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Crystallisation in the Energy Landscape

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If you have a question about this talk, please contact Dr Edward Cackett.

Is Glass a Liquid?

The observation that old windows are often thicker at the bottom than at the top has led to the popular myth that such glass has flowed over a matter of centuries, and hence glass is a liquid.

There are a number of materials that can not be simply classified as solids or liquids. When a liquid is cooled, it generally undergoes a transition at its freezing temperature to form a crystalline solid. However, it is possible to avoid this transition, lowering the temperature of a liquid below its freezing point, without it becoming solid. The most common method to form a ‘supercooled liquid’ is to cool the liquid very fast. On further cooling, the motion of the particles within the liquid decreases and eventually, when the structure appears frozen in experiments, the system can be described as a solid, specifically a noncrystalline solid or glass.

Crystal or glass?

The structure of a glass is liquid-like. From the point of view of each individual atom, there is order due to chemical bonding but there is no long-range order or symmetry as found in a crystalline solid. In contrast, the mechanical properties of a glass resemble those of a solid i.e. it does not flow. In fact the shape of old windows is due to the process of manufacture. It was hard to create a completely flat pane of glass and when installing windows, it was logical to place the thicker part at the bottom.

The Energy Landscape?

Particles interact with each other and this interaction has an energy cost or gain. The total energy of an arrangement of particles changes with the position of each individual particle. As each particle can move, this forms a multidimensional landscape. It is possible to move from one structure to another and energy changes en route. By studying the energy changes we can examine structures and the connections between different arrangements of atoms, and deduce how particular properties are related to underlying structure.

This talk is part of the Darwin College Sciences Group series.

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