COLLOIDAL SOLUTIONS 199
if an undercooled solution crystallized out needle-shaped crystals
throughout the solution so that flow of the resulting mass was
stopped except by breaking the crystalline structure. Such a
structure is a matter of slow growth, it may be partially destroyed
by purely mechanical means, and it arises from forces which are
of a polar nature. In view of this analogy we may speak of this
type of structure as polar, whereas the second type is non-polar.
In the second type of colloidal solution, typified by clay
suspensions those forces are absent which bring about the
setting of the gel. We have in the typical case merely particles
of suspended solid which affect to some extent the fluidity of
the solution, but as we shall see the amount of lowering of the
fluidity is very much less than when the structure is polar in
character. If the distribution of the particles is uniform, the
fluidity of the solution will be independent of time, agitation,
and previous treatment.
Suspensions.—For the simplest conceivable case of a solid
suspended in a liquid, we can imagine lamellae of solid parallel
to the direction of shear as discussed on page 104. If the
alternating lamellae are sufficiently numerous, the flow will
take place without separation of the components, even though the
fluidity of the one component is zero. The fluidity of the
suspension <t> is
« = cupi (69)
where a is the volume percentage of the medium whose fluidity
is <pi that is, the fluidity of the medium in the limiting case will
be decreased in exactly the ratio which the volume of the solid
bears to the total volume of the suspension.
If the lamellae have an irregular surface, this law becomes
invalid. If, for example, the lamellae are pierced by a number of
fine pores, the fluid will fill these pores, yet the stream lines will
not pass through the pores or be appreciably distorted by their
presence. The fluidity is then
4> = (a - d)vi (70)
where d is the fraction of the total volume which is pore-space.
The ordinary suspension consists of discrete particles, and
for the simplest case we may consider a sphere suspended in a
fluid of its own specific gravity. The shearing of the fluid,