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In 1851, George Gabriel Stokes derived an expression, now known as Stokes's law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes's law is derived by solving the Stokes flow limit for small Reynolds numbers of the Navier–Stokes equations.
The force of viscosity on a small sphere moving through a viscous fluid is given by:^{[1]}
where:
In SI units, F_{d} is given in newtons (= kg m s^{−2}), η in Pa·s (= kg m^{−1} s^{−1}), R in meters, and v in m/s.
Stokes's law makes the following assumptions for the behavior of a particle in a fluid:
Note that for molecules Stokes's law is used to define their Stokes radius.
The CGS unit of kinematic viscosity was named "stokes" after his work.
Stokes's law is the basis of the falling-sphere viscometer, in which the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes's law can be used to calculate the viscosity of the fluid. A series of steel ball bearings of different diameters are normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses glycerine or golden syrup as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. Several school experiments often involve varying the temperature and/or concentration of the substances used in order to demonstrate the effects this has on the viscosity. Industrial methods include many different oils, and polymer liquids such as solutions.
The importance of Stokes's law is illustrated by the fact that it played a critical role in the research leading to at least three Nobel Prizes.^{[2]}
Stokes's law is important for understanding the swimming of microorganisms and sperm; also, the sedimentation of small particles and organisms in water, under the force of gravity.^{[3]}
In air, the same theory can be used to explain why small water droplets (or ice crystals) can remain suspended in air (as clouds) until they grow to a critical size and start falling as rain (or snow and hail).^{[4]} Similar use of the equation can be made in the settlement of fine particles in water or other fluids.
At terminal (or settling) velocity, the excess force F_{g} due to the difference between the weight and buoyancy of the sphere (both caused by gravity^{[5]}) is given by:
with ρ_{p} and ρ_{f} the mass densities of the sphere and fluid, respectively, and g the gravitational acceleration. Requiring the force balance F_{d} = F_{g} and solving for the velocity v gives the terminal velocity v_{s}. Note that since the excess force increases as R^{3} and Stokes's drag increases as R, the terminal velocity increases as R^{2} and thus varies greatly with particle size as shown below. If the particle is falling in the viscous fluid under its own weight, then a terminal velocity, or settling velocity, is reached when this frictional force combined with the buoyant force exactly balances the gravitational force. This velocity v (m/s) is given by:^{[5]}
(vertically downwards if ρ_{p} > ρ_{f}, upwards if ρ_{p} < ρ_{f} ), where:
In Stokes flow, at very low Reynolds number, the convective acceleration terms in the Navier–Stokes equations are neglected. Then the flow equations become, for an incompressible steady flow:^{[6]}
where:
By using some vector calculus identities, these equations can be shown to result in Laplace's equations for the pressure and each of the components of the vorticity vector:^{[6]}
Additional forces like those by gravity and buoyancy have not been taken into account, but can easily be added since the above equations are linear, so linear superposition of solutions and associated forces can be applied.
For the case of a sphere in a uniform far field flow, it is advantageous to use a cylindrical coordinate system ( r , φ , z ). The z–axis is through the centre of the sphere and aligned with the mean flow direction, while r is the radius as measured perpendicular to the z–axis. The origin is at the sphere centre. Because the flow is axisymmetric around the z–axis, it is independent of the azimuth φ.
In this cylindrical coordinate system, the incompressible flow can be described with a Stokes stream function ψ, depending on r and z:^{[7]}^{[8]}
with u_{r} and u_{z} the flow velocity components in the r and z direction, respectively. The azimuthal velocity component in the φ–direction is equal to zero, in this axisymmetric case. The volume flux, through a tube bounded by a surface of some constant value ψ, is equal to 2π ψ and is constant.^{[7]}
For this case of an axisymmetric flow, the only non-zero component of the vorticity vector ω is the azimuthal φ–component ω_{φ}^{[9]}^{[10]}
The Laplace operator, applied to the vorticity ω_{φ}, becomes in this cylindrical coordinate system with axisymmetry:^{[10]}
From the previous two equations, and with the appropriate boundary conditions, for a far-field uniform-flow velocity u in the z–direction and a sphere of radius R, the solution is found to be^{[11]}
The viscous force per unit area σ, exerted by the flow on the surface on the sphere, is in the z–direction everywhere. More strikingly, it has also the same value everywhere on the sphere:^{[12]}
with e_{z} the unit vector in the z–direction. For other shapes than spherical, σ is not constant along the body surface. Integration of the viscous force per unit area σ over the sphere surface gives the frictional force F_{d} according to Stokes's law.