Optical microrheology using rotating laser-trapped particles
  We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure. 
Alexis I. Bishop, Timo A. Nieminen, Norman R. Heckenberg and Halina   Rubinsztein-Dunlop