Electric fields induce orientational order in particulate suspensions thereby rendering them birefringent. The decay rate of the birefringence following the field termination is characteristic of sample size, shape, and polydispersity. Theory is developed herein for the evaluation of a two-parameter function for the distribution of sizes of arbitrarily shaped particles in terms of their equivalent spherical diameters. An experimental procedure, involving the measurement of the birefringence decay characteristics in two well-defined experimental conditions, is outlined and used to check the theory for a colloidal suspension of polytetrafluoro-ethylene particles in water.
A B S T R A C T:Under the influence of a pulsed field, dilute clay sols become birefringent as the particles undergo orientational order. The rate of decay of the birefringence on removal of the field is characteristic of the particle geometry. Measurement of the decay rates under two specific experimental conditions provides sufficient information from which the particle-size distribution can be evaluated in terms of a two-parameter function. Experimental data are reported and analysed in terms of a log-normal distribution of particle sizes for attapulgite (rods), kaolinite (discs) and halloysite (ellipsoids) sols and compared with success to electron microscopic data. The ability of the method to determine size distributions in terms of the major dimensions of the clay particles, rather than those of the often used equivalent sphere, is highlighted.
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