Typically, the average velocity is directly proportional to the applied force, giving the equation: ∂ ∂ c ∂ ∂ t = ∇ ∇ ⋅ ⋅ ( D ∇ ∇ c ) − − ∇ ∇ ⋅ ⋅ ( ζ ζ − − 1 F c ) + R where F is the force, and ζ characterizes the friction or viscous drag. (The inverse ζ is called mobility.)