Structure and Dynamics in Freezing and Frozen Colloidal Suspensions from Direct Observations and X-ray Scattering.

摘要

Among common substances, water is unique because it can occur in its solid, liquid and vapor phases at temperatures and pressures typical of the Earth's surface. Each of these phases individually play important roles in the physical, chemical, and biological processes that shape the Earth's surface and atmosphere, as well as in the manufacturing, energy production, and agricultural activities that shape our lives. However, only rarely is one phase found in isolation from the other phases or other substances. More often, multiple phases of water coexist with each other and with foreign materials. The resulting interfaces give rise to interesting, new physical phenomena not present in the bulk phases.;My thesis focuses on understanding how ice interacts with other solid materials, such as rocks, dust, cells, or colloidal particles. My experiments comprise a joint direct imaging and X-ray scattering study of particle redistribution in freezing and frozen colloidal suspensions. By using an analog system of monodisperse, spherical colloidal particles in water, I eliminate much of the complexity found in natural or technological settings, and thus investigate the fundamental physics involved. Visible light imaging combined with static and dynamic X-ray scattering provide a unique view of particle rearrangement during and after freezing as well as unprecedented information about the structure and dynamics of the samples at the single particle scale.;After significant supercooling, the solutions freeze in two stages: an unstable first stage and a stable second stage. During the unstable stage, particles segregate into the regions between ice dendrites forming high-particle-density regions. The ice forces these particles into contact, creating aggregates. During the stable stage, the ice engulfs particles in the high-particle-density regions, whereas other particles are pushed ahead of the freezing front. After freezing, the polycrystalline ice coarsens and grain boundary motion scavenges particles from the bulk ice crystals to accumulate in the high density regions at grain boundaries. During coarsening, the particles move ballistically and their characteristic velocity increases with increasing temperature. These results have implications for debris entrainment into glaciers, dust migration in ice sheets, and the fabrication of complex materials.