Stokes Law formula:
  V = gr²(ρ_p-ρ_m )/18μ;
where:
ρ_p: Particle Density, in Kg/m^3
ρ_m: Medium Density, in Kg/m^3
r: Particle Diameter, in meter
V: Fall or Settling Velocity, in meter/second
μ: Medium Viscosity, in N.s/m^2
g: The acceleration due to gravity, is 9.80665 m/s^2

Stokes Law describes the steady flow of a continuous stream of fluid which is incompressible and has negligible viscosity.