Micrometer-sized colloidal particles have strong thermal motions and can form various phases, thus they can be used as a model of atoms. Their larger size and slow dynamics can be directly observed under optical microscopy. Therefore, colloidal model systems are ideal for investigating phase transitions, particularly on poorly understood kinetic processes.

In this thesis, we first introduce the NIPA colloidal particles, which size and attraction can be finely tuned by temperature. It provided a platform to study surface physic at the single-particle level (Chapter 2). We observe the pre-solid-solid transition at the free surface in equilibrium and the growth front.

In Chapter 3, we report the novel polymorphic crystalline layer on crystal surface, which is based on previous labmates Xip...[ Read more ]

Micrometer-sized colloidal particles have strong thermal motions and can form various phases, thus they can be used as a model of atoms. Their larger size and slow dynamics can be directly observed under optical microscopy. Therefore, colloidal model systems are ideal for investigating phase transitions, particularly on poorly understood kinetic processes.

In this thesis, we first introduce the NIPA colloidal particles, which size and attraction can be finely tuned by temperature. It provided a platform to study surface physic at the single-particle level (Chapter 2). We observe the pre-solid-solid transition at the free surface in equilibrium and the growth front.

In Chapter 3, we report the novel polymorphic crystalline layer on crystal surface, which is based on previous labmates Xipeng Wang’s simulations and Bo Li’s experiments about PMMA colloids. The surface of a thin-film 4△ NIPA crystal develops a layer of 4☐ lattice whose thickness increases in a power law as approaches the solid–solid transition point, analogous to premelting. Therefore, we name it as ”pre-solid-solid transition”. We show that surface crystals can appear during thermal equilibrium, melting, crystallization, and grain coarsening by forming a coherent interface. Additionally, we propose that a crystal surface can develop a crystalline layer and a liquid layer when the premelting and presolid- solid transition coexist.

In Chapter 4, we focus on the pre-solid-solid transition during NIPA crystal growth. We find under slow cooling, the crystallization front of 4△ lattice develops a layer of 4☐ lattice whose thickness increases in a power law as the crystallization temperature decreases and can reach over 40 layers, which is much thicker than other surface-wetting layers in various non-equilibrium processes.

In Chapter 5, we report the surprisingly large penetration depths of surface effects on crystals. Different parameters have different penetration depths, associated with the observed strong lattice dilation near the surface.