|MadSci Network: Chemistry|
If you try to make a fluid mixture of sand and water, the sand quickly settles to the bottom of the vessel through the action of earth gravity alone. This is because sand consists of relatively coarse grains. COLLOIDS are suspensions of very much smaller particles in liquids. In order to make them settle, acceleration much stronger than gravity is needed. ULTRACENTRIFUGES make use of centrifugal force by spinning the sample at high speed (much faster than your washing machine when it spins at the end of the washing cycle). To allow for the high rotation speed, air is removed from the spinning chamber, so there is less resistance. Accelerations 100'000 times stronger than gravity can be achieved. The separation of the colloidal particles is according to their size and their specific weight relative to that of the liquid (look up Stokes law). There exist devices allowing the observation of the optical properties of the sample during spinning. Ultracentrifuges are subject to stringent safety regulations, since a lot of energy is stored in the spinning rotor.
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