Formation of streamers in sedimentation

DR. C. E. MARSHALL has recently described a method (Proc. Roy. Soc. A, 126, pp. 427–439; 1930) by which he claims that the size distribution of particles in a weak suspension of clay or similar material with particles of 2μ–20μμ, diameter can be accurately determined. A high-speed laboratory centrifuge is used, each tube being nearly filled with a solution of cane-sugar or urea and a small quantity of the aqueous suspension (which has a lower density) is carefully poured on the top. A determination of the weight of sediment collected on the bottom of the tube in a measured time is used by Marshall to determine the weight of the particles which exceed a certain size, it being assumed that all the particles start from the top of the column at approximately the same time and settle through the solution in accordance with Stokes's Law. In discussing the method the author remarks: “It has not been found possible, so far, to apply these principles with any accuracy to the case of sedimentation under gravity. … Owing to slight variations in temperature or concentration the upper liquid sends down ‘streamers’ of suspension into the heavier liquid below.” In advocating the use of the high-speed centrifuge he says: “The boundary between the two liquids actually becomes more sharply denned as centrifuging proceeds, and even if ‘streamer’ formation has begun, the liquid of low density moves rapidly back into place under its action.”