The images (6.4 mm×4.8 mm) show the development of the front at two different times (≈16 s apart). Each image is a superposition of ten consecutive video frames (0.67 s in total) colour-coded from blue (through green and yellow) to red. In the fluid regime (left), where the diffusive particle motion is well pronounced, a more or less homogeneous mix of all colours is seen. In the crystalline regime (right), where the (average) particle positions are fixed, the last colour used (red) hides the others and hence dominates, leading to a clear visual identification of ordered crystalline structures. Globally, the crystalline region consists of two domains of different lattice orientation (or structure), with the ‘interfacial melting’ in between.