Electrophoretic measurements using the optical tweezers. (a) Schematic illustration of a charged colloidal particle held by an optical tweezer and driven by an oscillating electric field. The electric field direction is indicated for a particular instant of time. The measured γ E / γ S (b), magnitude of Q_eff (c), and μ _E (d) plotted as a function of applied electric field frequency for a 0.75- μ m radius polystyrene particle, held by a fixed optical trap in DI water (λ _D=96.1 nm) under two different electric field strengths. It is seen that all measured quantities are relatively independent of the frequency, indicating the quasi-static nature of the experiment.