Investigating the Snow Crystal
: A Computational Study of the Molecular Mechanisms of Hexagonal Ice Growth

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Hexagonal Ice (Ih) plays a number of essential roles in controlling and maintaining the natural environment on Earth. It is the predominant crystalline form of ice and permanently covers at least 10% of all land. Within the atmosphere ice forms cirrus clouds which are involved in the greenhouse e↵ect. The altitude and thickness of the clouds controls the balance of solar heating and infrared cooling from Earth to space, and is governed by the sizes and shapes of the constituent ice crystals. The shape of vapour grown ice crystals depends strongly on temperature and vapour pressure. At low vapour pressures, simple hexagonal prisms form with an aspect ratio that depends on the relative rate of growth of its two crystallographic surfaces - the basal and prism surface. The competing action of the two rates results in thin hexagonal plates at very low temperatures and elongated prisms at higher temperatures. Curiously, as temperature is increased further, thin plates form once again. The reasons why growth occurs preferentially along the basal and prism surfaces at different temperatures remain unknown. Progress in climate science is hindered by an incomplete grasp of the underlying mechanisms involved in ice growth in cirrus clouds. A comprehensive understanding of ice crystal growth is essential if we are to overcome the colossal climate change challenges we face.
In this project we investigate the basal and secondary prism surfaces of hexagonal ice at a range of temperatures between 240 K and 270 K using molecular dynamics and metadynamics. Metadynamics is an enhanced sampling technique that enables efficient sampling of rare events and yields an estimate of the free energy as a function of selected collective variables. Our simulations show the formation of a disordered quasi-liquid layer (QLL) on the surfaces of ice. The QLL mediates crystal growth and has a thickness which varies with temperature. We investigate how the ice/QLL and QLL/vapour interfaces influence the water adsorption potential, surface diffusion properties and growth shape. Our findings reveal that the outer surface structure at the QLL/vapour interface depends weakly on the underlying crystal lattice. The desorption energy costs are equivalent for the basal and prism surfaces, however, the crystal lattice impacts the dynamics within the QLL. Our results show that there are distinct diffusion energy barriers at the basal and prismatic QLL/ice interfaces. The implication of our work is that the QLL/ice interface is key to the overall kinetics of ice growth in vapour whereas the QLL/vapour interface plays a secondary role.
Date of Award2017
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorCarla Molteni (Supervisor) & Chris Lorenz (Supervisor)

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