Colloidal Crystals: Strain Enhancement and Coarsening

Abstract

Colloids are micrometer-size particles suspended in a fluid. The specific size gives them advantages of exhibiting Brownian motions due to thermal fluctuations and being able to be tracked in real time at single-particle level under optical microscope. Further with tunable interparticle interaction and adjustable density- and refractive index matching of colloids and the solvents, colloidal suspensions serve as model systems for fundamental questions in statistic physics and materials science. One basic colloidal model is near-hard-sphere model, where the volume fraction of colloids, ϕ, is the only tuning parameter. Crystallization of colloids occurs at relatively high ϕ, ϕ>0.49, where the diffusivity of colloids slows down dramatically and diverges at the glass transition point, ϕ=0.58. Similar kinetic conundrum applies in colloid-polymer mixtures, where the volume of colloids is replaced by polymers. Fundamental descriptions of physical process can be quite different if the kinetic arrest of structures is involved. In this thesis, we present two scenarios that crystallization is kinetically impeded using colloid-polymer mixtures, and two solutions to increase the mobility of single particles to facilitate the crystallization process. First, we show that crystallization arrested kinetically due to local crowding in colloidal gels can be substantially accelerated by oscillatory shear deformation, and that the degree of crystallization enhancement depends on the strain amplitude, the shear direction and the deformation history. The crystallization rate is significantly enhanced by alternating the shear oscillation in perpendicular directions. We demonstrate that the enhanced crystallization occurs in plastically deformed regions where the changes in the local packing density allow structural changes. Second, we report, leaving enough space for fluid phase between crystals, the long-time Ostwald ripening of crystals in fluids is recovered, for the first time. We reveal that its coarsening mechanism aligns well with the reaction-limited case in the classic Lifshitz-Slyozov-Wagner theory. By tracking the motion of the individual particles, we directly measure the crystal size changes, the kinetic factors and therefore the fluid-crystal interfacial energy.