  The controlled rotation of solid particles trapped in a liquid by an
ultrasonic vortex beam is observed. Single polystyrene beads, or clusters, can
be trapped against gravity while simultaneously rotated. The induced rotation
of a single particle is compared to a torque balance model accounting for the
acoustic response of the particle. The measured torque ($\sim 10$~pNm for a
driving acoustic power $\sim 40$~W/cm$^2$) suggests two dominating dissipation
mechanisms of the acoustic orbital angular momentum responsible for the
observed rotation. The first takes place in the bulk of the absorbing particle,
whilst the second arises as dissipation in the viscous boundary layer in the
surrounding fluid. Importantly, the dissipation processes affect both the
dipolar and quadrupolar particle vibration modes suggesting that the
restriction to the well-known Rayleigh scattering regime is invalid to model
the total torque even for spheres much smaller than the sound wavelength. The
findings show that a precise knowledge of the probe elastic absorption
properties is crucial to perform rheological measurements with manoeuvrable
trapped spheres in viscous liquids. Further results suggest that the external
rotational steady flow must be included in the balance and can play an
important role in other liquids.
