Brian Tighe

Sticky Matters: Dense attractive emulsions

Emulsions and related amorphous materials jam into a solid at a critical packing fraction where the droplets form long-lasting contacts. Processing and handling these soft solids efficiently requires a fundamental understanding of their mechanical properties.

The non-equilibrium jamming transition has been extensively studied in the context of a simple model system. The model consists of soft non-Brownian spheres that are placed in a (simulation) box and confined by an imposed stress. The resulting system has many features remiscent of an equilibrium phase transition, including a shear modulus that grows continuously as a power law above the critical packing fraction.

Numerical and theoretical studies of jamming have focused overwhelmingly on the case where the droplets are purely repulsive. This case is now fairly well understood. However, while it is possible to prepare purely repulsive emulsions in the lab, this is not the generic case. Researchers have implicitly assumed that the effect of attractions between droplets can be accounted for as a weak perturbation to the purely repulsive case. However, this assumption has not been tested.

I will report the results of a numerical study of "sticky jamming," i.e. systems of repulsive spheres inside a sticky shell. Using techniques from percolation theory, I will show how attraction pushes the jamming transition to lower packing fractions. More importantly, attraction changes the character of the jamming transition -- while repulsive jamming is a first order percolation transition, sticky jamming is continuous. Finally, I will show how stickiness alters the shear modulus. Using critical scaling analysis, I will identify the critical exponents that characterize the sticky jamming transition.