AMPLIFICATION OF THE LAW OF POISEUILLE 57 ^
tfj,
of error such as surface tension, slipping at the boundary, f
necking in of the lines of flow at the entrance of the capillary, |
eddy currents inside of the capillary, resistance to flow outside f
of the capillary, peculiar shapes to the ends of the capillary f|
affecting the magnitude of the kinetic energy correction have f
all been considered in detail. They may all be eliminated |
by using long, narrow capillaries with a low velocity of flow. |
The fluidity of compressible fluids may be obtained by the p
same Law of Poiseuille but the volume of flow is approximately 1
the mean of the volume at the entrance and at the exit of the f
capillary Fl + V'2. I
Plastic solids in their flow do not obey the Law of Poiseuille f
and their study is deferred until Chapter VIII. Many attempts 14
have been made to measure the viscosity of soft solids. The |
fluidity of such a substance is not a constant quantity but f
falls off rapidly although regularly as the radius of the capillary "
falls below a certain point. This is not stoppage of the capillary
of the ordinary sort due to extraneous particles, but rather a new
type of flow. The terms fluidity and viscosity will therefore be
avoided when referring to plastic substances in order to avoid
confusion and a sharp criterion given by which a soft solid may be |f
distinguished from a true fluid, just as Reynolds' criterion ff
enables one to distinguish between viscous and turbulent flow. 4J