Acoustic Propulsion of Nano- and Microcones: Dependence on the Viscosity of the Surrounding Fluid

Abstract

This article investigates how the acoustic propulsion of cone-shaped colloidal particles that are exposed to a traveling ultrasound wave depends on the viscosity of the fluid surrounding the particles. Using acoustofluidic computer simulations, we found that the propulsion of such nano-and microcones decreases strongly and even changes sign for increasing shear viscosity. In contrast, we found only a weak dependence of the propulsion on the bulk viscosity. The obtained results are in line with the findings of previous theoretical and experimental studies.

Propulsion of bullet- and cup-shaped nano- and microparticles by traveling ultrasound waves

“…The latter is characterized by the particle's rotational diffusion coefficient D R = (k B T 0 /ν s )(H −1 ) 66 = 8.47 s −1 , corresponding to rotation in the x 1 -x 2 plane, where k B is the Boltzmann constant. Brownian rotation with this rotational diffusion coefficient is associated with a reorienta-tion of the particle on the timescale 0.12 s. On the other hand, the angular velocity ω = −0.12 s −1 corresponds to a reorientation by 90°in about 13 s. This shows that the Brownian rotation is dominant and the rotational acoustic propulsion can be ignored here, which is in line with the findings of previous studies that addressed other kinds of ultrasound-propelled particles [44,49,[52][53][54].…”

“…As a consequence, the pressure is laterally increased and above and below the particle decreased. Flow fields with a qualitatively similar structure (but for convex particle shapes without the small vortices) have already been observed for solid or hollowed-out half-sphere-shaped particles [44] and for solid [49,[52][53][54] or hollowed-out [44] cone-shaped or triangular particles. In the present case, the centers of the large vortices above the particle are slightly nearer to the particle's center of mass than those below the particle.…”

“…Some studies have addressed hemi-spherical cups (nanoshells) [19,37,44], which can be interpreted as the limit of a bullet-shaped particle with hemi-spherical ends for decreasing length of the cylindrical part between the two ends. There are also studies on cone-shaped particles [44,49,[52][53][54], a gear-shaped particle [21,25,31], and microspinners [55].…”

“…For the hemi-spherical-cup particles [19,37,44], the opposite propulsion direction as for bullet-shaped particles with hemi-spherical ends was found. A problem additional to this limited insight into the shape-dependence of the acoustic propulsion is the fact that all except for a few [32,49,[52][53][54][55] previous studies addressed particles in a standing ultrasound wave, whereas traveling ultrasound waves are much more relevant for the envisaged future applications of acoustically propelled particles [44,49,52].…”

An acoustically controlled helical microrobot

“…The formation of the latter has already been observed to be common in acoustically propelled microparticles (36,37,41). The vortices near the fins form two helicoidal vortex tubes that wind around the particle.…”