In this paper, we present detailed and systematic experimental results on the
sedimentation of solid particles in aqueous solutions of polyox and polyacrylamide. The tilt
angles of long cylinders falling in these viscoelastic liquids were measured. The effects of
particle length, particle weight, particle shape, liquid properties and liquid temperature were
determined. In these experiments, the cylinders fall under gravity in a two-dimensional bed. No
matter how or where they are released they will center themselves between the close walls and
fall steadily in a configuration in which the axis of the cylinder is at a fixed angle of tilt with the
horizontal. A discussion of the tilt angle may be framed in terms of competition between viscous
effects, viscoelastic effects and inertia. When inertia is large the particles settle with their
broadside perpendicular to the direction of fall. When inertia is small viscoelasticity dominates
and the particles settle with their broadside parallel to the direction of fall. The tilt angle varies
continuously from 90o
when viscoelasticity dominates to 0o
when inertia dominates. The balance
between inertia and viscoelasticity was controlled by systematic variation of the weight of the
particles, the concentration of polyox in water and of the temperature of the solution. Particles
will turn broadside-on when the inertia forces are larger than viscous and viscoelastic forces.
This orientation occurred when the Reynolds number was greater than some number not much
greater than one in any case, and less than 0.1 in Newtonian liquids and very dilute solutions.
The appearance of a tilt angle, however, appears to be most strongly correlated with values of a
viscoelastic Mach number M=U/c where U is the terminal velocity and c is the shear wave speed
measured with the shear wave speed meter (Joseph, [1990]). Strong departure of the tilt angle
from θ=90o
begins at about M=1 and ends with θ=0
o
when 1