An isolated falling sphere

The movement of an isolated single solid sphere under the influence of gravity in a liquid is described by Stokes law 

Ap is the density difference between the spheres and the liquid, rj is the viscosity of the liquid, and g is the acceleration due to gravity. 

If the moving sphere is itself a liquid, immiscible with the liquid through which it is moving, then the velocity is given by the expression above multiplied by  

If the sphere is falling in a tube of radius R, it slows down relative to the Stokes velocity, and an approximate correction factor for the amount of retardation is given by (1 - 2.1R/r [1- R/ry]) for R/r 0.32. 

A cloud of solid spheres moving in a Newtonian liquid imder the influence of gravity travels with a velocity of [4] 

---