Real space study of pattern formation in freezing colloidal suspensions

Thesis

We investigate the interaction of colloidal particles with ice-water interfaces. To this end, the experimental setup we use consists of a directional solidification stage, a light microscope, and thin sample cells, which allows for the possibility of particles to be viewed at the single particle level, and thermally in two dimensions (2D). First, the interaction of single particles with a moving ice-water interface is studied. We show that speed, ice growth facets and grain boundaries are important parameters in determining the way in which particles are incorporated into the ice. Secondly, once a particle is trapped, as along as it is not too cold, it is still free to move. This is due to the presence of premelted liquid trapped within the ice, surrounding the particles in a thin layer. Particle movement is measured by use of tracking algorithms, and related to theoretical predictions, which use a balance between a thermomolecular and a viscous force to describe the motion. The extracted premelted layer thickness and its dependence on the undercooling implies that for the particle--ice system, non--retarded van der Waals interactions dominate. However, we find that the forces in the model should be modified for particles trapped in grain boundaries, and along cellular instabilities, or if impurities are present. Thirdly, the thermomolecular motion is also key to understanding how segregated ice, ice which is devoid of particles, can form in concentrated suspensions of particles, where the pore space between particles can also stay liquid below the bulk melting temperature. We study segregated ice growth both at the macroscopic scale and at the particle level. Regular ice lenses form in a regime defined by the temperature gradient and cooling rate, and these formations cause large structural changes in freezing colloidal suspensions. We explore how changing conditions such as the packing, size, shape and polydispersity of the particles, and the impurity concentration affect this process. Finally, due to the confinement of the system, the particles are in quasi 2D, whereas the ice--water system is in 3D. Consequently, we investigate the ice-water meniscus in 3D, both theoretically and experimentally using laser scanning confocal microscopy. The capillary length we measure is of the order of a colloidal particle.

Authors: Schollick, J

• DOI: 10.5287/ora-kzbxyg5d5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford