A method to estimate the orthokinetic coalescence effieciency of polydisperse droplets in a water/oil emulsion is developed. The method is based on comparing results of experimentally measured and theoretically calculated light transmittance intensities. The experimental values were found by measuring the signal through a water/oil emulsion sheared in a microcouette apparatus. The evolution of the number of droplets with time is followed by solving numerically Smoluchowski's equation. The number of droplets and their radii, as well as the optical properties of the emulsion, were used to calculate the light transmitted intensities as a function of time. The orthokinetic coalescence efficiency was assumed to be a function of the droplet radius ratio. The model assumes that the orthokinetic coalescence efficiency is a maximum when the radius ratio is 1 and that there exists a limiting radius, αc, above which no coalescence occurs. The effect of αc and hence the surface potential of the droplets on the orthokinetic coalescence efficiency was investigated. A very weak dependence on αc for a polydisperse system and no dependence for uniform distributions were found. The effect of the initial droplet size distributions on the orthokinetic coalescence efficiency was examined. For the water-in-oil emulsion studied, it was found that the orthokinetic coalescence efficiency is a weak function of the initial droplet size distribution.
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