  Previous studies on ultrasound-propelled nano- and microparticles have
considered only systems where the particle orientation is perpendicular to the
direction of propagation of the ultrasound. However, in future applications of
these particles, they will typically be able to attain also other orientations.
Therefore, using direct acoustofluidic simulations, we here study how the
propulsion of cone-shaped nano- and microparticles, which are known to have a
particularly efficient acoustic propulsion and are therefore promising
candidates for future applications, depends on their orientation relative to
the propagation direction of a traveling ultrasound wave. Our results reveal
that the propulsion of the particles depends strongly on their orientation
relative to the direction of wave propagation and that the particles tend to
orient perpendicularly to the wave direction. We also present the
orientation-averaged translational and angular velocities of the particles,
which correspond to the particles' effective propulsion for an isotropic
exposure to ultrasound. Our results allow assessing how free
ultrasound-propelled colloidal particles move in three spatial dimensions and
thus constitute an important step towards the realization of the envisaged
future applications of such particles.
