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.
Sedimentation rate
Consider the sedimentation of an'uncharged particle of mass m and
specific volume v in a liquid of density p. The driving (or sedimenting)
force on the particle, which is independent of particle shape or
solvation, is m (I — vp)g, where g is the local acceleration due to
gravity (or a centrifugal field). The factor (1 - vp) allows for the
buoyancy of the liquid. The liquid medium offers a resistance to the
motion of the particle which increases with increasing velocity.
Provided that the velocity is not too great, which is always the case
for colloidal (and somewhat larger) particles, the resistance of the
liquid is, to a first approximation, proportional to the velocity of the
sedimenting particle. In a very short time, a terminal velocity, dx/df,