CHAPTER X
SUPERFICIAL FLUIDITY
The viscosity of a liquid may change, and it may change
in a quite extraordinary manner, as the boundary of the liquid
is approached. This must of necessity result wherever the
surface tension is such as to bring about a change in concentra-
tion at the boundary. We should therefore naturally expect
soap and saponin solutions to show this phenomenon. Experi-
mentally this field of study has not been much explored although,
as we shall attempt to show, the promise of reward is very great
and the need of such study in industry is pressing. However,
Stables and Wilson (1883) have proved that a saponin solution
has a viscosity at the surface which is 4,951 as compared with
3.927 for the surface of pure water. The viscosity was measured
by the oscillations of a circular nickel-plated brass disk, of 7.625
cm diameter and 0.2 cm thickness, which was suspended in the
liquid by means of a wire 119.8 cm long. As soon as the solution,
was allowed to rise 0.15 crn above the disk the viscosity fell to Its
normal value.
The viscosity found by Stables and Wilson indicates that
the surface layer of a supposedly dilute solution may nevertheless
have a viscosity which is over a thousand-fold that of water
at 20°C (1,260 cp) -or about the viscosity of castor oil. But for
very small stresses, the viscosity may be still higher, for it
is to be particularly noted that in a saponin solution a pendulum
does not oscillate isochronously. Thus in one experiment
with vibrations of large amplitude, Stables and Wilson found the
time of vibration to be 10.52 seconds, whereas with small ampli-
tudes the time of vibration was 9.73 sec. This would indicate
that with very small stresses the viscosity might be found to be
infinite, which would mean that we are here again dealing with.
plastic flow.
The experiments of Stables and Wilson need confirmation and
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