Measurement of small electrophoretic mobilities by light scattering and analysis of the amplitude weighted phase structure function

Abstract

A novel signal processing scheme for the analysis of laser light scattered by small particles allows the measurement of very small collective particle motion in the presence of large diffusive motion. The technique uses a symmetric real fringe Doppler system with frequency shift. Amplitude and phase of the high pass filtered Doppler intensity signal are measured simultaneously and processed into an amplitude weighted phase structure function. An analysis of this function yields diffusion coefficient and, in a sinusoidal electric field,electrophoretic mobility. If present, a constant velocity motion, e.g., due to thermal convection, is simultaneously determined. The technique overcomes a principal limit of conventional light scattering techniques and allows the use of small fields or the measurement of small zeta potentials. Experimental results on latex systems and macroemulsions demonstrate possible improvements.