Shear Rupturing of Droplets in Complex Fluids

We have experimentally studied the shear-induced rupturing of viscous droplets in viscoelastic complex fluids. Remarkably, a premixed emulsion of large, polydisperse droplets can be ruptured into monodisperse emulsions of uniform colloidal droplets. The monodispersity becomes most pronounced when the premixed emulsion is viscoelastic and has a shear-thinning viscosity. Since viscoelastic materials may fracture, we reduce the gap of our shear cell to ensure a spatially uniform strain rate for rupturing. We observe monodispersity whether the viscoelasticity arises from the suspending fluid (e.g., concentrated surfactant solution) or droplet deformation as in compressed emulsions. Our observations suggest that the monodispersity results from droplet rupturing alone and that the capillary instability is inhibited by the partial elasticity of the complex fluid. We use the monodispersity to study how the droplet size depends upon the shear rate and composition.