Shear-induced droplet deformation: Effects of confined geometry and viscoelasticity

Deformation and breakup of droplets is a key mechanism in the flow-induced evolution of the microstructure of biphasic liquid–liquid systems, such as emulsions and polymer blends. Here, the flow behavior of isolated droplets in simple shear flow, which is a challenging problem with important applications (though simplified by the absence of droplet–droplet interactions), will be considered. The focus of this review is on two topics which have recently attracted much attention in the literature, i.e., the effects of a confined geometry and of viscoelastic fluid components. The former case applies when droplet size is comparable to the gap between the confining walls. In such conditions, a range of peculiar phenomena, which are not found in unbounded flow, are observed, including a significant slowing down of droplet dynamics, a breakup mechanism leading to a nearly monodisperse fragment distribution, and the possibility of breaking up droplets at a viscosity ratio between dispersed and continuous phase above 4. Possible applications of these studies are in the area of microfluidics, where droplets can act as carriers or microreactors for process intensification. Concerning the effects of viscoelastic components, most studies have been carried out in the case where either one of the two phases (i.e., droplet and matrix) displays significant elasticity with shear rate independent viscosity. While matrix viscoelasticity enhances droplet orientation towards the flow direction, in the opposite case new (and still unexplained) breakup modes along the vorticity axis of the shear flow have been observed at high shear rates.