Capped colloids as light-mills in optical traps

Custom-designed colloidal particles in optical tweezers act as light-mills in a fluid. In particular, aqueous suspensions of capped silica 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 mu m 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 centre 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.