292

FLUIDITY AND PLASTICITY

dent upon the amount of the shearing force, the values of
viscosity as such are quite illusory. For example, a 1.39 per
cent solution of nitrocellulose in acetone gave a fluidity of
52.49 using a pressure of 403.6 g per cm2 whereas the fluidity
apparently fell to 51.31 at 214.5 g per cm2 and to 50.76 at 62.96
g per cm2. These results are similar to those of Glaser (cf.
Table XVIII).

It is quite possible to measure the plasticity of materials of
this kind in the viscometer shown in Fig. 29. It is merely
necessary to measure the flow at two or more pressures and then

50

40

c: 30
o

20

345
Concentration

FIG. 90. — Friction-weight-concentration curve for colloidal dispersions
nitrocellulose in acetone measured in dynes per square centimeter.

of

compare the volume of flow with the shearing stress. The
volume of flow-shearing stress curves obtained in this way by
Hyden are linear in every case. The mobility is found to in-
crease with the temperature in a nearly linear manner and the
mobility falls off very rapidly with increasing concentration of
colloid, approaching the zero value asymptotically. Both of these
results are similar to those for clay suspensions (cf. pages 220
and 221). The equations of these curves have not yet been
obtained.
The friction in nitrocellulose solutions increases rapidly with
increasing concentration of colloid, as shown in Fig. 90. As the
temperature is raised the friction is decreased in a linear manner,