When freezing unidirectionally at low velocity, particles pile up at the ice-water interface. After sufficient accumulation of the particles, it destabilises the interface and bulk ice differentiates into smaller ice-fingers. The image was coloured with the orientation of particles, the kaleidoscopic pattern suggests the particles forms clusters with different orientations.
Particle induced instability at ice-water interface —

When freezing unidirectionally at low velocity, particles pile up at the ice-water interface. After sufficient accumulation of the particles, it destabilises the interface and bulk ice differentiates into smaller ice-fingers. The image was coloured with the orientation of particles, the kaleidoscopic pattern suggests the particles forms clusters with different orientations.

https://fluids.ac.uk/files/12_Siyu_Chen_submission_instability_Siyu-Chen_1024w.1543871261.jpg