THE PLASTICITY OF SOLIDS

221

zero friction, Fig. 79. Evidently these two concentrations are
identical, and the concentration of zero fluidity or of zero friction
is a fundamental constant of the material giving important
information in regard to its nature, it being intimately related to
the size of the particles and to the adhesion between them.
The flow of a given material is defined completely by a knowl-
edge of the friction and mobility, but when the concentration of
the suspension is changed, a knowledge of the concentration of
zero fluidity is necessary in order to estimate the effect produced

25 50 75

VOLUME: PERCENTAGE CLAY

100

FIG. 80.—[Relation between mobility and volume concentration of solid in clay

suspensions.

upon the friction and mobility. It therefore seems probable that
the concentration of zero fluidity is a variable which is inde-
pendent of both the friction and the mobility.
Finally, we may add that the mobility of suspensions de-
creases very rapidly with increasing concentration of solid
as indicated by measurements of the author which are plotted in
Pig. 80. Clay suspensions were used having a concentration of
zero fluidity of 19 per cent by volume. The mobility starts at a
very large but undetermined value and quickly falls to a very
small value in a concentration of about 50 per cent by volume.
The friction on the other hand, starts at zero in the 19 per
cent mixture and rises steadily and in an apparently linear man-
ner as the concentration is increased as seen in Fig. 79.