Electric charge measurement on a single microparticle using thermodynamic analysis of electrostatic forces

Abstract

A method is proposed for analyzing the electric charge at the shear plane of a single microparticle. A particle dispersed in solution is three-dimensionally trapped with a focused laser beam, and an electrostatic field is applied with microelectrodes. The thermal motion of the particle is sequentially measured with a quadrant photodiode, such that the potential energy exerted on the particle can be estimated by thermodynamic analysis. A potential profile of an electrostatic force is obtained as the energy difference between two potential curves observed with and without the electrostatic field. The strength of the force exerted on a 4.5 μm polystyrene latex particle in 5×10 −4 M NaCl solution was determined to be 1.6×10 −13 N , which corresponded to the net charge of 5.8×10 −17 C .