Centrifugation: Basic Principle and Types Used in Experimentation

A basic guide to understand the nature of centrifugation along with its types.

Centrifugation is a method for separation of mixtures by the application of centrifugal force. The force is generated inside stationary equipment by introducing a high-velocity fluid stream tangentially into a cylindrical-conical chamber, forming a vortex of considerable intensity. The high velocity required at the inlet of a liquid cyclone is obtained with standard pumps.  With increasing the effective gravitational force on a test tube causes the centrifugation to move more rapidly and completely cause the precipitate or pellet to gather on the bottom of the tube.

 The remaining solution is properly called the supernatant. The supernatant liquid is withdrawn with a Pasteur pipette. The particles settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity. The rate of centrifugation is specified by the acceleration applied to the sample, typically measured in revolutions per minute (RPM) or g.

There are two major types of centrifuges which are sedimenters and filters. A sedimenting centrifuge contains a solid-wall cylinder rotating about a horizontal or vertical axis. An annular layer of liquid of fixed thickness is held against the wall by centrifugal force because this force is so large compared with that of gravity. Hence, the liquid surface is essentially parallel with the axis of rotation regardless of the orientation of the unit. Heavy phases “sink” outwardly from the center, and less dense phases “rise” inwardly. The heavy solid particles collect on the wall and must be continuously removed. On the other hands, a filtering centrifuge operates on the same principle as the spinner in a household washing machine.

 The basket wall is perforated and lined with a filter medium such as a fine screen. Liquid passes through the wall are impelled by centrifugal force, leaving behind solids on the filter medium. The filtration rate increases with the centrifugal force and with the permeability of the solids. Some compressible solids do not filter well in a centrifuge because the particles deform under centrifugal force and the permeability of the solids is greatly reduced. The amount of liquid adhering to the solids after they have been spun also depends on the centrifugal force applied.

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