Single colloid electrophoresis.

Optical tweezers enable one to trap a single particle without any mechanical contact and to measure its position and the forces acting on it with high resolution (+/-4 nm, +/-160 fN). Taking advantage of a specially designed microfluidic cell the electrophoretic response of the colloid under study and the electroosmotic effect on the surrounding medium are determined using the identical colloid. The former is found to be by more than one order of magnitude larger than the electroosmotic effect. It is shifted in phase with respect to the external field, hence giving rise to a complex electrophoretic mobility which can be theoretically described by a strongly damped driven harmonic oscillator model. By exchanging the medium surrounding the colloid it is possible to deduce the (KCl) concentration dependence of the single colloid electrophoretic response. The results are compared with conventional Zetasizer measurements.