Probing Quench-Dependent Gelation in Thermoresponsive Attractive Colloids

We investigate the effects of thermal quench on gelation in thermoresponsive, phase separating colloids. As a model system, we employ recently established oil-in-water nanoemulsions which contain thermosensitive telechelic polymers in the continuous phase and undergo temperature-induced colloidal gelation via polymer-mediated interdroplet attractions. [1] Modeling the effective interactions with a detailed statistical mechanical model for polymer bridging, we establish the equilibrium colloidal phase behavior of the system. Under appropriate conditions, the system forms colloidal gels in regions of fluid-fluid phase instability, where gelation and spinodal decomposition compete to direct the arrested gel structure. [2] Here, time-dependent quenches of various depths and rates are performed along isochores of volume fraction from 0.1 to 0.4, and linear viscoelasticity and optical microscopy are used to track the evolution of rheology and structure in the system. We find that gelation depends significantly on both the depth and rate of quenching into the gelled state. For example, the kinetics of gelation vary by several orders of magnitude with relatively minor changes in the quench rate, even for relatively shallow quenches into the spinodal region. Furthermore, we find that the onset of solid-like viscoelastic behavior for sufficiently slow quenches is consistently deeper than the predicted equilibrium coexistence line, suggesting the presence of an attractive glass transition line within the spinodal region. These results offer a starting point for more rationally controlling colloidal gelation and phase separation, and underscore the active interplay between viscoelastic phase separation and glassy dynamics in the formation of dense colloidal gels. 

[1] Helgeson, M. E.; Gao, Y.; Moran, S. E.; Lee, J.; Godfrin, M.; Tripathi, A.; Bose, A.; Doyle, P. S. Soft Matter 2014, 10, 3122-3133. 

[2] Gao, Y.; Kim, J.; Helgeson, M. E. Soft Matter 2015, 11, 6360-6370.