Optically trapped particles can be used as probes to study the mechanical properties of substances on a microscopic scale. Such experiments have been performed on colloids, single bio-molecules such as DNA and proteins, and components of living cells. A particularly promising type of probe particle is the micro-rotor - an optically trapped and powered microscopic rotating particle. Such a probe allows steady-state motion, and is ideal for the measurement of viscosity on a microscopic scale. The change in polarisation of the trapping beam due to scattering by the probe particle can be used to measure the optical torque acting on, and the rotation of, the probe particle. We present results from experiments showing that it is possible to rotate small calcite crystals adhering to the membrane of a cell in vitro, and measure the applied torque and rotation speed
