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The products we stir, pump, spread,
pour, and spray are usually thought
of as liquids. Surprisingly, however,
it can be demonstrated that they live a
quiet double life as liquids when observed,
but retreat into a little-known soft-solid
character when left to their own devices.
Because this double life imparts many
desirable processing, storage, and performance
attributes to manufactured products,
it is important to understand it.
Viscosity measurement example
An interesting viscosity measurement
paradox helps to illustrate this point.
Consider two skin treatments: a foot
cream that applies easily and rubs out to
a silky finish, and a soothing emollient
for diaper rash and eczema, known for
its thick and heavy feel when applied
to the skin. The prescribed method for
performing a viscosity check is straightforward.
A rotational benchtop viscometer
is equipped with a T-bar spindle and
Helipath stand (Brookfield Engineering, Middleboro, MA) that is well suited
for soft solids, and a quick, single-speed
viscosity test is run to confirm the relative
viscosity of each cream (see Figure
1). The measurement results were unexpected:
thin foot cream: 13,740 poise
(or 1,374,000 cP), and thick emollient:
10,520 poise (or 1,052,000 cP). The viscometer was working well, and the
method is an established one for these
products. How can a high-viscosity emollient
deliver a lower measured viscosity
value than the foot cream?
On closer inspection of the undisturbed
samples in their tubs, a different picture
emerges: The low-viscosity foot cream
actually appears to have a degree of firmness
not seen in the creamier emollient.
In other words, the relative consistencies
depend on whether one is poking the
material or spreading it. This is an example
of the double life of structured liquids.
Products such as suspensions, emulsions,
and gels possess structure—internal continuous
networks due to colloidal interactions,
polymer entanglements, and associations—
that impart solid-like characteristics to the
product. An example of these solid-like
characteristics can be seen clearly in mayonnaise.
Consider a large spoonful of mayonnaise
on a plate. It does not appear to be
flowing at all. Furthermore, if the mound of
mayonnaise is pushed gently with a spoon,
it deforms slightly but bounces back to its
original shape when the spoon is removed.
This is due to the elastic deformation of
the mayonnaise on the application of stress
and its subsequent recovery on the removal
of the imposed stress. In other words, the
mayonnaise exhibits solid-like behavior.
However, this type of behavior is seen only
when there is gentle handling of the mayonnaise.
If pushed a little harder, the structure"breaks" and the mayonnaise flows,
demonstrating liquid-like behavior. If the
applied stress is reduced, the gel-like structure
recovers and solidity promptly returns.
A two-state solution
These two states—the deformable and elastic
soft solid under gentle stresses and the
free-flowing liquid that emerges at higher
stresses—are drastically different. Further,
these "liquids," as we tend to label them,
spend most of their low-stress, at-rest life
cocooned in solidity in a bottle or tub during
storage on a shelf. For only a few moments,
when they are pumped, spread, stirred, or sprayed, they experience life as a liquid,
hence the double life of structured liquids.
The soft-solid structure of these products
should not be overlooked or ignored.
Structure contributes to many critical performance
attributes, such as:
- Texture, both of foods in the mouth and topical products on the skin
- Sag and slump resistance
- Penetration
- Dripping and draining characteristics.
Thus, a measurement of soft-solid structural properties,
such as rigidity and strength, is very valuable. Herein lies
the problem: A measurement method for liquids (i.e.,
viscosity) is not suitable for measuring solids. Similarly,
a method for measuring solids (i.e., rigidity, strength, or
firmness) is meaningless when applied to a liquid. The
best way to deal with this is simply to perform both structure and flow tests to obtain a more meaningful, comprehensive
characterization of the product.
Flow test methods
Figure 2 - a) Test method known as shear rate
ramp. b) Viscosity profile for shear thinning
material (pseudoplastic behavior). c) Viscosity
profile for shear thickening material (dilatant
behavior).
Most companies perform a QC check for viscosity
with a single-point measurement. This means using
one spindle at a defined speed and reporting a single number, as explained above. Formulators
will run a flow curve test for viscosity
during R&D to create the product. This
involves testing the material at different
rotational speeds to observe how viscosity
will change as a function of shear rate. Figure
2 shows graphs of the test method and
possible outcomes. Sometimes, QC will be
required to run more than a single-point
test to ensure optimal quality of product.
Choice of spindle for the viscosity test is
typically made by R&D. Disk-type spindles
are most commonly used, provided there
is plenty of sample available for testing (at
least 0.5 L). When sample availability is
limited (<20 mL), coaxial cylinder geometry
(cylindrical spindle inside of a cylindrical
chamber) is an option. For even
smaller sample volumes (<2 mL), cone/
plate geometry is the alternative. Figure 3
shows images of each spindle type. There
are many other spindle geometries, such
as the T-bar mentioned earlier, that are
appropriate for certain types of materials.
The two sample materials described above
were also tested with the cone/plate geometry.
The shear rate selected was 250 sec–1,
which correlates with applying the creams to
the skin with a steady rubbing action. Test
results were as follows: thin foot cream: 543
cP, and thick emollient: 3460 cP.
Figure 3 - a) Disk-type spindle. b) Coaxial
cylinder spindle geometry. c) Cone/plate
spindle geometry. d) Vane spindle.
The cone/plate method provides the means
to simulate the higher shear rate of rubbing,
and the test data confirm that, as expected,
the thin foot cream exhibits much lower
viscosity than the thick emollient.
Structure test methods
For years, many viscometer users have, in
fact, unknowingly performed structure tests
to very good effect. The T-bar viscosity is
actually a measure of structural strength. The
key parameter is the torque required for the
T-bar spindle, driven by the Helipath stand,
to cut a path through a soft-solid sample. As
such, it is more related to yield stress-the
stress that must be applied to disrupt structure
and initiate significant flow-than a
true viscosity under shear. What other established
approaches can be taken to comprehend
the structure of a material?
Material strength
Two common methods are available to
measure this value:
- Use the flow curve data from a standard viscosity test, plot shear stress
versus shear rate, and extrapolate back
to zero shear rate (see Figure 4a). This
technique gives a dynamic yield stress
because it infers the result after the fluid
has already been allowed to flow.
- Use an instrument that measures static
yield stress directly. This means that
the material must be measured in an
at-rest condition. Controlled stress rheometers
have this capability (see Figure
4b). The technique is to apply a gradually
increasing torque to the spindle and
detect the moment the spindle begins
to rotate.
Figure 4 - a) Dynamic method for determining the yield stress of a material. b) Static method for determining the yield stress of a material.
Rigidity/firmness
Figure 5 - Graph of material tested for rigidity or firmness.
When applying an increasing force to
a material in order to initiate flow, the
elastic resistance to being deformed
is the characteristic of interest. Consider
a jar full of jelly. Pushing the jelly
down gently onto the surface with a
finger causes the jelly to deform, but
it returns to its original shape when
the finger is removed. When pushed
harder the resistance feels greater, but
the jelly still returns to its original
condition after the finger is removed.
The amount of resistance detected is
directly proportional to the speed with
which the finger is pressed into the
jelly. Two methods are used to obtain
this measurement:
- Using a standard viscometer, a vane spindle (see Figure 3d) is immersed into
the material, the spindle is rotated at a
very slow speed (0.1-1.0 rpm), and shear
stress or torque versus angle of rotation
is plotted. The slope of the line is a measurement
of rigidity (also thought of as
firmness or stiffness) (see Figure 5).
- Another type of instrument, known
as a texture analyzer (see Figure 6),
pushes a probe down into the material
at a defined speed and measures the
force resistance during penetration.
The force is plotted as a function of
the distance that the probe penetrates
into the material. The slope of the line
is again a measurement of rigidity.
Conclusion
Fortunately, these tests are easy to perform
and the instrumentation is a modest
investment. In fact, most companies
already have a rotational viscometer with
variable speed capability. Now it is simply
a matter of becoming familiar with the
different spindles that can be used with
this instrument and practicing the tests
described above. The ultimate goal, of
course, is to generate product that has
even better quality (consistent flow properties
and structure) than what has been
manufactured to date.
Mr. Cunningham is a Consultant, Rheology
School, Hampshire, U.K.; tel.: +44 (0) 1730
829858; e-mail: [email protected]; www.
rheologyschool.com. Mr. McGregor is Marketing
Manager, Brookfield Engineering Laboratories,
11 Commerce Blvd., Middleboro, MA 02346,
U.S.A.; tel.: 508-946-6200; fax: 508-946-6262;
e-mail: [email protected].
AMERICAN
Please check out our Rotational Viscometer section for more information or to find manufacturers that sell these products.