How a centrifuge separates particles by density

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Discussion Overview

The discussion centers around the mechanism by which a centrifuge separates particles based on their density. Participants explore the role of centrifugal force, the behavior of particles in a fluid during spinning, and the effects of angular acceleration versus constant speed operation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about whether more massive particles remain closer to the axis of rotation due to greater mass resisting centrifugal acceleration.
  • Another participant notes that denser particles have a tendency to settle but are hindered by low terminal velocity and turbidity, suggesting that spinning creates artificial gravity to help particles overcome these factors.
  • A question is raised about the applicability of these principles to horizontally spinning centrifuges, specifically whether the centrifugal force acts as a weight in the horizontal direction and how it interacts with the liquid's pressure.
  • One participant describes the centripetal force applied by the vial's bottom and how denser particles accumulate there due to their mass during the spinning process.
  • Another participant simplifies the concept by comparing centrifuge operation to the natural separation of oil and vinegar when left still, suggesting that centrifugation accelerates this process.

Areas of Agreement / Disagreement

Participants present multiple competing views and questions regarding the mechanics of centrifugation, indicating that the discussion remains unresolved with no consensus reached on certain aspects.

Contextual Notes

Some assumptions about the behavior of particles in different orientations of centrifuges and the definitions of forces involved remain unaddressed, leading to potential gaps in understanding the full mechanics of the process.

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I am confused on how a centrifuge separates particles by density. In http://en.wikipedia.org/wiki/Centrifuge" , 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 separation 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.
 
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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.
 


turin said:
Since the particles are denser than the fluid, they have a natural tendency to settle to the bottom...The spinning motion creates extra "artificial gravity" and so increases the terminal velocity of the particles, allowing them to overcome the turbidity.

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?

Thank you for you reply.
 


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.
 


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.
 

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