Surface and bulk melting of colloidal glasses

Abstract

The nature of glass transition, whether it is thermodynamic or dynamic, is a major puzzle in science. A similar challenge exists in glass-to-liquid transition, i.e., glass melting. In this thesis, we study surface premelting, surface melting, and bulk melting of colloidal glasses using video microscopy with the single-particle resolution. The background and experimental systems are reported in Chapters 1 and 2. In Chapter 3, using a thermally attraction-tunable colloidal system, we epitaxially grow stable colloidal glasses in a process similar to physical vapor deposition. We find that in the surface region, the structural and dynamic parameters saturate at different depths, which define a surface liquid layer and an intermediate glassy layer. The power-law growth of both layers and the melting front behaviors at different heating rates and for mono- and multi-layer systems are similar to crystal premelting and melting, suggesting that crystal premelting and melting can be generalized to amorphous solids. Ordinary glasses have been expected to melt catastrophically within the bulk, but the melting process has been poorly explored. In Chapter 4, we investigate ordinary glass melting through a thermally size-tunable colloid system, and directly visualize the melting processes of ordinary colloidal glass with the single-particle kinetics. The glasses melt through liquids nucleation, similar to the bulk melting of crystal. Although the structure is amorphous, the nucleation occurs heterogeneously from the preexisting structural “defects” identified by machine learning. We observe that nucleation rate increases with density of structural “defects” and heating temperature. Thus, glass with a high “defect” density or under high heating temperature melts catastrophically. Our results suggest that the glass melting scenario depends on the glass stability (i.e. ”defect” density), which unifies the melting of ordinary and ultrastable glasses.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Yilong HAN (Supervisor)