Centrifuge Principle
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To separate particles suspended in a liquid from the liquid itself, one of the simplest methods is to let the mixture sit for a while and allow gravity to do the work. Heavier particles will settle at the bottom, while lighter ones will float to the top. For example, if we denote the volume of the particles as v, their density as Ï, the density of the liquid as σ, and the acceleration due to gravity as g, the force F acting on the particles is:
F = gravity - buoyancy = Ïvg - σvg
When Ï > σ, the force F is downward, causing the particles to sink. Conversely, when Ï < σ, the force becomes upward, making the particles float.
However, when the particles are very small or their density is close to that of the liquid, the separation process can take a long time. In such cases, it becomes impractical to rely solely on gravity. To speed up the separation, centrifuges are commonly used in laboratories.
1-2-1 Centrifugation Principle
As shown in Figure 1-2-1, when particles in a solution move in a circular path around an axis, they experience centripetal acceleration. This motion creates a centripetal force, which is balanced by an apparent centrifugal force in a non-inertial frame of reference. Although this centrifugal force doesn't exist in the inertial system, it behaves similarly to a gravitational field within the rotating system.
In the gravitational field, the centripetal force acting on a particle is given by:
F = ma = Ïvω²R (1-1)
Where:
- Ï â€“ the density of the particles
- v – the volume of the particles
- R – the radius of the circular motion
- ω – the angular velocity of the rotation
The centrifugal force acting on the particles is equal in magnitude but opposite in direction to the centripetal force. Therefore, its value can also be calculated using equation (1-1).
From this formula, it's clear that particles located at different distances from the axis (different values of R) experience varying levels of centrifugal force and resistance. As a result, they move away from the axis at different rates. The centrifugal force essentially acts like artificial gravity, with its strength depending on the mass of the particles, the radius of rotation, and the angular velocity. This principle allows different materials to be separated based on their density and mass, forming layers according to their properties. This is the fundamental concept behind centrifugation, and centrifuges operate based on this very idea.
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