Bragg rafts

If a large number of hemispherical bubbles of equal size are produced on the surface of a bath of soap solution, using a uniform pressure jet of air, they will tend to form a plane hexagonal lattice, or honeycomb lattice, of bubbles (Plate 4.12(a)). Lawrence Bragg first used such rafts of bubbles to show that two dimensional lattices of bubbles have many features in common with three dimensional atomic crystalline lattices. These rafts have become known as Bragg rafts. 

A defect can be created in the bubble lattice by increasing, or decreasing, the size of one of the bubbles (Plate 4.12(c)). An interstitial bubble, also, is easily introduced into the Bragg raft. 

Fig. 1.5 A bubble raft illustrating the nature of a dislocation. The region of misfit near Y can be seen. (After Bragg and Nye )...

Early in the history of crystal dislocations, the lack of resistance to motion in pure metal-like crystals was provided by the Bragg bubble model, although it was not taken seriously. By adjusting the size of the bubbles in a raft, it was found that the elastic behavior of the raft could be made comparable with that of a selected metal such as copper (Bragg and Lomer, 1949). In such a raft, it was further found that, as expected, the force needed to form a dislocation is large. However, the force needed to move a bubble is too small to measure. 

Dislocations may be demonstrated by means of a soap bubble analogy. This consists of a raft of small bubbles, all the same size, generated on the surface of a soap solution. The bubbles represent atoms and are subjected to two forces just as the atoms in a metal are. Surface tension causes the bubbles to attract each other and form a dense array, while pressure within the bubbles prevents them from approaching each other closer than a characteristic distance. When a raft of bubbles is formed on a fluid surface, grain boundaries are evident as well as dislocations (Fig. 8.10). When such a raft of bubbles is sheared, deformation is seen to occur as dislocations move across the crystal. While the forces at work are not identical to those associated with atoms, the soap bubble model is a useful analogy. A film has been produced by Sir Lawrence Bragg who devised the soap bubble analogy, and this film lends considerable credibility to the relatively sophisticated dislocation concept. 

Dislocations occur in lattices other than those of atomic-scale crystals. The best known examples, no doubt, are those in the experiments of Bragg and Nye with rafts of soap bubbles [1]. Their work illustrated the geometry and kinematics of the edge dislocations in these two-dimensional hexagonal lattices, and vividly revealed how macroscopic plastic deformation of crystals is effected by the motion of large numbers of such dislocations. Ahead of the definitive identification of moving dislocations in atomic crystals by electron microscopy [2], Bragg and Nye s experiments had convinced many of the reality and the potential of what had initially been a purely theoretical concept [3-5]. 

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