Rheometers: not just for rheology any more

Rheometers: not just for rheology any more
Looking beyond rheology
Gavin Braithwaite
Cambridge Polymer Group,
56 Roland Street, Suite 310
Boston, MA 02129
Cambridge
Polymer Group, Inc.
Testing, Consultation, and Instrumentation for Polymeric Materials
7-17 Presentation (10/1/2010)
Rheology
•
Modern shear rheometers exceptionally robust tools
– Wide torque and strain range
• Many orders of magnitude
– Robust control systems with wide dynamic range
– Well understood flow-fields
– Industry accepted instrumentation and models
•
Other geometries
– Capillary
– Vane
– Extensional
•
Simplifying flows aids characterization
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Quantitative characterization
•
Simplifying flow-fields aids analysis
– Shear flows
•
•
•
•
•
•
Viscosity
Shear thinning/thickening
Elasticity
Temporal evolution
Relaxation times
Yield stress
– Pipe flow
• Extensional properties
• Melts
– Extensional rheometers
• Breakup times
• Relaxation times
– Reduce flow-fields and deformations to tractable situations
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“Psycho-rheology”
•
Where analytical rheology doesn’t provide the
complete answer
– Rheology invaluable as a method for reliably
analyzing and ranking materials
– Consumer perception arises from the overall
response of the material
• “psycho-rheology”
•
Real-world usage is rarely one deformation
– “performance” based tests often useful in linking
“real world” experience with fluid properties
– Tests that inherently use multiple relevant
deformations can sometimes provide better
insight in to consumer perception
– Almost certainly non-linear
– Less “transferable”
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Case studies
1. Food products
– Differentiating milk products
2. Consumer healthcare
– Tactile feel of personal care fluids
3. Cardiovascular applications
– Implantation of catheters
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Case 1: Food products
•
Perception of food “feel” is driven by all senses
– Partially sight and smell but substantially taste and “feel”
– Consumer test panels are costly and time consuming
•
•
•
•
•
•
Intensive training required
Subjective
Can have difficulty describing differences
Outcomes can be ambiguous
Need big sample group
Rheology provides tools
– rapid and cost effective screening
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Milk rheology
•
Bovine milk similar composition, with breed variations
– Fat
• Holstein/Friesian - 3.6wt%, Jersey - 5.2 wt%
– Proteins
• 3.4-3.9 wt%
– Lactose
• ~5 wt%
•
Proteins act to stabilize fat globules
– Strongly influence feel and behavior
– Agglomeration and separation important
– Other solids impact shear viscosity
•
•
Normally considered Newtonian
Motivation: Replacement of fats and sugars
– Desirable for health reasons
– Need to preserve consumer perception
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Experimental
•
AR-G2 cone-and-plate
– 60 mm 1º SS cone, 25 ºC
– Stepped shear 0.1-1000 s-1
•
Milk (fresh)
–
–
–
–
•
•
•
Whole (~4% fat)
Skimmed (2% fat)
Skimmed (1% fat)
Non-fat (0.5 wt% fat)
Starch solutions
Sugar solutions
Proprietary food additives
– Consumer testing indicates best alternative
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Microalgal flour
•
Microalgal biomass contains nutritionproviding materials
–
–
–
–
–
carotenoids
dietary fiber
tocotrienols and tocopherols
varying lipid compositions
low levels of saturated lipids
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Milk
Error bars SD for three runs
0.1
whole milk (G2)
2% fat (G2)
1% skimmed (G2)
Shear viscosity [Pa.s]
non-fat (G2)
Fat wt% down
0.01
0.001
0.1
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10
Shear rate [s-1]
10
100
1000
Cambridge Polymer Group
Impact of composition
•
•
Milk normally considered “newtonian”
Stabilized fat globules
– Deformable spheres
– Hydrodynamic interactions dominate
•
Einstein/Taylor/Schowalter etc
– Spheres in Newtonian solution
– Packing fraction depends on proteins
and sugars (~10%)
– Not mono-disperse
– Globules prone to cluster
•
C.W. Macosko Rheology: Principles, Measurements, and
Applications, VCH Publishers Inc., New York, 1994.
Shear rate
5 s-1
10 s-1
0.1 s-1
Critical response for mouth-feel
– Shear thinning with zero-shear plateau
– Fat provides viscosity
– Fat % does not change behavior
Fat volume fraction
Kyazze, G. and Starov, V., Viscosity of Milk: Influence of
Cluster Formation. Colloid Journal 66(3),316-321 (2004)
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Reducing fat
Alginate flour
Algal Flour
whole milk
2% fat
1% skimmed
non-fat
1% starch
2% starch
3% starch
5% starch
Shear Viscosity [Pa.s]
10.000
1.000
0.100
0.010
0.001
0.1
1.0
10.0
100.0
1000.0
-1
Shear rate [s ]
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Conclusions
•
Consumer testing implies
– Algal flour closest to milk response
– Shear rheology indicates starch is the best
•
Complex fluids can yield deceptively simple responses
– Milk (stabilized fat globules)
• Shear thinning
• Shear rate response controlled by fat content plus proteins and sugars
– Choosing “dominant” deformation does not always allow replacement of
ingredients
• Milk “feel” expected to be dominated by shear viscosity
• Corn syrup, algal flour and starch all provide reasonable rheological
responses
• But rheology does not provide separation between systems
– So where is the difference?
• Wrong deformation?
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Case 2: Consumer Products
•
Consumer products represent a massive market in US
– Emulsions, emollients, moisturizers and personal lubricants
•
Perception of efficacy influenced by “feel” and “look” of system
– Complex interplay of
•
•
•
•
•
•
Viscosity
Yield stress
Absorption
Wetting
Elasticity
Loading
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Personal Lubricants
Firmness
90
80
Wetness
70
Thickness
Aqueous
Silicone
Emulsion
60
50
40
1
30
2
20
4
10
Spreadability
Residue Thickness
0
5
6
8
9
10
Runniness
Slipperiness
Stickiness
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Consumer ranking (selected)
9
Aqueous
Silicone
Emulsion
8
7
6
1
5
2
4
4
5
6
3
8
9
2
10
1
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Average
Spreadability
Runniness
Stickiness
Thickness
Firmness
0
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Small Amplitude Oscillatory Shear
1000
1000
Aqueous
Silicone
Emulsion
100.0
100.0
1
10.00
9
4
1
4
9
2
5
6
10
1.000
0.1000
0.01000
0.1000
1.000
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10.00
Ang. frequency (rad/s)
17
100.0
1.000
G'' [Pa]
G' [Pa]
10.00
0.1000
0.01000
1000
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Yield Stress
1000
100.0
9
1
Aqueous
Silicone
Emulsion
4
1
4
9
2
5
6
10
Viscosity [Pa.s]
10.00
1.000
0.1000
0.01000
1.000E-3
0.01000
0.1000
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1.000
10.00
Shear stress [Pa]
18
100.0
1000
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Capillary Breakup
•
Sample
Thermo Haake CaBER
Laser micrometer
Diameter [mm]
10
1
1
9
9
4
4
1
4
9
2
5
6
10
0.1
0.01
0
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4
Time [s]
19
6
8
10
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Observations
•
High ranking materials appear to have
–
–
–
–
•
High low-shear viscosity and low high-shear viscosity
High shear viscosity seems to be more important
Elasticity less important
Extensional properties appear related
What is missing?
– “slipperiness” (lubricity)
• Related to shear viscosity and surface chemistry
• “Thin” film with gap governed by shear properties
• Coefficient of Friction
– “stickiness” (tack)
• Related to elasticity and adhesion
• Large contact area, dependent on pull speed and fluid properties
• Tack test
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Lubricity
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CoF on a rheometer
Spring
Annulus
Surface
1
Kavehpour and McKinley “Tribo-rheometry: from gap-dependent
rheology to tribology” Tribology Letters (2004) 17(2) 327-335
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Coefficient of Friction
Coefficient of Friction []
•
CoF fixture on AR-G2. Controlled normal stress (82 kPa) and rotation rate
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Hydrogel
0.0
-0.1 0.1
1
4
9
2
5
1, 4, 9
8
6
1
10
100
Coefficient of Friction []
Velocity [rad/s]
1.2
10
water
9
1
1.0
4
0.8
1
9
0.6
2
4
0.4
0.2
5
8
Neoprene
6
0.0
0.1
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Velocity [rad/s]
23
10
100
10
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Comparisons
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CoF fixture on AR-G2. Controlled normal stress (82 kPa) and rotation rate (0.3 rad/s)
0.7
Hydrogel
Neoprene
Coefficient of Friction []
0.6
0.5
0.4
0.3
0.2
0.1
0
1
4
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2
5
8
24
6
10
water
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Tackiness
•
Combination of accurate vertical position and normal force allow
tack to be measured on a conventional rheometer
– AR-G2 has a “fast sampling” mode that allows 250 Hz
– Squeeze/pull-off allows tack-like test using parallel plates
Velocity
Normal force
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Tackiness
•
4 cm parallel plate loaded to fixed gap and pulled at 500 micron/s
0
300
600
900
1200
1500
1
9
-5
1800
4
2100
9
2
5
2400
8
6
10
4
0.5
1
-10
0
Force [N]
Force [N]
1
-0.5
300
600
900
-1
-1.5
-15
-2
-2.5
-20
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Position (relative to bottom plate) [mm]
Position (relative to bottom plate) [mm]
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Work of Adhesion
0.006
Work of adhesion [J]
0.005
0.004
0.003
0.002
0.001
0
1
4
9
2
5
8
6
10
Sample
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Conclusions
Correlation between physical and sensory measures
Intimate Health products, (p<0.05)
Firmness
Thickness
Residue
Slipperiness
Stickiness
Runniness
Spreadability
Controllability
Wetness
Cooling
Shear Viscosity
Oscillatory Viscosity
Lubricity
Extensional Viscosity
Adhesion
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Case 3: Implantation of catheters
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•
•
•
•
Cardiovascular catheters are used for
access, surgery and drug delivery
Usually inserted through the femoral
artery and then guided to their destination
Device is “steered” through the arteries
along tortuous pathways and around
sharp corners
Guidewire is used to direct catheter
Surgical feel of device influenced by
– Coating friction on walls
– Level of wetting/lubrication
– Varying contact area due to bends
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Testing of catheters
•
Surgical feel of the device controlled by
–
–
–
–
–
•
Physiological fluids present
Absorption of species
Wettability of the coating
Intrinsic coefficient of friction
Bending elasticity of the composite catheter
ASTM F2394: Expandable Vascular Stents
– Tortuous path for testing of catheter insertion and rotation
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Friction and lubricity of catheters
•
Effective friction a function of pull-force and displacement
– AR1000 provides accurate measures of both
• Testing through “friction pad” decouples tortuosity
• Testing through fixture allows effect of tortuosity to be determined
Direct measure of friction
between guidewire and
model surface
Friction Pad
Mode 1
AR1000
Measure of friction
between guidewire and
catheter in tortuous
contour
Catheter
guidewire
Mode 2
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Mode 1: Friction pad
2.5
2
Pull force [N]
Start-up
1.5
Client
Competitor
Analysis
1
Competitor
Client
0.5
0
0
10
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30
Time [s]
32
40
50
60
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Mode 2: pull-force in catheter (dry)
1.4
1.2
Dry:
Competitor (straight)
Competitor (tortuous)
Client (straight)
Client (tortuous)
1.2
0.8
p < 0.003
1
Pull force [N]
Pull force [N]
1
0.6
0.4
0.8
0.6
p < 0.001
Straight
0.4
0.2
0.2
0
Client
Competitor
Client
Competitor
0
0
5
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15
20
Time [s]
33
25
30
35
40
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Mode 2: pull-force in catheter (wet)
0.45
Dry:
0.4
0.35
0.3
p < 0.001
0.07
0.06
p < 0.001
Straight
0.05
Pull force [N]
Pull force [N]
0.08
Competitor (straight)
Competitor (tortuous)
Client (straight)
Client (tortuous)
0.25
0.2
0.04
0.03
0.02
0.01
0.15
0
Client
0.1
Competitor
Client
Competitor
0.05
0
0
10
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30
Time [s]
34
40
50
60
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Conclusions
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•
•
In a crowded market place surgeon impressions drive sales
Reducing “impressions” to quantitative numbers allows direct
competitive comparisons
Using conventional rheological techniques and instruments in
unconventional manners allows differentiation of materials in
physiologically relevant conditions
– Test structure that is familiar to users
– Numbers that are easily correlated
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Lessons and comments
•
•
Understanding the rheology of fluids critical for understanding
consumer perception and use
But mode of use can be just as important
– Usage rarely simple shear
– Perception is therefore governed by response to variety of deformations
•
•
•
•
•
•
•
Simple shear
Compression
Extension
Pipe-flow etc
Collating data in more relevant configurations can provide useful
correlations with field data
Superb force and position control of a conventional rheometer still
useful
Sometimes the tests need to be modified to help act as consumer
screening tests
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Thank you
Cambridge Polymer Group is a contract
research laboratory specializing in
polymers and their applications. We
provide outsourced research and
development, consultation and failure
analysis as well as routine analytical
testing and custom test and
instrumentation design.
Cambridge Polymer Group, Inc.
56 Roland St., Suite 310
Boston, MA 02129
(617) 629-4400
http://www.campoly.com
[email protected]
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