After doing the washing up a few days ago, I returned to the sink to find a raft of bubbles had formed on the surface of the water. All the bubbles were roughly equal size, and they had aligned themselves into a close-packed lattice, as the photo shows. (Sorry about the quality of the photo – the bubbles were small and the light was poor – and I didn’t have a tripod – but I did my best. )
Look closely, however, and you’ll see some defects on this lattice. If you follow a row of bubbles, you’ll see in places that a row ends abruptly or another one is inserted. This is a ‘dislocation’. Also, you’ll see single point defects (a bubble missing) and grain boundaries – where parts of the raft with different directions of the bubble rows meet.
All these defects are found in real materials – though of course in a real crystal there is a third dimension which complicates things a bit more. For example, there is a ‘screw dislocation‘ that can occur in a 3d lattice which has no analogue in 2d.
Making a bubble raft is a good way of teaching about crystal structure and crystal defects – the tricky bit is getting all the bubbles about the same size. I’m not sure how it happened in this case, but it was worth grabbing the camera and having a closer look.