F = 6π.η.r.v, where F is the force exerted on a sphere of radius r which is moving through a fluid of viscosity η with a relative velocity v; this equation holds at low velocities which are free from turbulence (called the stokes region).
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2216 (https://doi.org/10.1351/pac199062112167)