Drop breakup in dilute Newtonian emulsions in simple shear flow: New drop breakup mechanisms

We investigate drop breakup in dilute Newtonian emulsions in simple shear flow using high-speed video microscopy over a wide range of viscosity ratio (0.0017 < lambda d <3.5), focusing on high capillary number (Ca up to 12Ca(c), Ca-c, is the critical capillary number). The final drop size distribution of emulsions is found to be intimately linked to the drop breakup mechanism, which depends on Ca and h. Drop breakup is caused by end pinching at Ca < 2Ca(c). For Ca > 2Ca(c), breakup dynamics are strongly controlled by lambda. For 0.1 < lambda < 1, capillary instability is the dominant drop breakup mechanism, and thread radius and wavelength at breakup are independent of the initial drop sizes. Fairly monodisperse emulsions are obtained, and the average drop size is inversely proportional to the shear rate. For 1.0 < lambda < 3.5, long wavelength capillary instability generates large satellite drops, resulting in emulsions with bimodal distribution. For lambda < 0.1, a new drop re-breaking mechanism is observed, producing polydisperse emulsions. The polydispersity increases with decreasing lambda. The capillary number based on the thread radius at breakup CaT is about 2.5Ca, and shows a minimum at lambda = 1.0. The measured CaT agrees with slender body theory for lambda < 0.1. Drops deform pseudo affinely for 0.1 < lambda < 1.0, but deformation deviates from being pseudo affine otherwise. (c) 2007 The Society of Rheology.