particles motion in a viscous fluid bounded by cylinders

We study the force on a spherical rigid particl moving in a viscous fluid bouded by cylinders.
A sphere sedimenting in a stagnant fluid inside a vertical cylinder is always forced towards the axis. And a sphere sedimenting between two concentric vertical cylinders is forced towards the equilibrium position. When a cylinder, whose radius is 0.01 times the radius of the outer cylinder is placed at the axis of the cylinder, the lateral equilibrium position of the sphere is at a distance of about 0.3 times the outer cylinder radius from the axis.
Two spheres sedimenting symmetrically about the axis of a cylinder are forced towards the equilibrium position at a distance from the axis of about 0.4 times the cylinder radius.