The coalescence efficiency of two Newtonian droplets submerged in a Newtonian fluid subjected to a simple shear flow was investigated experimentally and theoretically. The experimental investigation was based on observing collisions between two droplets under a microscope. The theoretical investigation considered three drainage models: immobile, partially mobile and mobile interfaces. Both the experimental results and the theoretical analysis showed that a critical approach angle exists below which the colliding droplets separate. Above this critical angle the collision leads to coalescence. Knowledge of the critical angle permits calculation of the coalescence efficiency. The dependence of the coalescence efficiency on various dimensionless groups such as the flow number, the capillary number and the viscosity ratio was studied. The theoretical analysis indicated that the coalescence efficiency decreases as the capillary number and the flow number increase. The experimental results showed that the coalescence efficiency goes through a minimum as the value of the flow number increases. The discrepancy between the experimental and the theoretical results was attributed to some mechanism that enhances coalescence and that is not accounted for in the equation used for the critical thickness for film rupture. Both the experimental and the theoretical results indicated that the coalescence efficiency decreases as the viscosity ratio decreases.
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