  Among the many types of artificial motile nano- and microparticles that have
been developed in the past, colloidal particles that exhibit propulsion when
they are exposed to ultrasound are particularly advantageous. Their properties,
however, are still largely unexplored. For example, the dependence of the
propulsion on the particle shape and the structure of the flow field generated
around the particles are still unknown. In this article, we address the
propulsion mechanism of ultrasound-propelled nano- and microparticles in more
detail. Based on direct computational fluid dynamics simulations and focusing
on traveling ultrasound waves, we study the effect of two important aspects of
the particle shape on the propulsion: rounded vs. pointed and filled vs. hollow
shapes. We also address the flow field generated around such particles. Our
results reveal that pointedness leads to an increase of the propulsion speed,
whereas it is not significantly affected by hollowness. Furthermore, we find
that the flow field of ultrasound-propelled particles allows to classify them
as pusher squirmers, which has far-reaching consequences for the understanding
of these particles and allows us to predict that they can be used to realize
active materials with a tunable viscosity that can exhibit suprafluidity and
even negative viscosities. The obtained results are helpful, e.g., for future
experimental work further investigating or applying ultrasound-propelled
colloidal particles as well as for theoretical approaches that aim at modeling
their dynamics on mesoscopic scales.
