Colloids and Fine Particles

The ratio of the surface area to the volume of a spherical particle can be calculated from the diameter of the sphere as follows (where x is the particle diameter)  

The mass of a particle is directly related to its volume by its density. As particle size decreases the influence of surface forces (described in Section 5.3) dominate the behaviour of the powders and suspensions relative to body forces which depend on the particles mass. Body forces are easy to understand because they are simply a result of Newton s laws of motion, where the force depends on the mass and acceleration via F = ma. The quintessential example of a body force is that of a particle settling under the influence of gravity (F = mg) as described in Chapter 2. 

Introduction to Paiticle Technology, 2nd Edition Martin Rhodes 2008 John Wiley Sons Ltd. ISBN 978-0-470 -01427-1 

Surface forces may result in either attraction or repulsion between two particles depending on the material of which the particles are composed, the fluid type and the distance between the particles. Generally, if nothing is done to control the interaction between particles, they will be attracted to each other due to van der Waals forces which are always present. (The few rare cases where the van der Waals forces are repulsive are described in Section 5.3.1.) The dominance of attraction is the reason why fine powders in air are usually cohesive. 

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W.R. Bowen and F. Jenner, Theoretical descriptions of membrane filtration of colloids and fine particles an assessment and review, Adv. Colloid Interface Sci. 56 (1995) 141-200. 

Separation operations are frequently classified according to the size of particles. No precise size definitions exist for terms such as ultra-, micro-, nano-, colloidal, and fine particle filtration. Different authors use different nomenclature in classifying separations in terms of size. A simplified classification is shown in Table 22.1. 

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