  Custom-designed colloidal particles in an optical tweezers act as light-mills
in a fluid. In particular, aqueous suspensions of capped colloids, in which
half of the surface is covered with metal layers, are investigated. Due to
their asymmetry, the capped colloids can act as rotators when exposed to
intense laser fields. Particles of 4.7 micrometer in diameter are observed
rotating around the focus of a laser beam. For low intensities, particles
become trapped close to the spot of highest laser intensity. Above a threshold
value of about 4 mW in total beam intensity, the particles move away from the
center of the focus and start to rotate at frequencies of about 1 Hz. The
balance of forces due to light pressure and hydrodynamic forces gives a
constant rotation rate. The speed of the spinning particle increases linearly
with laser power to above 2 Hz until the particles are ejected from the focus
for intensities higher than 7 mW. Magnetic caps introduce further possibilities
to tune the rotation rates.
