A neat study that involves placing colloidal particles on curved oil-glycerol interfaces reveals a new form of crystal defect. The defect is called a pleat, by analogy to the age-old type of fabric fold. See Letterp.947
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.