Chemical Engineering November 2010 - 28

Viscosity
Measurement
Department Editor: Scott Jenkins
M
easurement of fluids' viscosity in
the chemical process industries
(CPI) can represent a useful
" product dimension " for manufacturers.
Understanding a material's flow characteristics
is valuable in predicting several
parameters relevant to many CPI processes,
including pumpability, pourability,
performance in a dipping or coating
operation, ease of handling or processing.
The relationship between rheology
(study of the flow of matter) and other
properties often makes viscosity measurement
a sensitive and convenient method
for detecting changes in other product
parameters, such as density, stability,
solids content or molecular weight.
Viscosity Framework
Viscosity is a measure of a fluid's internal
friction. Caused by intermolecular attraction,
viscosity can be thought of as
resistance to flow. This friction becomes
apparent when a layer of fluid is made to
move in relation to another layer. More
friction requires more force to effect this
movement, called shear. Shearing occurs
when fluids undergo physical movement
or distribution, such as in pouring,
spreading, spraying and mixing.
Flow of layers
Isaac Newton defined viscosity by considering
the model represented in Figure 1.
Two parallel planes of fluid of equal area
A are separated by a distance dx and are
moving in the same direction at different
velocities V1 and V2.
dv
V2
A
F
V1
A
dx
Newton assumed that the force required
to maintain this difference in speed
was proportional to the difference in
speed through the liquid, or the velocity
gradient. The velocity gradient, dv/dx,
represents the change in speed at which
the layers move with respect to each
other. It describes the shearing the liquid
experiences, and is thus called shear rate
[1]. Its units are reciprocal seconds (s-1).
The term F/A indicates the force per
unit area required to produce the shearing
action (dynes/cm2).
Viscosity is the ratio of shear stress to
shear rate.
Fluid behavior
Many fluids, like water or
gasoline, exhibit Newtonian
behavior, which is to say that their
viscosity remains constant with
varying shear rates. Viscosity of a
Newtonian fluid depends only on
temperature and pressure, but not
on the forces acting on the material.
For Newtonian fluids, plotting
shear stress versus shear rate (a
rheogram) yields a straight line
that passes through the origin. The
slope is equal to the viscosity.
However, many materials in
the CPI behave in non-Newtonian
ways, so that the rate of shear is
not linearly proportional to the
corresponding stress. For many
applications in the CPI, expressions
of viscosity as a single value
fail to capture the full picture
of the many factors that affect
viscosity. Non-Newtonian viscosity
Table 1. ViscosiTies of coMMon
MaTerials
Material name Temperature
(°C)
Ethanol
Water
Water
Water
Sulfuric acid
Blood
20
20
100
25
Motor oil (SAE 30) 20
37
Corn syrup
Milk
Acetone
Olive oil
Honey
Glycerin
behavior include the following:
* Pseudoplasticity (shear-thinning)
occurs when a fluid's viscosity
decreases with increasing shear
rate. Many emulsions, polymer melts
and solutions, paints, blood and some
solid suspensions exhibit this property
Peanut butter
Air
Glass
25
20
Ethylene glycol 25
20
20
20
20
20
18
20
Viscosity
(cP)
1.1
1.79
1.0
0.28
24.2
~450
3-4
1,380
~3
16.1
0.30
81
10
1,420
~250,000
0.019
1018 - 1021
* Thixotropy is a situation where a fluid's
viscosity decreases over time under
constant shear stress. Clay suspensions
used as drilling muds, mayonnaise and
some paints and inks behave this way
* Rheopectic behavior is the less-common
opposite of thixotropy - shear stress
increases at constant shear rate. A gypsum
suspension in water is an example
* Dilatant (shear-thickening) fluids show
increasing viscosity with increasing
shear rate. Dilatant behavior is observed
in starch suspensions in water,
quicksand and in some high molecularweight
polymers used in drilling muds
Viscosity measurement
When testing materials that flow, it is
important to think about how the material
will be processed and handled when in
use. Analytical procedures for simulating
the shearing action with an instrument is
the key to predicting flow behavior.
Rotational viscometers are a common
tool, wherein a spindle with a defined
geometry is inserted into the fluid to be
measured. The spindle rotates at various
fixed speeds, shearing the material at constant
shear rates. The viscometer measures
the torque resistance experienced by the
spindle at different rotational speeds.
As temperatures increase, most materials
exhibit a decrease in viscosity. To ensure
the consistency of results, it is critical
that the temperature be closely defined
when making viscosity measurements.
Rheometers are related, but relatively
more complex, instruments that function
across a very wide range of shear rates,
enabling the simulation of real processes
that occur over vastly different timescales,
such as in sedimentation and spraying.
Yield stress
For many CPI applications, yield stress
is an important parameter to measure.
Yield stress is the force required to cause
a material to begin flowing. For example,
yield stress represents the force that must be
overcome when a pump is switched on. The
startup torque required for a pump must be
calculated to ensure proper sizing.
Controlled stress rheometers are the tools
of choice in measuring yield stress. The
method is to run a " shear ramp, " where
increasing torque is applied to the spindle
until rotation of the instrument is observed.
Such a test can yield a numerical value
that can be used by process engineers to
determine yield stress of the material. This
information can, in turn, be used in pump
sizing calculations for startup torque and
full flow conditions.
References
1. Brookfield engineering educational website,
www.brookfieldengineering.com/education,
2010.
2. McGregor, Robert G., Viscosity: The basics,
Chem. Eng., August 2009, pp. 34-39.
3. " Perry's Chemical Engineering Handbook, " 7th
ed. McGraw Hill, 1997.
http://www.brookfieldengineering.com/education
Chemical Engineering November 2010

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