Materials science: Pleated crystals

In 1947, William Bragg and John Nye had a simple yet brilliant idea¹: to model atoms using bubbles. Bubbles are neatly spherical yet soft, and if many small bubbles, all of the same size, are blown onto the surface of a soapy film, they assemble into a hexagonal array (raft) that mimics the atomic arrangements of flat, two-dimensional crystals. On page 947 of this issue, Irvine and colleagues² present an extension of the 'bubble-raft technique' to visualize the behaviour of two-dimensional crystals along curved surfaces.

Bubble rafts are ideal tools for studying crystals. They are imperfect: exactly like crystals, they can contain defects such as vacancies, impurities, dislocations and grain boundaries. In two-dimensional crystals, dislocations are point defects (line defects in three dimensions) formed by the termination of a row or column (a plane in three dimensions) of periodically aligned atoms (Fig. 1). Dislocations are important because, if they move, they shift matter by one atomic-lattice spacing about their trajectory; in three dimensions, dislocations are the main mechanism by which metals deform permanently. They are also important in crystalline materials because arrays of dislocations form the building blocks of subgrain boundaries — grain boundaries separating grains that are misoriented by only a few degrees¹.

This image shows one of Bragg and Nye's original bubble rafts¹, assemblies of bubbles on the surface of a solution consisting of water, glycerine, oleic acid and triethanolamine that can be used to model two-dimensional crystals. Every other horizontal bubble row has been coloured green to facilitate the identification of the column of bubbles that terminates at a dislocation (red). The bubbles that form the bottom of this column (and hence the dislocation) are two opposite disclinations, as predicted by theory. They have a different number of nearest neighbours from bubbles elsewhere: 5 and 7 rather than 6. Horizontal bubble rows tilt downwards slightly to the right of the dislocation. An array of similar dislocations, regularly spaced one over the other, creates a boundary across which the crystal is tilted by a constant angle: this is a two-dimensional subgrain boundary. Irvine and colleagues² study the interaction between the curvature of a two-dimensional crystal and the disclinations or dislocations it contains, and find short subgrain boundaries, which they call pleats. (Image reproduced from ref. 1.)

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