where is the fluid's viscosity, and is the local pressure. In the absence of sources or sinks,

completes the flow's description. Eq. (14) is a good approximation for flows at low Reynolds numbers for which viscous damping dominates inertial effects - typical conditions for small collections of diffusing spheres.

Flows vanish on solid surfaces thus setting boundary conditions for solutions to Eqs. (14) and (15). Once these boundary conditions are satisfied, we can calculate the viscous drag on a particle by integrating the pressure tensor

over its surface: