The motion of particles in liquid media

In this chapter the thermal motion of dissolved macromolecules and dispersed colloidal particles will be considered, as will their motion under the influence of gravitational and centrifugal fields. Thermal motion manifests itself on the microscopic scale in the form of Brownian motion, and on the macroscopic scale in the forms of diffusion and osmosis. Gravity (or a centrifugal field) provides the driving force in sedimentation. Among the techniques for determining molecular or particle size and shape are those which involve the measurement of these simple properties. 

The motion of colloidal particles in an electric field is treated separately in Chapter 7. 

Before these kinetic properties are discussed in any detail, some general comments on the laws governing the motion of particles through liquids are appropriate. 

For spherical particles the frictional coefficient is given by Stokes law 

Therefore, if p2 is the density of a spherical particle (in the dissolved or dispersed state (i.e. P2 = 1/v)), then 

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