UC Santa Barbara

Colloids are used to form processable soft solids that are ubiquitous in everyday materials like foods, industrial building materials, coatings and biomedical products. Despite their prevalence, there are limited experimental methods to probe the underlying colloidal scale interactions in these materials. These underlying interactions give rise to unique structure and properties. By modulating the interaction potential between colloids both equilibrium phase transitions can be triggered and non-equilibrium `arrested states' such as gels and glasses can be formed. However, there is a lack of experimental methods available for faithfully mathematically representing these interactions which prevents a better understanding of the phase behavior and processing of these materials.

To address this problem, we use a model experimental colloidal system to develop a new approach for determining a unique interaction potential that can be used to replicate the systems equilibrium and non-equilibrium phase behavior in particle simulations. The new approach uses a non-equilibrium comparison test between experiments and simulation to down-select candidate potentials (extracted from dilute experimental structural measurements) that differ in shape but share the same equilibrium structure. We demonstrate that the conditions for non-equilibrium arrest by colloidal gelation are sensitive to both the shape of the interaction potential and the thermal quench rate. We exploit this sensitivity to select the potential that best matches the non-equilibrium behavior between simulation and experiments.

With a better understanding of the underlying colloidal scale interactions in our experimental system, we then focus on developing new methods to precisely establish locations of non-equilibrium states to generate a comprehensive phase diagram and to elucidate the underlying combination of mechanisms that lead to the emergence of gelation. Our method employs systematically varied quenches of the colloidal fluid over a range of volume fractions to identify minimal conditions for gel solidification. The method is applied to experimental and simulated systems to test its generality toward attraction potentials of varied shape. Using structural and rheological characterization, we show that all gels incorporate elements of percolation, phase separation and glassy arrest, where the quench path sets their interplay and determines the shape of the gelation boundary. We find that the slope of the gelation boundary corresponds to the dominant gelation mechanism, and its location approximately scales with the equilibrium fluid critical point. These results are insensitive to potential shape, suggesting that this interplay of mechanisms is applicable to a wide range of colloidal systems.

Building on our understanding of colloidal phase behavior, we next investigate how programmed quenches to the gelled state can be used to effectively tailor gel structure and mechanics. Specifically, we look at how changing the quench rate from the fluid phase into the gel region can be used to vary the gels elastic modulus by over two orders of magnitude. These results suggest an important parameter for control of the structure and mechanics of gels is the ratio of the timescale of the thermal quench relative to the timescale for phase separation. We also investigate how the structure and mechanics change in time after performing instantaneous quenches to different quench depths in the gel region.

We then used our model colloidal system to develop a new technique for templating porous polymeric materials. These porous materials have a diverse range of applications in materials for water filtration, insulation, biomedical tissue scaffolds and catalyst supports.

Finally we present some future directions on developing time-temperature-transformation (TTT) diagrams to better inform thermokinetic processing of colloidal materials. We also briefly discuss the development of new techniques to locate the attractive glass line inside the phase instability region of the colloidal phase diagram.