Consider, for instance, particles suspended in a viscous fluid in a gravitational field. Gravity tends to make the particles settle, whereas diffusion acts to homogenize them, driving them into regions of smaller concentration. Under the action of gravity, a particle acquires a downward speed of v = μmg, where m is the mass of the particle, g is the acceleration due to gravity, and μ is the particle's mobility in the fluid. George Stokes had shown that the mobility for a spherical particle with radius r is μ μ = 1 6 π π η η r , where η is the dynamic viscosity of the fluid. In a state of dynamic equilibrium, and under the hypothesis of isothermal fluid, the particles are distributed according to the barometric distribution ρ ρ = ρ ρ o exp ⁡ ⁡ ( − − m g h k B T ) , where ρ − ρ o is the difference in density of particles separated by a height difference, of h = z − − z o , k B is the Boltzmann constant (the ratio of the universal gas constant, R, to the Avogadro constant, N A), and T is the absolute temperature.