# How a centrifuge separates particles by density

1. Feb 3, 2010

### GRB 080319B

I am confused on how a centrifuge separates particles by density. In http://en.wikipedia.org/wiki/Centrifuge" [Broken], which appears to have something to due with an applied force or centrifugal force. It would seem like the more massive particles would remain closer to the axis of rotation due to their greater mass resisting centrifugal acceleration. Does any of the seperation occur during the angular acceleration of the device up to operating angular speed, or does it all occur while spinning at constant speed?

Any help will be greatly appreciated.

Last edited by a moderator: May 4, 2017
2. Feb 3, 2010

### turin

Re: Centrifuge

Since the particles are denser than the fluid, they have a natural tendency to settle to the bottom, but they are prevented from this due to a low terminal velocity and turbidity of the suspension. The spinning motion creates extra "artificial gravity" and so increases the terminal velocity of the particles, allowing them to overcome the turbidity.

3. Feb 3, 2010

### GRB 080319B

Re: Centrifuge

Does this still apply to a horizontally spinning centrifuge? Is the "weight" of the denser particles greater in the horizontal direction as the centrifuge spins, and this overcomes the pressure of the liquid (due to the liquids "weight" in this direction), so the denser particles sink sideways? Is the "weight" the centrifugal force, and if so what applied force is the "weight" a reactive force to?

4. Feb 3, 2010

### rcgldr

Re: Centrifuge

As the centrifuge spins, the "bottom" of the vial applies a centripetal force to the contents within the vial. The contents of within the vial respond with a reactive centrifugal force. Since this force is related to the mass of the particles, the denser particles will tend to flow towards and accumulate at the "bottom" of the vial during this process.

5. Feb 3, 2010

### Staff: Mentor

Re: Centrifuge

A centrifuge increases g-forces, amplyfing the effect of gravity, that's all. So if you're not sure what will happen in a centrifuge, just consider what will happen to the contents of the container if they are sitting still on a table. Ie, shake up some Italian dressing (oil and vinegar) and set it on a table. What happens? The oil and vinegar slowly separate. Put it in a centrifuge? They separate faster.