In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers (i.e. very small particles) in a viscous fluid. Stokes' law is derived by solving the Stokes flow limit for small Reynolds numbers of the Navier–Stokes equations:
Statement of the law
The force of viscosity on a small sphere moving through a viscous fluid is given by:
where Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle, μ is the dynamic viscosity, R is the radius of the spherical object, and V is the flow velocity relative to the object. In SI units, Fd is given in Newtons, μ in Pa·s, R in meters, and V in m/s.
Stokes' law makes the following assumptions for the behavior of a particle in a fluid:
The CGS unit of kinematic viscosity was named "stokes" after his work.
Stokes' 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' 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' law is illustrated by the fact that it played a critical role in the research leading to at least 3 Nobel Prizes.
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). Similar use of the equation can be made in the settlement of fine particles in water or other fluids.
Terminal Velocity of Sphere Falling in a Fluid
with ρp and ρf the mass density of the sphere and the fluid, respectively, and g the gravitational acceleration. Demanding force balance: Fd = Fg and solving for the velocity V gives the terminal velocity Vs. Note that since buoyant force increases as R3 and Stokes drag increases as R, the terminal velocity increases as R2 and thus varies greatly with particle size as shown below. If the particle is falling in the viscous fluid under its own weight due to gravity, then a terminal velocity, or settling velocity, is reached when this frictional force combined with the buoyant force exactly balances the gravitational force. The resulting terminal velocity (or settling velocity) is given by:
where V is the flow settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ), g is the gravitational acceleration (m/s2), ρp is the mass density of the particles (kg/m3), ρf is the mass density of the fluid (kg/m3) and μ is the dynamic viscosity (kg /m*s).
Steady Stokes flow
- p is the fluid pressure (in Pa),
- u is the flow velocity (in m/s), and
- ω is the vorticity (in s−1), defined as
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.
Flow around a sphere
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.
with ur and uz 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.
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
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:
with ez 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 Fd according to Stokes' law.
Other types of Stokes flow
- Stokes flow
- Einstein relation (kinetic theory)
- Scientific laws named after people
- Drag equation
- Equivalent spherical diameter
- Deposition (geology)
- Batchelor (1967), p. 233.
- Dusenbery, David B. (2009). Living at Micro Scale, p.49. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
- Dusenbery, David B. (2009). Living at Micro Scale. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
- Hadley, Peter. "Why don't clouds fall?". Institute of Solid State Physics, TU Graz. Retrieved 30 May 2015.
- Lamb (1994), §337, p. 599.
- Batchelor (1967), section 4.9, p. 229.
- Batchelor (1967), section 2.2, p. 78.
- Lamb (1994), §94, p. 126.
- Batchelor (1967), section 4.9, p. 230
- Batchelor (1967), appendix 2, p. 602.
- Lamb (1994), §337, p. 598.