Particle charges and particle-substrate forces by optical transients

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Abstract
A novel optical technique based on the observation of time‐dependent changes in the intensity of light scattered from particles during their motion in a liquid containing contrasting medium is outlined. Principles of the method and theoretical models are presented for sweepout and transit experiments which permit determination of particle sizes, mobilities, charges, particle‐substrate interaction forces, and also the removal times of particles from the substrates. Average values and at least the second moments of these quantities are determined. The method was tested on several nonaqueous dispersions with particular attention being paid to titanium dioxide particles in a hydrocarbon fluid. Particle mobilities were measured over a wide range of electric fields and found to increase with field. Particle charges were found roughly proportional to particle size and the average particle‐substrate interaction forces were found to be ∼1.4×10−10 N in reasonable agreement with a theoretical estimate of the electrostatic interaction forces. Dynamics of charged particles subject to high‐frequency fields was also investigated.

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