Effect of confinement on droplet breakup in sheared emulsions

The breakup of Newtonian droplets in a Newtonian matrix during shear flow is investigated in a counterrotating parallel plate device. For bulk conditions, the critical capillary number for breakup is known to be only determined by the viscosity ratio. Here, we show that the critical capillary number is also affected by the degree of confinement: for low viscosity ratios, confinement suppresses breakup, whereas for high viscosity ratios, confinement promotes breakup. This way, above a critical value for the degree of confinement, even droplets with a viscosity ratio larger than 4, which are unbreakable by shear in a bulk situation, can be broken in a simple shear flow field.