Dramatic droplet deformation through interfacial particles jamming.

Droplets of one fluid in a second, immiscible fluid are typically spherical in shape due to the interfacial tension between the two fluids. Shear forces can lead to droplet deformation when they are subjected to flow, and these effects can be further modified when the droplet is stabilized by a surfactant due to a flow-induced gradients in the surfactant concentration. An alternative method of stabilizing droplets is through the use of colloidal particles, whose stabilization behavior is intrinsically different from molecular surfactants. Under the same flow condition, a gradient of particle concentration can result in the jamming of particles in regions with a high packing density, making the interface solid-like, albeit only under compression and not tension. However, how this asymmetry in the surfactant properties alters the droplet shape under shear is unknown. Here, we show that shear of particle-stabilized droplets can lead to a remarkable array of shape deformations as the droplets flow through a constrained microchannel. The shear-induced migration of particles on the surface results in the formation of an elastic shell at the back of the droplet, which can wrinkle and invaginate, ultimately leading to a unique core-shell structure. The shapes depend on the Peclet number of the flow, reflecting the balance of shear forces that drive the particles and diffusion that randomizes them. These findings highlight the consequences of the asymmetry in the forces between the particles and provide a unique method to controllably create droplets with a vast array of different shapes.