Stokes’ Law describes the drag force experienced by a small spherical object moving slowly through a viscous fluid. It applies under conditions of laminar (non-turbulent) flow, where the fluid motion around the object is smooth and orderly.
According to Stokes’ Law, the drag force acting on the sphere is directly proportional to:
- the fluid’s viscosity,
- the sphere’s radius, and
- the sphere’s velocity relative to the fluid.
This means that as the viscosity or speed increases, the resistance (drag) also increases.
Conditions for Stokes’ Law to Apply:
- The object must be small and spherical.
- The motion must be slow, with a low Reynolds number (indicating laminar flow).
- The fluid must be Newtonian (constant viscosity).
Examples:
- Settling of pollen grains in water or dust particles in air.
- Oil droplets in emulsions and blood cells moving through plasma.
- Used to determine particle size in sedimentation and viscosity measurements.
Stokes’ Law is vital in fields like aerosol physics, biomedical engineering, and environmental science, wherever particles move through a viscous medium.