Try looking up Stoke's law which can be applied to centrifugation. It describes the time taken for particles to settle in a solvent for a given particle density . You can convert rpm to G (gravity) using the dimensions of your centrifuge. You also need know the distance that your particles need to travel through the solvent.

Stokes‟ law, the settlement velocity, Ws (m s ^ -1), can be applied to establish the time taken for the NPs to settle to the bottom, working on the assumption that particles are spherical.

Ws = (( Ps - Pw ) * gd^2 ) / 18μ

Where Ps is the density of nanoparticle (e.g. TiO2 = 4 kg m ^-3), Pw is the density of water (0.999 kg m^-3); μ is the dynamic viscosity of water (1.3 10-3 kg m^-1 s^-1 at 10 o C), g is gravity (9.8 m s-2), d = diameter of the particles in metres.

Dear Samha

It depends on kind of nano-particles and their surroundings as solvent.

Although the RPM is commonly used, it is very inaccurate. Important is the resultant force acting on the particle. So on different machines (with different radius) with the same RPM values you will obtain different results. Quite clearly it is explained e.g. in the following article. It is therefore important to know the centrifuge radius (distance of sample from the center of the rotor).

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711687/

You can look at this article, might help you.

http://onlinelibrary.wiley.com/doi/10.1002/9780470027318.a1502/abstract

Try looking up Stoke's law which can be applied to centrifugation. It describes the time taken for particles to settle in a solvent for a given particle density . You can convert rpm to G (gravity) using the dimensions of your centrifuge. You also need know the distance that your particles need to travel through the solvent.

Stokes‟ law, the settlement velocity, Ws (m s ^ -1), can be applied to establish the time taken for the NPs to settle to the bottom, working on the assumption that particles are spherical.

Ws = (( Ps - Pw ) * gd^2 ) / 18μ

Where Ps is the density of nanoparticle (e.g. TiO2 = 4 kg m ^-3), Pw is the density of water (0.999 kg m^-3); μ is the dynamic viscosity of water (1.3 10-3 kg m^-1 s^-1 at 10 o C), g is gravity (9.8 m s-2), d = diameter of the particles in metres.

v = d2 (p–L) 3 g/

18 n

v = sedimentation rate or velocity of the sphere

d = diameter of the sphere

p = particle density

L = medium density

n = viscosity of medium

g = gravitational force

Thank you all so much

A few practical examples of using Stoke's equation you can find in the attached document..

The correct term is force acting on the particle not rpm. RPM for different holder will be same but the force acting on the particles would be different.

And the separation force depends on the stability of colloid. More stable colloid need more force to get separated.

to use Stoke's law we need the density of the nanoparticles. Anyone does know how to obtain the anisotropic bimetallic nanoparticles' density?or is there another method for calculation the minimum centrifuge speed?

Hi all,

Forgive me if I ask something stupid. When I looked at the stoke's equation and I cannot understand one thing: It seems to me that when particles size get smaller, the required speed is also reduced, that sounds contradictory to what the real situation is.

Hi Zhuoran

The settling velocity is directly proportional to square of size of the NP. It means we the particles will take much more time to settle hence, to separate the supernatant from the precipitate we need to provide very high rpm (with higher time).

I want to centrifuge bimetallic NPs of size ~ 4 nm (in water). Can anyone suggest me the method to do it. I have tried to precipate it using ethanol first and then centrifuge. Is there any other method to do so?