Title: Phase Behavior, Processing, and Properties of Thermoresponsive Colloidal Gels

Advisor: Matt Helgeson

Abstract: 

Colloidal gels — soft, solid-like networks of attractively “bonded” particles — are foundational materials for technologies in a number of emergent industries such as additive manufacturing and biomaterial production. The ability to engineer macroscopic colloidal gel properties through controlled processing of microscopic substituent particles presents an enticing opportunity for multiscale material design. However, current processing capabilities are limited due to the lack of dynamically tunable interparticle interactions in existing systems. To advance our understanding toward such capabilities, in this work we develop thermoresponsive colloidal systems to manipulate the formation, structure, and mechanics of colloidal gels by using the equilibrium phase instability and non-equilibrium arrest kinetics.

Serving as our model system are nanoscale oil droplets dispersed in water, or nanoemulsions, that exhibit thermally tunable interdroplet interactions mediated by thermoresponsive polymers. To determine the relationship between equilibrium phase behavior and non-equilibrium gelation processes, we use an effective interdroplet potential and generate mean field predictions of the pseudo-one component colloidal phase behavior. We find that the predictions agree with both experimental measurements as well as coarse-grained molecular dynamics simulations of gels formed via arrested phase separation. These results provide a scientific foundation for rationalizing observations of arrested colloidal gels within thermodynamic state space. Furthermore, the results provide critical evidence that near-equilibrium behavior can still be recovered within the non-equilibrium glassy state, which establishes material design boundaries for engineering colloidal gels. 

To understand the dynamic gelation processes at play during arrested phase separation, we also study colloidal gel elasticity and kinetics inside the phase boundary. By simultaneously probing mechanics and structure, we develop an isothermal transformation diagram to delineate the sequence of transitions that occur en route to arrest, which identifies processing windows for sculpting the mesostructure of nascent gels. From structural imaging and analysis, we resolve the quench-dependent kinetic rate of gel coarsening that can be quantitatively explained with a mean-field description of subcritical viscoelastic phase separation. Controlling these kinetics leads to a range of arrested gel microstructures accessible by differential thermal quenching. From linear viscoelastic measurements, we find that existing particle-scale theories prove insufficient to explain the observed quench-dependent gel modulus, demonstrating that gel network elasticity depends on heterogeneous structure in the region of arrested phase separation. Altogether, these findings advance the broader goal of controlling large-scale gel formation and mechanics by understanding and controlling particle-scale interactions — an understanding which will be critical to the continued development of colloidal gels in their applications.