Page 65 - Instrumentation Reference Book 3E

50 Measurement of viscosity

2.5 Measurement of the
extensiona I viscosity
Many industrial processes, especially in the poly-
mer industries, involve a high extension-flow
component and there is an acknowledged need
for extensional viscometers. The construction of
Diol such devices is, however, fraught with difficulties.
-_-_-_ --Fluid level- - - - For example, it is difficult to generate an exten-
r rate range. Indeed, many of the most popular and
sional deformation over a sufficient deformation-
sophisticated devices for work on polymer melts
Spindle
(such as those constructed at B.A.S.F. in Ger-
many) cannot reach the steady state required to
in (2.4). Therefore, they are, as yet, of
,AI determine the extensional viscosity qE defined
unproven utility. A full discussion of the subject
Disc
Figure 2.8 Schematic diagramof the Brookfield of extensional viscometry within the context of
viscometer. polymer-melt rheology is provided by Dealy
(1982).
was probably one of the first forms of viscometer In the case of more mobile liquid systems, it is
ever used and today there are more than 50 difficult to generate flows which approximate to
versions of the so-called flow cups. steady uniaxial extension and the most that can
Often, results from flow cups are simply reasonably be hoped for is that instruments will
expressed as “time in seconds” (e.g., “Redwood provide an estimate of a fluid’s resistance to
seconds”) but for Newtonian liquids these can be stretching flows (see, for example, Chapter 1 of
converted to kinematic viscosity v through an Walters (1980)). With this important proviso, the
(approximate) formula of the form Ferguson Spin-Line Rheometer is a commer-
v = At - B/t (2.17) cially available instrument which can be used on
mobile liquids to provide extensional viscosity
where A and B are constants which depend on the information.
cup geometry (see, for example, Walters and
Barnes (1980)).
The second term on the right-hand side of
(2.17) is essentially a kinetic-energy correction. 2.6 Measurement of viscosity
For Newtonian liquids, A and B can be deter- under extremes of temperature
mined by carrying out standard experiments on and pressure
liquids of known kinematic viscosity.
A major disadvantage of the standard Ford cup Any of the techniques discussed above can
so far as non-Newtonian liquids are concerned is be adapted to study the effect of temperature
that only one time can be taken, i.e., the time taken and pressure on viscosity, provided the apparatus
for the cup to empty. Such a measurement leads to can accommodate the extremes prevailing.
a single (averaged) viscosity for a complicated It is important to emphasize that viscosity
deformation regime and this is difficult to interpret is very sensitive to temperature. For example,
consistently for rheologically complex fluids. the viscosity of water changes by 3 percent per
Indeed, liquids with different rheologies as regards Kelvin. It is therefore essential to control the
shear viscosity, extensional viscosity, and elasticity temperature and to measure it accurately.
may behave in an identical fashion in a Ford- Pressure is also an important variable in some
cup experiment (Walters and Barnes, 1980) so studies. In the case of lubricating oils, for
that shop-floor instruments of this sort should be example, high pressures are experienced during use
used with extreme caution when dealing with non- and it is necessary to know the pressure-depend-
Newtonian liquids. The same applies to the ence of viscosity for these fluids.
“flow-around-obstacle” viscometers of Figure 2.7. At temperatures near absolute zero measurements
Typical examples of this type of viscometer are have been concerned with the viscosity of liquid
the Glen Creston falling-ball viscometer and the helium. Recently, special techniques have been
Hoeppler rolling-ball instrument (see, for example, developed. Webeler and Allen (1972) measured
Van Wazer etal. (1963) and Cheng (1979). Rising- the attenuation of torsional vibrations initiated
bubble techniques may also be included in this by a cylindrical quartz crystal. Vibrating-wire
category. viscometers have also been used. The resonant