UC Santa Barbara

Colloidal suspensions, consisting of particles of nano to micron scale dispersed in a medium, experience interparticle interactions that profoundly impact the macroscopic properties of soft materials, including stability, structure formation, and dynamics. A mechanism to dynamically tune these interactions can provide new avenues for self assembly and material processing. However, while traditional colloidal interactions have been well understood, it has been theoretically challenging to develop frameworks for nonequilibrium systems where interactions are dynamically evolving on the order of colloidal timescales.

In this thesis, I combine theoretical, experimental, and computational efforts to understand out-of-equilibrium colloidal interactions that are modulated by external stimuli. These nonequilibrium interactions result from microscopic relaxation timescales which are intrinsic to the colloidal system and may be controlled to vary suspension-level properties such as viscosity and morphology. First, as a proof of concept, I demonstrate precise control of dynamic pair interactions using surface-mobile polymer-coated colloids that are inspired by biological cell membranes. I show that entropically-driven surface rearrangement of polymers at colloidal contact interfaces enable an effective, dynamic interaction which is controllable over a range of pico-Newton forces and seconds timescales. Later in the thesis, I extend the theoretical framework to attractive colloids and show that polymer entropic effects regulate structure formation and phase stability in the context of colloidal self assembly. Unlike traditional interactions, we show that surface-mobile polymers act as dynamic surfactants and allow colloids to acquire anisotropic shape throughout the assembly process. Microscopic polymer distributions impose unique geometric constraints between colloids that precisely control their packing in lamellar, string, and vesicle superstructures. Then, to understand the material properties of the suspension, I develop a first-principle framework that captures the multiscale coupling between microscopic timescales and macroscopic transport properties. Using microrheology in a bidisperse suspension as a case study, we demonstrate that effective depletion interactions between driven colloids are sensitive to particle timescales out of equilibrium and cannot be predicted by equilibrium-based pair potentials. We show that the interplay between Brownian relaxation timescales of different species plays a critical role in governing the viscosity of multi-component suspensions. Finally, in the context of biological systems, I use Brownian Dynamics simulations and polymer theory to investigate crowding effects on reconstituted and living cell membranes and characterize spatial heterogeneities. Overall, several systems are presented which exhibit multiscale, nonequilibrium interactions, and a framework connecting local dynamics to macroscopic material properties is developed.