Engineering Colloidal Microstructures for Functional Properties: Crystals and Gels

Abstract

In this dissertation, we report the microstructural engineering of colloidal crystals and gels to alter their macroscopic functional properties, including, specifically, the optical properties of crystals and the rheological properties of gels. Confocal laser scanning microscopy (CLSM) is used to characterize microstructures at the single particle level. Small-angle light scattering (SALS) and rheology are used separately to quantify the macroscopic light diffraction responses and mechanical properties.

External fields can accelerate colloidal self-assembly, yet the accelerated kinetics can negatively impact the quality of ordered crystal structures. We show that cyclically applied external electric fields can improve the crystallinity of colloidal crystals by annealing local disorder. We find that the optimal off-duration for maximum annealing is approximately one-half of the melting half lifetime of the assembled crystal. The annealing efficacy depends on the creation of mobile defects while avoiding additional immobile defects during field off-duration. Molecular dynamics simulations show that the optimal off-duration for maximum annealing, normalized by crystal melting time, is insensitive to particle interaction details. This research provides a simple and efficient strategy to rapidly create perfect crystals as well as the mechanistic insights into how defects annealing is the key to the phenomena.

From CLSM, we find that ellipsoids with aspect ratio 2.0 melt into disordered structures 5.7 times faster compared to spheres. On the other hand, ellipsoids with the same aspect ratio self-assemble into ordered crystals at a similar rate to spheres. By molecular dynamics simulations, we find that it is the Brownian rotation of the ellipsoids contributes to faster melting kinetics relative to spheres. Insights from the project can be applied to reconfigurable self-assembly manipulation.

Physical gelation of colloids produces elastic structures that are used to stabilize formulation. However, rheological control is greatly limited by the universality of the arrested spinodal decomposition mechanism of colloidal gelation. Volume fraction and interparticle bonding are the limited tools to control gel modulus. We demonstrate, through manipulation of particle shape, that we can expand the design space of available elastic states. Gels formulated from discoids exhibit expanded elasticity states that are shifted in volume fraction from the universality of sphere gels by a factor of as much as 15 in volume fraction and 20 in elastic modulus. We apply the predictive model of particle gelation and explain this efficient generation of elasticity into a series of factors dependent on fractal dimension, backbone topology, and non-central forces, with each independently measured and quantified. Our study reveals a new strategy for designing sustainable gels materials with tailored rheological and mechanical properties, particularly elasticity at ultra-low volume fractions.