Stokes' law

In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on sphere/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:^[1]

Statement of the law

The force of viscosity on a small sphere moving through a viscous fluid is given by:

$F_d = 6 \pi\,\mu\,R\,V\,$

where F_d 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, F_d 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:

• Laminar Flow
• Spherical particles
• Homogeneous (uniform in composition) material
• Smooth surfaces
• Particles do not interfere with each other.

Note that for molecules Stokes' law is used to define their Stokes radius.

The CGS unit of kinematic viscosity was named "stokes" after his work.

Applications

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.^[2]

Stokes' law is important to understanding the swimming of microorganisms and sperm; also, the sedimentation, under the force of gravity, of small particles and organisms, in water.^[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.

Terminal velocity of sphere falling in a fluid

Creeping flow past a falling sphere in a fluid (e.g., a droplet of fog falling through the air): streamlines, drag force F_d and force by gravity F_g.

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:

$F_g = \left( \rho_p - \rho_f \right)\, g\, \frac{4}{3}\pi\, R^3,$

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 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, 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]

$V = \frac{2}{9}\frac{\left(\rho_p - \rho_f\right)}{\mu} g\, R^2$

(vertically downwards if ρ_p > ρ_f, upwards if ρ_p < ρ_f ), where g is the gravitational acceleration (m/s²), ρ_p is the mass density of the particles (kg/m³), ρ_f is the mass density of the fluid (kg/m³) and μ is the dynamic viscosity (kg /m*s).

Steady Stokes flow

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]

$\begin{align} &\nabla p = \eta\, \nabla^2 \mathbf{u} = - \eta\, \nabla \times \mathbf{\boldsymbol{\omega}}, \\ &\nabla \cdot \mathbf{u} = 0, \end{align}$

where:

• 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  $\boldsymbol{\omega}=\nabla\times\mathbf{u}.$

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]

$\nabla^2 \boldsymbol{\omega}=0$   and   $\nabla^2 p = 0.$

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.

In this cylindrical coordinate system, the incompressible flow can be described with a Stokes stream function ψ, depending on r and z:^[7][8]

$u_z = \frac{1}{r}\frac{\partial\psi}{\partial r}, \qquad u_r = -\frac{1}{r}\frac{\partial\psi}{\partial z},$

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]

$\omega_\varphi = \frac{\partial u_r}{\partial z} - \frac{\partial u_z}{\partial r} = - \frac{\partial}{\partial r} \left( \frac{1}{r}\frac{\partial\psi}{\partial r} \right) - \frac{1}{r}\, \frac{\partial^2\psi}{\partial z^2}.$

The Laplace operator, applied to the vorticity ω_φ, becomes in this cylindrical coordinate system with axisymmetry:^[10]

$\nabla^2 \omega_\varphi = \frac{1}{r}\frac{\partial}{\partial r}\left( r\, \frac{\partial\omega_\varphi}{\partial r} \right) + \frac{\partial^2 \omega_\varphi}{\partial z^2} - \frac{\omega_\varphi}{r^{2}} = 0.$

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]

$\psi = - \frac{1}{2}\, u\, r^2\, \left[ 1 - \frac{3}{2} \frac{R}{\sqrt{r^2+z^2}} + \frac{1}{2} \left( \frac{R}{\sqrt{r^2+z^2}} \right)^3\; \right].$

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:^[1]

$\boldsymbol{\sigma} = \frac{3\, \eta\, u}{2\, R}\, \mathbf{e}_z$

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' law.

Other types of Stokes flow

See also

• Stokes flow
• Einstein relation (kinetic theory)
• Scientific laws named after people
• Drag equation
• Viscometry
• Equivalent spherical diameter
• Deposition (geology)

Notes

1. Batchelor (1967), p. 233.
2. Dusenbery, David B. (2009). Living at Micro Scale, p.49. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
3. Dusenbery, David B. (2009). Living at Micro Scale. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
4. Hadley, Peter. "Why don't clouds fall?". Institute of Solid State Physics, TU Graz. Retrieved 30 May 2015.
5. Lamb (1994), §337, p. 599.
6. Batchelor (1967), section 4.9, p. 229.
7. Batchelor (1967), section 2.2, p. 78.
8. Lamb (1994), §94, p. 126.
9. Batchelor (1967), section 4.9, p. 230
10. Batchelor (1967), appendix 2, p. 602.
11. Lamb (1994), §337, p. 598.

References

• Batchelor, G.K. (1967). An Introduction to Fluid Dynamics. Cambridge University Press. ISBN 0-521-66396-2.
• Lamb, H. (1994). Hydrodynamics (6th ed.). Cambridge University Press. ISBN 978-0-521-45868-9. Originally published in 1879, the 6th extended edition appeared first in 1932.