Gravitational field, particle migration

For example, at n = 1200 rpm = 20 liter/sec and r = 0.5 m, the settling velocity in the centrifuge is almost 28 times greater than that in free settling. Note that the above expressions are applicable only for Re > 500. For small particles. Re < 2, migration toward the wall is laminar. The proper settling velocity expression for the gravitational field is... 

It can be shown that for fine particles (dp < 10 (tm) in an electrostatic precipitator the migration velocity of the particle is usually much higher than the corresponding particle terminal velocity in the gravitational field. For this reason, fine particles are commonly separated by electrostatic precipitation. 

When dealing with the sedimentation of colloidal particles, it is principally necessary to regard the Brownian motion of the particles, which results in diffusive particle transport and, thus, acts against the migration in the gravitational or centrifugal field. The relevance of the Brownian motion can be roughly estimated by means of a Peclet-number ... 

Field flow fractionation (FFF) can also be used for microbial cell separation. In the FFF technique, a field (may be gravitational, centrifugal, thermal-gradient, electrical, magnetic, etc.) is applied perpendicular to the fluid flow, causing particles to migrate with different velocities. Fields of sedimentation, diffusion, and electrical diffusion are manipulated to optimize the separations of microbes. Separation of Pseudomonas putida and E. coli has been achieved by hyperlayer FFF. Fractions of the whole cells were collected after the separation at different time intervals, dif-... 

The reverse effect of electrophoresis arises when small particles which are suspen ded m a liquid ate forced to move, say by gravitation or by an ultrasonic or a centrifugal Reid. Then an electric field is generated in the direction of the movement and a potential difference can be measured between two electrodes in the liquid, one at the top, the other at the bottom of the vessel This potential difference is called migration potential or potential of failing particles and the effect is indicated as the DoRN-effect The experiments are difficult and as yet only few results have been obtained... 

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