Dynamic Electrophoretic Mobility of Colloidal Particles Measured by the Newly Developed Method of Quasi-elastic Light Scattering in a Sinusoidal Electric Field

We developed a new method for measuring the dynamic electrophoretic mobility μ* and apparent diffusion coefficient D of colloidal particles in a sinusoidal electric field by employing the heterodyne method of light scattering. The frequency of the scattered light is sinusoidally modulated due to the Doppler effect if there is no Brownian motion of colloidal particles. The undesirable influence of Brownian motion is removed by squaring the time-domain signal, and μ* is obtained from the second and fourth harmonic components of signal detected with a lock-in amplifier. On the other hand, the influence of electrophoresis is excluded to obtain D by calculating the autocorrelation function of the component at the same frequency of the applied electric field. This method is applied to a suspension of latex particles, and the complex spectrum of μ* is successfully measured for the first time in a wide frequency range below 50 kHz. The obtained spectrum of μ* shows relaxation, and the apparent diffusion coefficie...