Micrometre-sized colloidal particles can be viewed as large atoms with tailorable size, shape and interactions. These building blocks can assemble into extremely rich structures and phases, in which the thermal motions of particles can be directly imaged and tracked using optical microscopy. Hence, colloidal particles are excellent model systems for studying phase transitions, especially for poorly understood kinetic and non-equilibrium processes.

In this thesis, we report a dye-induced attraction between colloidal spheres which can be finely tuned by temperature. It opens the way to study surface physics in colloidal model systems (Chapter 2). We epitaxially grew high-quality colloidal crystals with free surfaces, and observed the surface premelting, grain-boundary-mediated 2D...[ Read more ]

Micrometre-sized colloidal particles can be viewed as large atoms with tailorable size, shape and interactions. These building blocks can assemble into extremely rich structures and phases, in which the thermal motions of particles can be directly imaged and tracked using optical microscopy. Hence, colloidal particles are excellent model systems for studying phase transitions, especially for poorly understood kinetic and non-equilibrium processes.

In this thesis, we report a dye-induced attraction between colloidal spheres which can be finely tuned by temperature. It opens the way to study surface physics in colloidal model systems (Chapter 2). We epitaxially grew high-quality colloidal crystals with free surfaces, and observed the surface premelting, grain-boundary-mediated 2D melting and isostructural solid-solid transitions at the single-particle level for the first time. Analogues to the surface liquid in premelting, we discovered a layer of square lattice on the surface of the bulk triangular lattice.

In Chapter 3, we focus on the surface premelting and melting in colloidal crystals. We find that monolayer colloidal crystals exhibit incomplete premelting, with a constant liquid-layer thickness. In contrast, two- and three-layer crystals exhibit conventional complete melting, with the thickness of the surface liquid diverging as approaching the melting point. Incomplete premelting in the monolayer crystals is triggered by a bulk isostructural solidsolid transition and truncated by the first order homogeneous bulk melting. The interplay between surface premelting, bulk melting and solidsolid transitions challenges both premelting and melting theories.

In Chapter 4, we report the discovery of a thin layer of ☐-lattice formed on the surface of the Δ-lattice of a colloidal thin-film crystal. The surface crystal wets both the bulk crystal and the surface liquid, thus effectively reduce the interfacial energy. The wetted structure also exists in the epitaxial growth process. The discovered phenomenon provide a novel way to tune the surface properties of crystals.