Optically induced electrokinetic concentration and sorting of colloids

We demonstrate an optically induced ac electrokinetic technique that rapidly and continuously accumulates colloids on a parallel-plate electrode surface resulting in a crystalline-like aggregation. Electrothermal hydrodynamics produce a microfluidic vortex that carries suspended particles toward its center where they are trapped by local ac electrokinetic hydrodynamic forces. We characterize the rate of particle aggregation as a function of the applied ac voltage, ac frequency and illumination intensity. Hundreds of polystyrene particles (1.0 mu m) suspended in a low conductivity solution (2.4 mS m(-1)) were captured at a range of voltages (5-20 V-pp) and frequencies (20-150 kHz) with an optical power of approximately 20 mW. This technique was not restricted to near infrared (1064 nm) illumination and was also demonstrated at 532 nm. The sorting capability of this technique was demonstrated with a solution containing 0.5 mu m, 1.0 mu m and 2.0 mu m polystyrene particles. This dynamic optically induced technique rapidly concentrates, sorts and translates colloidal aggregates with a simple parallel-plate electrode configuration and can be used for a variety of lab-on-a-chip applications.