Poiseulle
flow creates a parabolic profile of velocities with the maximum at the center
of the tube and zero at the walls. The Java exercise graphs from the center line of the capillary to the outer wall. Please move the slider bars and try the check box to see what they observed. There is a minor glitch in the code (see if you can find it). Half of the greatly enlarged capillary is represented with the center line at the left of the figure and the wall at 500 micrometers away at the right of the figure.
Particle migration across streamlines is very interesting, but the observance of a concentric annulus as the ultimate location of particles adds spice to the experiment. The discoverers called it the "tubular pinch zone". When particles are prevented from rotating by drilling to insert heavy material to weight one side, they move to the center. This means that the tubular pinch zone or annulus is the region where migration forces due to streamlines are equal to Magnus forces due to rotation. (Magnus forces explain the trajectory of a spinning ping-pong ball.)
Migration across streamlines depends on the particle diameter cubed. That is why you saw in the computer exercise little effect for a 1-micron particle and an overwhelming effect for particles more than about 15 microns diameter. Very large particles distort the streamlines. We have measured velocities approaching 0.5 millimeter per second, and this is far greater than Brownian motion. In a stirred tank, it is meaningless to consider any ordered velocity fields, but there must be velocity gradients orders of magnitude more intense than those in the capillary of our exercise. It seems that particle migration in velocity gradients must play a major role in mass transfer. Some other situations where particle migration is important are the distribution of blood cells into capillaries that branch off from blood vessels where the cells have migrated away from the walls, clear zones when suspensions flow around obstacles, and infection of body organs. Some areas of the liver seldom are infected, and this may be explained by particle trajectories. Mudpuddle watchers see clear zones as the fluid suspension accellerates and decellerates passing rocks and other obstacles.