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

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 mum polystyrene latex particle in 5x10(-4) M NaCl solution was determined to be 1.6x10(-13) N, which corresponded to the net charge of 5.8x10(-17) C. (C) 2002 American Institute of Physics.