Expanding Fluidized Beds
By using a solvent counterflow (i.e., fluidization) to prevent sedimentation of charged silica spheres, we have established a steady state polycrystalline colloidal suspension of uniform concentration. With this technique we varied the steady state concentration of a single sample by simply changing the flow rate.
For the published paper see:
Crystalline Fluidized Beds, M.A. Rutgers, J.Z. Xue, E.
Herbolzheimer, W.B. Russel, and P.M. Chaikin, Physical Review E,
51: 4674-4678, 1995.
The experimental setup is as follows:
A fluid reservoir (a) supplies a
pressure drop across a glass capillary (b). The reservoir's height controls the
pressure and thereby the flow rate. The fluidized bed (c) consists of a
rectangular glass tube (1.2 cm X 0.2 cm X 10cm) with an alumina
membrane (d), permeable to fluid only, at the bottom. The liquid flows through
the colloidal suspension (e) and out of the cell (f) where it accumulates in a
long thin tube (g). The flow rate, measured from the rate at which the tube
fills, is typically less than 1 cc every 10 days.
What follows is a sideways movie, compressed from about two days, of an expanding fluidized bed. The dense area at the bottom undergoes some curious instabilities. I never got around to studying this in greater detail. What may appear as layers at the top of the fluidized bed (right is top, flow is from right to left) is actually due to video compression blocking artifacts. I had to do this in order to keep the movie at a reasonable size.
The following figure shows several of the unstable events with an actual timeline below. They happen rather quickly, and the patterns are surprisingly reproducible.
