study these steady, nonlinear phenomena for a case of two elastic spheres in a standing wave. We show that if one or both spheres are smaller or comparable to the viscous boundary layer, the microstreaming close to the pressure node can lead to an interparticle attraction along the direction of the pressure gradient of the wave. Similar behavior is observed when, instead of size, density of one of the spheres is sufficiently larger relative to the other sphere. These findings could promote the acoustic manipulation of nanoparticles and bacteria.

Influence of particle shape and material on the acoustic radiation force and microstreaming in a standing wave.

In view of its influence on the acoustic radiation force, we investigate the microstreaming around a small solid elastic particle in an ultrasonic standing wave in dependence of its material properties and shape. The configuration is axisymmetric, making it accessible to numerical methods, such as the finite element method. The results reveal a transition from viscous scattering- to microstreaming-dominated acoustic radiation force that depends on the particle density. When a deviation of the particle shape from a sphere becomes smaller than the viscous boundary layer thickness, we show that the influence of the shape on the viscous contributions to the acoustic radiation force diminishes, allowing the use of theoretical models for a spherical particle. However, extreme asymmetric shape perturbations, such as crowns with sharp edges, can give rise to noticeable viscous contributions for a dense particle that is larger than the viscous boundary layer thickness. We also introduce a hybrid analytical model for the acoustic radiation force on a spherical particle that accounts for the microstreaming and particle compressibility and shows a good agreement with numerical simulations for an arbitrary particle size and density.

Acoustic radiation force acting on a heavy particle in a standing wave can be dominated by the acoustic microstreaming.

We numerically investigate the contribution of the microstreaming to the acoustic radiation force acting on a small elastic spherical particle placed into an ultrasonic standing wave. When an acoustic wave scatters on a particle the acoustic radiation force and the microstreaming appear as nonlinear time-averaged effects. The compressible Navier-Stokes equations are solved up to second order in terms of the small Mach number using a finite element method. We show that when the viscous boundary layer thickness to particle radius ratio is sufficiently large and the particle is sufficiently dense, the acoustic microstreaming dominates the acoustic radiation force. In this case, our theory predicts migration of the particle to the velocity node (pressure antinode).