CambridgePolymer Group, Inc.
Testing, Consultation, and Instrumentation for Polymeric Materials
Cambridge Polymer Group, 56 Roland Street, Suite 310
Boston, MA 02129
7-17 Presentation (10/1/2010)
Rheometers: not just for rheology any more
Looking beyond rheology
Gavin Braithwaite
Rheology
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• 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
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 nutrition-providing materials – carotenoids– dietary fiber– tocotrienols and tocopherols– varying lipid compositions– low levels of saturated lipids
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0.001
0.01
0.1
0.1 1 10 100 1000
She
ar v
isco
sity
[Pa.
s]
Shear rate [s-1]
whole milk (G2)
2% fat (G2)
1% skimmed (G2)
non-fat (G2)
Milk
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Error bars SD for three runs
Fat wt% down
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
• Critical response for mouth-feel – Shear thinning with zero-shear plateau– Fat provides viscosity– Fat % does not change behavior
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Fat volume fraction
Shear rate5 s-1
10 s-1
0.1 s-1
Kyazze, G. and Starov, V., Viscosity of Milk: Influence of Cluster Formation. Colloid Journal 66(3),316-321 (2004)
C.W. Macosko Rheology: Principles, Measurements, and Applications, VCH Publishers Inc., New York, 1994.
Reducing fat
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0.001
0.010
0.100
1.000
10.000
0.1 1.0 10.0 100.0 1000.0
Shear rate [s-1]
Shea
r Vis
cosi
ty [P
a.s]
Alginate flourAlgal Flourwhole milk2% fat1% skimmednon-fat1% starch2% starch3% starch 5% starch
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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• 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
Case 2: Consumer Products
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Personal Lubricants
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0102030405060708090Firmness
Thickness
Residue Thickness
Slipperiness
Stickiness
Runniness
Spreadability
Wetness
124568910
AqueousSiliconeEmulsion
Consumer ranking (selected)
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0
1
2
3
4
5
6
7
8
9Fi
rmne
ss
Thic
knes
s
Stic
kine
ss
Run
nine
ss
Spr
eada
bilit
y
Ave
rage
1
2
4
5
6
8
9
10
AqueousSiliconeEmulsion
0.1000 1.000 10.00 100.0 1000Ang. frequency (rad/s)
0.01000
0.1000
1.000
10.00
100.0
1000
G' [
Pa]
0.01000
0.1000
1.000
10.00
100.0
1000
G''
[Pa]
1
10
2
4
56
9
Small Amplitude Oscillatory Shear
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1
4
9
AqueousSiliconeEmulsion
Yield Stress
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AqueousSiliconeEmulsion
1
4
9
0.01000 0.1000 1.000 10.00 100.0 1000Shear stress [Pa]
1.000E-3
0.01000
0.1000
1.000
10.00
100.0
1000
Visc
osity
[Pa.
s]
1
10
2
4
6
9
5
0.01
0.1
1
10
0 2 4 6 8 10Time [s]
Diam
eter [m
m]
14925610
Capillary Breakup
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• Thermo Haake CaBER
0.01
0.1
1
10
0 2 4 6 8 10Time [s]
Diam
eter [m
m]
14925610
Sample
Laser micrometer
1
4
9
4
9
4
9
4
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
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1 Kavehpour and McKinley “Tribo-rheometry: from gap-dependent rheology to tribology” Tribology Letters (2004) 17(2) 327-335
Spring
Annulus
Surface
Coefficient of Friction
• CoF fixture on AR-G2. Controlled normal stress (82 kPa) and rotation rate
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-0.10.00.10.20.30.40.50.60.7
0.1 1 10 100Coe
ffici
ent o
f Fric
tion
[]
Velocity [rad/s]
1
4
9
2
5
8
6
10
water
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100
Coe
ffici
ent o
f Fric
tion
[]
Velocity [rad/s]
1
49
2
5
8
6
10
Hydrogel
Neoprene
1, 4, 9
4
9
1
Comparisons
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1 4 9 2 5 8 6 10 water
Coe
ffici
ent o
f Fric
tion
[]
Hydrogel Neoprene
• CoF fixture on AR-G2. Controlled normal stress (82 kPa) and rotation rate (0.3 rad/s)
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
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Normal force
Velocity
Tackiness
• 4 cm parallel plate loaded to fixed gap and pulled at 500 micron/s
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-20
-15
-10
-5
0300 600 900 1200 1500 1800 2100 2400
Forc
e [N
]
Position (relative to bottom plate) [mm]
1 4 9 2 5 8 6 10
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
300 600 900
Forc
e [N
]
Position (relative to bottom plate) [mm]
1
4
9
Work of Adhesion
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0
0.001
0.002
0.003
0.004
0.005
0.006
1 4 9 2 5 8 6 10
Wor
k of
adh
esio
n [J
]
Sample
Conclusions
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Correlation between physical and sensory measures Intimate Health products, (p<0.05)
Shear Viscosity
Oscillatory Viscosity
Lubricity
Extensional Viscosity
Adhesion
FirmnessThicknessResidueSlipperinessStickinessRunninessSpreadabilityControllabilityWetnessCooling
Case 3: Implantation of catheters
• 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
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guidewire
AR1000
Catheter
Mode 2
Friction Pad
Mode 1
Direct measure of friction between guidewire and model surface
Measure of friction between guidewire and catheter in tortuous contour
0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60
Pull
forc
e [N
]
Time [s]
Mode 1: Friction pad
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Start-up
AnalysisCompetitorClient
CompetitorClient
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25 30 35 40
Pull
forc
e [N
]
Time [s]
Dry:Competitor (straight)Competitor (tortuous)Client (straight)Client (tortuous)
Mode 2: pull-force in catheter (dry)
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0
0.2
0.4
0.6
0.8
1
1.2
Client Competitor Client Competitor
Pull
forc
e [N
]
p < 0.001Straight
p < 0.003
Mode 2: pull-force in catheter (wet)
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0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 10 20 30 40 50 60
Pull
forc
e [N
]
Time [s]
Dry:Competitor (straight)Competitor (tortuous)Client (straight)Client (tortuous)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Client Competitor Client CompetitorPu
ll fo
rce
[N]
p < 0.001Straight
p < 0.001
Conclusions
• 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 310Boston, MA 02129(617) 629-4400http://[email protected]
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