Stokes Sedimentation Calculator
This App uses Stokes’ law to estimate the rate at which a dispersion will settle. It will help you to predict how settling will depend on properties of your formulation such as
- Viscosity, shown as η
- Density of dispersed and continuous phases, shown as ρ
- Particle radius, r
You will need to know or be able to estimate these properties plus a few more such as volume fraction φ.
When you have particles of radius r of density ρp in a liquid of viscosity η and density ρl and a gravitational force g then the particles will fall at a velocity v and cover a distance h in time t. The formula is the Stokes equation:
`g_(rel)=1+.000000001118 r_(cent) RPM^2`
The normal formula uses g for gravity but if the particle is spinning at a given RPM in a centrifuge of radius rcent then the effective force is larger, grel (relative to g) and the settling is faster. The 2.18 in the formula is the familiar 2/9 from the Stokes equation multiplied by g=9.81. The significance of φ is explained below.
If your particle isn't spherical it has an aspect ratio, AR, (long dimesion/short dimension greater than 1, and the Stokes equivalent radius is reduced. If AR21 = Sqrt(AR²-1) then the effective radius, reff is given by1:
If ρp is less than ρl as in a typical oil in water emulsion then the "settling" velocity is negative and t is the time taken for the emulsion particle to rise by h.
So far, we have assumed that the particles don't interfere with themselves. A simple, but effective approximation from Richardson and Zaki tells us that the velocity is reduced by a factor of (1-φ)4.65 where φ is the volume fraction. Clearly this means that as particles start to settle and get locally more concentrated, the rate will reduce. This app is just to give a general idea of the effects in the early stages of settling.
There is one more factor. The argument goes that if the particle is very small then thermal motion (kT) is enough to keep it moving randomly so it will not settle. This is controlled by a Gravity Number, Gr given by:
`Gr = (0.384.2πr^4Δρg)/(3kT)`
The 0.384 comes from some assumptions about relative particle sizes and is a maximum value for a size ratio of 1.66:1. For simplicity, T is assumed to be 298K. The "Too small to settle" box goes to "Yes" when Gr < ~1 For the case of 1g, most nanoparticles will not settle easily. But as you spin the rotor, it is harder for thermal motions to resist the higher g.
It happens that this argument is bogus, but in practice smaller particles with the normal vibrations, convection currents etc. can resist settling for some considerable time. To see why it is bogus, explore the Gravitational Sedimentation app.
Notes and limitations
Although measuring densities and viscosities is not too hard, you are unlikely to have a single, known radius. In any case, as particles start to settle, they get slowed down either according to Richardson and Zaki or by other effects such as flocculation according to Buscall-White theory. So the point of the app is to give a feel for the sort of formulation options available to you, such as changing the viscosity and density of the liquid.
1B. R. Jennings and K. Parslow, Particle Size Measurement: The Equivalent Spherical Diameter, Proc. Royal Soc. London. Series A, Mathematical and Physical Sciences, 419, No. 1856. 1988, 137-149