THE PLASTICITY OF SOLIDS

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The result of defloeculation is to greatly increase the mobility.
Thus one-tenth of 1 per cent of potassium carbonate raised the
mobility from 1.17 to 5.11 which is an increase of over 330 per
cent, a truly remarkable effect.
SEEPAGE AND SLIPPAGE
"When the shearing force is just a little less than the friction,
there is generally a certain amount of flow which is due to two
different causes. In the first place, under ordinary conditions
of flow the pressure tends to cause the medium to seep through
the material. With this filtration phenomenon there is a local
change in concentration and therefore a change in the char-
acter of the flow. Seepage is unimportant when the medium is
viscous and the suspended particles are small as in paint.
The second difficulty is due to slippage, which comes from lack
of sufficient adhesion between the material and the shearing
surface. The shearing surface is wet with the liquid medium and
the smooth surface affords little opportunity for the attachment
or interlocking of the particles. The result is that there is a
layer of liquid between the .shearing surface and the main body of
the suspension and flow takes place in this layer according to
the laws of viscous rather than plastic flow. Green (1920)
has observed this phenomenon in paint under the microscope,
the material moving as a solid rod until the shear reaches a
certain value when it begins to move in telescoping layers.
This slippage causes the rate of flow-shear curve to be no longer
linear when the rate of flow is small and the curve passes through
the origin.
Difficulties due to seepage and slippage can be overcome
by using sufficiently high pressures, so that the viscous flow
factor will become negligible. In this case there should be a
linear relation between shear and rate of flow.
HYDRAULIC FLOW AND THE PLASTIC STATE
So far as known to the author, no one has yet used rates
of flow high enough to bring about eddy currents, which are so
troublesome in the case of liquids. But there is the same
necessity for using long narrow tubes for measuring the flow,