An unexpected phenomenon observed in particle settling: oscillating falling spheres

We have observed the phenomenon of oscillating particle settling. A particl e settling in berate cross-linked hydroxy propyl guar in a pipe with a rela tively large particle/pipe diameter ratio regularly accelerates and deceler ates while it is settling. The phenomenon can be described by a particle th at settles slower and slower until at a certain point the particle suddenly accelerates as if it is settling in a Newtonian fluid and then start slowi ng down again. This cycle regularly repeats itself a number of times before the bottom of the pipe is reached. The phenomenon has been reproduced a nu mber of times with different fluids. (C) 1999 Elsevier Science B.V. All rig hts reserved.