Particle banding in rotating fluids: a new pattern-forming system

During experiments carried out several years ago to grow dendritic crystals while they are levitated in a tube containing rotating supersaturated solution, a curious banding process was observed, in which the crystals were seen to accumulate preferentially in periodic positions within the tube. We have now confirmed that this is a hydrodynamic pattern-forming process, caused by an interaction between the particles and the flow in the tube. The process is independent of the type of particle (large, small, round, irregular, density above or below that of the fluid) and a well-defined banding period occurs which is independent of the angular velocity of the fluid within a wide range. Experimental observations and a tentative theoretical explanation are presented.