(533c) Heterogeneity and Large-Scale Dynamics in Nanoemulsion Colloidal Gels

Nanoemulsions are known to form “organohydrogels”, solid-like phases with rheology similar to structurally arrested states in attractive near hard sphere suspensions. In this work, we use comprehensive experimental studies to reveal the mechanism of gelation in these systems, providing insight into the mechanics of both soft and hard-sphere colloidal gels at high volume fraction. The use of materials with thermosensitive interdroplet attractions allows for careful examination of the gel transition by combining a number of techniques, including bulk rheology, scattering, and optical microscopy. In particular, we employ newly developed dynamic microscopy techniques to track both the structure, dynamics, and topology of large-scale density fluctuations in the gelling fluid across a wide range of length and time scales in situ.

We find that the nanoemulsion gels exhibit strong structural and dynamical heterogeneity at length scales significantly larger than the characteristic size of interdroplet clusters. At the onset of gelation, strong cluster-cluster correlations appear, indicating the formation of colloid-rich and colloid-poor regions in the fluid. As gelation proceeds, the growth and relaxation of these correlations reveal that gelation proceeds by arrested phase separation vis-à-vis spinodal decomposition. Specifically, the dynamics of cluster-cluster correlations are ballistic above a characteristic length scale, which grows linearly with time until complete structural arrest is observed. We track the dependence of arrest as a function of both the colloid volume fraction and degree of quenching into the spinodal region, and find that arrest approximately corresponds with the point at which the colloid-rich region approaches the attractive jamming transition. These results suggest rules for controlling both the porosity and mechanical properties of a large class of particulate gels formed in polymer-colloid mixtures.