(276g) Particle-Induced Drop Bridging in Pickering Emulsions

It is well-known that particles can adsorb at fluid/fluid interfaces and can stabilize emulsions; such emulsions are called Pickering emulsions. Recent experiments show that particles can simultaneously adsorb at two fluid-fluid interfaces, thereby .bridging. across - and stabilizing - a thin fluid film between the two interfaces. We hypothesize that a similar phenomenon should occur in Pickering emulsions: particles should be able to bridge across drops, causing them to stick to each other and form non-coalescing drop clusters. Furthermore, such clusters should cause a large change in rheological properties of the Pickering emulsion.

We present results on emulsions comprised of polyethylene oxide drops dispersed in a polyisobutylene matrix, with hydrophobic silica particles adsorbed at the interface. This is an ideal model system for elucidating the effects of particle bridging by microscopy and rheology. These emulsions were subjected to controlled shear flow conditions and studied by optical microscopy. We find that the particles can indeed bridge across PEO and glue them together. The resulting cluster morphologies include tightly-aggregated drops, loose networks of drops, as well as chains of drops.

We also studied the rheological properties of such emulsions. In the absence of particles, the rheological properties of these emulsions are very similar to those of similar emulsions at the same drop volume fraction. However upon addition of particles, particle-induced drop clustering causes large changes in the rheology: the viscosity of the emulsions, as well as the strain recovery after cessation of shear, reduces significantly. Most importantly, their linear viscoelastic properties reveal gel-like behavior at low frequencies indicating that particle-induced bridging imparts the emulsion solid-like characteristics.