COLLOIDAL SOLUTION'S 211
smaller. ZBtiglia (1908) has found that the fluidity of milk is
lessened wfron the milk is "homogenized" by being squirted
against an ^Sate plate, thereby increasing the number of fat
globules. The apparent decrease in fluidity with errmlsification
finds excelled practical examples in the manufacture of solid
lubricants and of certain household products such as mayonnaise,
<e whipped cream/7 and beaten egg albumen. In engine grease
less than 1 "PBT cen"^ °f water emulsified by means of a solution of
soap with, mineral oil produces a salve-like grease. Such bodies
h.a,ve the properties of solids and may also be considered in
connection- with the plasticity of solids.
The question inevitably arises, " How is it possible that water
with a high- fluidity can decrease the fluidity of a heavy oil,
or air decrease the fluidity of albumen, so that the resulting
product, einxtilsion or foam as the case may be, has the rigidity
of a solid ? To answer this question it is necessary to return to
the consideration of our simple case of lamellse of different liquids
at; right angles to the direction of shear. The theory that
viscosities of emulsions are additive, will account for the fluidity
being less t>han the fluidity of the homogeneous mixture but it will
in no way account for the case we have here where the fluidity
of the emulsion is less than the fluidity of either component.
As the shear progresses, it is to be noted, Fig. 34, that the
la,mellse a,re greatly elongated. But in immiscible liquids
this thinning out of the layers is opposed by the surface tension
which tends "bo keep the surface area a minimum. If therefore
the shearing force is less than the maximum force arising from
the surface "tension, continuous deformation will not result.
There will be a certain amount of temporary deformation but
this too will disappear as soon as the shearing force is removed.
In other words, the substance shows not only rigidity but also
elasticity; if the shearing force is greater than "the elastic limit,"
continuous deformation wjQl take place, but since we are dealing
with immiscible liquids, the lamella will not be thinned out indefi-
nitely, bu/b torn into portions which will gather into drops under the
influence of surface tension. Thus in an emulsion, shear tends
to make tlie droplets continually smaller, and consequently to
raise the "viscosity. This corresponds to the "cold working"
of metals. This effect is opposed by the spontaneous coalescence