  The manipulation of microscopic objects requires precise and controllable
forces and torques. Recent advances have led to the use of critical Casimir
forces as a powerful tool, which can be finely tuned through the temperature of
the environment and the chemical properties of the involved objects. For
example, these forces have been used to self-organize ensembles of particles
and to counteract stiction caused by Casimir-Liftshitz forces. However, until
now, the potential of critical Casimir torques has been largely unexplored.
Here, we demonstrate that critical Casimir torques can efficiently control the
alignment of microscopic objects on nanopatterned substrates. We show
experimentally and corroborate with theoretical calculations and Monte Carlo
simulations that circular patterns on a substrate can stabilize the position
and orientation of microscopic disks. By making the patterns elliptical, such
microdisks can be subject to a torque which flips them upright while
simultaneously allowing for more accurate control of the microdisk position.
More complex patterns can selectively trap 2D-chiral particles and generate
particle motion similar to non-equilibrium Brownian ratchets. These findings
provide new opportunities for nanotechnological applications requiring precise
positioning and orientation of microscopic objects.
