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Fast and precise measurements of particle charge with optical trapping electrophoresis

We subject micrometer-sized, optically trapped colloidal particles in a non-polar liquid to a sinusoidally varying electric field, and measure their resulting movement. From this movement, we calculate the electrophoretic mobility and charge of the particle in the liquid. The use of high frequencies of the electric field (well above the corner frequency of the optical tweezers) allows us to estimate the electrical charge of colloidal particles with an accuracy of the order of the electron charge in a time interval of only 10 ms. This technique can be used to provide valuable information about the dynamics of the poorly understood processes that lead to the charge on colloidal particles in non-polar liquids.