UC Santa Barbara

The ability to engineer macroscopic properties of colloidal gels through controlled processing of their microscopic substituents presents an opportunity to improve soft material technology in emergent fields such as additive manufacturing and biomaterial production. However, the fundamental underpinnings of colloidal gel formation and behavior are not comprehensively developed to a degree that would facilitate new material design. To build toward such an understanding of colloidal gels, in this dissertation we use a model thermoresponsive colloidal system to investigate (1) the formation of colloidal gels with respect their interparticle potential, phase behavior, and gel arrest kinetics, and (2) the resultant properties of colloidal gels as observed through their structure, viscoelasticity, and yielding.

The system of interest to this work comprises nanoemulsion dispersions with interactions mediated by thermoresponsive polymers. To model the effective interdroplet attractions arising from thermoresponsive behavior, we employ a hard sphere two-Yukawa interaction potential and determine model parameters from experimental scattering measurements. The interaction potential is subsequently used to make mean field predictions of the effective pseudo-one component colloidal phase behavior by means of variational perturbation methods, which are found to agree with measurements from sedimented phase separating gels as well as comparable coarse-grained molecular dynamics simulations. These results are noteworthy because they provide evidence that near-equilibrium behavior can still be recovered underneath the non-equilibrium glassy arrest line.

Additionally, we report qualitatively distinct gelation kinetics between colloidal gels formed at volume fractions below and above the predicted spinodal boundary. To explain gelation kinetics inside the phase coexistence region, we use rheo-microscopy which enables simultaneous characterization of gel linear viscoelasticity and microstructure. Through this characterization, we map out an isothermal transformation diagram to identify the sequential transitions that occur en route to prolonged arrest for varying thermal quenches. From rheological analysis of linear elasticity, we find that conventional nondimensionalization proves insufficient to collapse the quench dependence of gel elasticity, demonstrating a need for further investigation into the appropriate length and energy scales for scaling colloidal gel rheology. From imaging analysis of gel structuration, we find that late-stage coarsening rates exhibit a sigmoidal quench dependence akin to the effective interdroplet attraction strength, which can be predicted from a simple mean-field description of sub-critical viscoelastic phase separation.

Finally, we describe novel experimental techniques and metrics to evaluate colloidal gel structure during the process of yielding during large amplitude oscillatory shear deformation. Combining intracycle and intercycle analyses of microstructure and rheology reveals a three-stage process of network rupture, softening, and alignment that the yielding gel undergoes. Establishing this methodology provides a robust experimental platform for forthcoming studies into the direct influence of gelation and shearing conditions on the complex yielding process.