Talk:Sedimentation coefficient

Introduction

This article used to redirect to the svedberg article. But because the unit (svedberg) and the measure (sedimentation coefficient) got all mixed up there, I split up the article and moved part of it here. This may need some further cleaning up and editing though.

In particular, it would be nice to have an explanation of why one would want to calculate or measure the sedimentation coefficient of a particle. I know of at least two reasons: 1) To be able to estimate sedimentation time when the s of a particle is known (see the article clearing factor, which I just created), and 2) to derive physical properties of the particle by measuring s (in a known medium). I'm afraid I do not know enough about analytical ultracentrifugation to say how to do the latter though. :-) Anyone?

Also, it would be good to mention the relationship between the sedimentation coefficient and the density of a particle (instead of just saying that bigger particles have higher values). And the correction of s needed for other media than water and other temperatures than 20 degrees should probably also be mentioned. --> [Definitely mention density instead of saying "bigger particles." Bigger in shape (is in large and spread out VS compact) mean the particle would be slower, and the s coefficient lower. More dense, however, means it would be faster. Dense=/=big.]

Lvzon (talk) 23:45, 11 September 2008 (UTC)

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Ah, I think I've found the correction formula to convert between the standard and actual sedimentation coefficients ($s_{20,w}$ in water at 20 degrees, and s). Not sure it's correct though, could someone please check this?

$s_{20,w}=s\cdot {\frac {1-V_{20,w}\cdot \rho _{20,w}}{1-V\cdot \rho }}\cdot {\frac {\eta }{\eta _{20,w}}}$

$V$ is partial specific volume

$\rho$ is density

$\eta$ is the viscosity coefficient

Lvzon (talk)

Buoyancy

Currently the article states, "The centrifugal force is given by ... mrω2. ... When the two forces (viscous force and the centrifugal force) balance ...".

In most cases the buoyancy of the suspending fluid is also significant. The force balance is
F_drag + F_buoyancy = F_inertia.
F_drag = F_inertia - F_buoyancy
Denote the volume of a particle by V_particle and density by ρ.
For a spherical particle at low Reynolds number, Stokes law is F_drag = 6πηr₀v. For a specific volume of particle, this force increases as the shape varies from a sphere.
For any shape of particle, F_inertia = ρ_particleV_particlerω² and F_viscosity = ρ_fluidV_particlerω². Therefore
6πηr₀v = (ρ_particleV_particlerω²)-(ρ_fluidV_particlerω²).
6πηr₀v = (ρ_particle-ρ_fluid)V_particlerω²
v = ((ρ_particle-ρ_fluid)V_particlerω²)/(6πηr₀)
For the spherical particle the volume is (4/3)πr₀³.
v = ((ρ_particle-ρ_fluid)(4/3)πr₀³rω²)/(6πηr₀)
v = ((ρ_particle-ρ_fluid)(4/18)r(r₀ω)²)/η
Regards, PeterEasthope (talk) 02:58, 11 August 2015 (UTC)