(698a) Invited Talk: Generation of Stable Non-Spherical Capsules with Controlled Interfacial Coverage of Surface-Active Particles

The complex interfaces of particle-stabilized drops and bubbles have a significant effect on the rheology of emulsions and foams. Common high-shear emulsification methods offer no direct control over the rate of droplet generation, the final droplet size distribution, or the composition of the interface, all of which directly impact the rheology of the final emulsion. In this talk, a simple microfluidic device is used to generate monodisperse droplets with independent control of droplet size, production rate, and the degree of particle loading on the interface. Droplets are formed on demand at a T-junction via pressure control of each fluid phase. The freshly formed droplets travel along the axis of a circular capillary containing a surface-active particle suspension. The residence time of the droplets within the capillary and the transport of particles in the thin films and fluid volumes between droplets determine the coverage of particles on the interface. The flow along the axis of the droplet sweeps adsorbed particles to the rear of the droplet, leading to a highly concentrated region of the interface that does not relax upon exiting the confined geometry of the microchannel. The length of the concentrated region increases with particle coverage, producing capsules with shapes from pears to maracas to cylinders. The capsules are monodisperse, can be produced at high rates, and remain stable for tens of hours after formation. Measurements of the dilatational and shear interfacial rheology help rationalize the observations of highly stable non-spherical shapes. Thus, careful control of transport in microscale devices allows for production of emulsions with tunable morphology, interfacial rheology, and bulk rheology.