A Local Structural Mechanism For Dynamical Arrest

March Meeting 2010

A Local Structural Mechanism For Dynamical Arrest Low-energy clusters of micron-sized colloidal particles form a percolating network causing gelation in an experiment. Gelation is an example of dynamical arrest (vitrification), for which a structural mechanism has long been sought.


Abstract

V2.00004: "Locally Favoured Structures and Dynamic Arrest"

Presented Thursday, March 18, 2010

Paddy Royall
University of Bristol
Bristol, United Kingdom

A Local Structural Mechanism For Dynamical Arrest

Despite its solid appearance, glass is actually a ‘jammed’ state of matter that moves unbelievably slowly. Like cars in a traffic jam, atoms in a glass can’t reach their destination because the route is blocked by their neighbours, so it never quite becomes a ‘proper’ solid.

For more than 50 years most scientists have tried to understand just what glass is. Work so far has concentrated on trying to understand the traffic jam however we argue that glass, or in our case, a gel, ‘fails’ to crystallize the special local structures that form before the constituent particles have had time to crystallise.

The problem is you can’t watch what happens to atoms as they cool because they are just too small. So using micron-sized particles called colloids that mimic atoms, but are just large enough to be visible using state-of-the-art microscopy, we cooled some down and watched what happened.

What we found was that the gel these particles formed also ‘wants’ to be a crystal, but it fails to become one due to the formation of icosahedra-like structures exactly as Sir Charles Frank had predicted 50 years ago, which are shown in various colours. It is the formation of these structures that underlie jammed materials and explains why a glass is a glass and not a liquid or a solid.

Knowing the structure formed by atoms as a glass cools represents a major breakthrough in our understanding of meta-stable materials and will allow further development of new materials such as metallic glasses.

References

Royall CP Williams SR, Ohtsuka, T and Tanaka, H, 'Direct observation of a local structural mechanism for dynamical arrest', Nature Materials, 7556-561 (2008).

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