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Basics in Rheology Theory Oscillation TA Rheometers Linear Viscoelasticity Instrumentation Setting up Oscillation Tests Choosing a Geometry Transient Testing Calibrations Applications of Rheology Flow Tests Polymers Viscosity Structured Fluids Setting up Flow Tests Advanced Accessories
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Rheology: The study of stress-deformation relationships
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Rheology is the science of flow and deformation of matter The word ‘Rheology’ was coined in the 1920s by Professor E C Bingham at Lafayette College in Indiana Flow is a special case of deformation The relationship between stress and deformation is a property of the material
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Top plate Area = A x(t) Velocity = V0 Force = F
y y Bottom Plate Velocity = 0 x F Shear Stress, Pascals σ = A
x(t) σ γ = η = Shear Strain, % Viscosity, Pa⋅s y γ
γ -1 γ = Shear Rate, sec t
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Ω r θ
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Stress (σ) ߪൌ మ ൈM ഏೝయ r = plate radius h = distance between plates γ ߛൌೝ ൈθ M = torque (μN.m) Strain ( ) θ = Angular motor deflection (radians) Ω= Motor angular velocity (rad/s) ೝ Ω Strain rate (ߛሶ) ߛሶ ൌ ൈ
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ARES G2 DHR
Controlled Strain Controlled Stress Dual Head Single Head SMT CMT
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Dual head or SMT Single head or CMT Separate motor & transducer Combined motor & transducer
Displacement Measured Transducer Sensor Torque (Stress) Measured Strain or Rotation Non-Contact Drag Cup Motor
Applied Torque Sample (Stress)
Applied Strain or Direct Drive ^ƚĂƚŝĐWůĂƚĞStatic Plate Rotation Motor
Note: With computer feedback, DHR and AR can work in controlled strain/shear rate, and ARES can work in controlled stress. TAINSTRUMENTS.COM :KDWGRHVD5KHRPHWHUGR"
Rheometer – an instrument that measures both viscosity and viscoelasticity of fluids, semi-solids and solids
It can provide information about the material’s: Viscosity - defined as a material’s resistance to deformation and is a function of shear rate or stress, with time and temperature dependence
Viscoelasticity – is a property of a material that exhibits both viscous and elastic character. Measurements of G’, G”, tan δ with respect to time, temperature, frequency and stress/strain are important for characterization.
A Rheometer works simply by relating a materials property from how hard it’s being pushed, to how far it moves
by commanding torque (stress) and measuring angular displacement (strain) by commanding angular displacement (strain) and measuring torque (stress)
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From the definition of rheology,
the science of flow and deformation of matter or the study of stress (Force / Area) – deformation (Strain or Strain rate) relationships.
Fundamentally a rotational rheometer will apply or measure: 1. Torque (Force) 2. Angular Displacement 3. Angular Velocity
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In a rheometer, the stress is calculated from the torque.
The formula for stress is: σ сΜ п <σ Where σ = Stress (Pa or Dyne/cm2) Μ = torque in N.mor gm.cm
<σ = Stress Constant
The stress constant, <σ, is a geometry dependent factor
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In a SMT Rheometer, the angular displacement is directly applied by a motor.
The formula for strain is: γ с<γ п θ йγ сγ п ϭϬϬ where γ = Strain
<γ = Strain Constant θ = Angular motor deflection (radians)
The strain constant, <γ, is a geometry dependent factor
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Describe In Spec Correctly
σ M ⋅ Kσ G = γ = θ ⋅ Kγ
Geometric Rheological Constitutive Raw rheometer Shape Parameter Equation Specifications Constants
The equation of motion and other relationships have been used to determine the appropriate equations to convert machine parameters (torque, angular velocity, and angular displacement) to rheological parameters.
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In a SMT rheometer, the angular speed is directly controlled by the motor).
The formula for shear rate is:
ߛሶ ൌܭఊ ൈ: where ߛሶ = Shear rate <γ = Strain Constant Ω = Motor angular velocity in rad/sec
The strain constant, <γ, is a geometry dependent factor
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Describe In Spec Correctly σ ⋅ η = = M Kσ γ Ω ⋅ Kγ
Geometric Rheological Constitutive Raw rheometer Shape Parameter Equation Specifications Constants
The equation of motion and other relationships have been used to determine the appropriate equations to convert machine parameters (torque, angular velocity, and angular displacement) to rheological parameters.
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Specification HR-3 HR-2 HR-1 Bearing Type, Thrust Magnetic Magnetic Magnetic Bearing Type, Radial Porous Carbon Porous Carbon Porous Carbon Motor Design Drag Cup Drag Cup Drag Cup Minimum Torque (nN.m) Oscillation 0.5 2 10 ,ZͲϮ ,ZͲϯ Minimum Torque (nN.m) Steady 51020 ,ZͲϭ Shear Maximum Torque (mN.m) 200 200 150 Torque Resolution (nN.m) 0.05 0.1 0.1 Minimum Frequency (Hz) 1.0E-07 1.0E-07 1.0E-07 Maximum Frequency (Hz) 100 100 100 Minimum Angular Velocity (rad/s) 0 0 0 Maximum Angular Velocity (rad/s) 300 300 300 Displacement Transducer Optical Optical Optical encoder encoder encoder Optical Encoder Dual Reader Standard N/A N/A Displacement Resolution (nrad) 2 10 10 Step Time, Strain (ms) 15 15 15 Step Time, Rate (ms) 5 5 5 DHR - DMA mode (optional) Normal/Axial Force Transducer FRT FRT FRT Maximum Normal Force (N) 50 50 50 Motor Control FRT Minimum Force (N) Oscillation 0.1 Normal Force Sensitivity (N) 0.005 0.005 0.01 Maximum Axial Force (N) 50 Minimum Displacement (μm) 1.0 Normal Force Resolution (mN) 0.5 0.5 1 Oscillation Maximum Displacement (μm) 100 Oscillation Displacement Resolution (nm) 10 Axial Frequency Range (Hz) 1 x 10-5 to 16
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Force/Torque Rebalance Transducer (Sample Stress) Transducer Type Force/Torque Rebalance Transducer Torque Motor Brushless DC Transducer Normal/Axial Motor Brushless DC Minimum Torque (μN.m) Oscillation 0.05 Minimum Torque (μN.m) Steady Shear 0.1 Maximum Torque (mN.m) 200 Torque Resolution (nN.m) 1 Transducer Normal/Axial Force Range (N) 0.001 to 20 Transducer Bearing Groove Compensated Air
Driver Motor (Sample Deformation) Maximum Motor Torque (mN.m) 800 Orthogonal Superposition (OSP) and Motor Design Brushless DC DMA modes Motor Bearing Jeweled Air, Sapphire Motor Control FRT Displacement Control/ Sensing Optical Encoder Strain Resolution (μrad) 0.04 Minimum Transducer Force (N) 0.001 Oscillation Minimum Angular Displacement (μrad) 1 Maximum Transducer Force 20 Oscillation (N) Maximum Angular Displacement (μrad) Unlimited Minimum Displacement (μm) 0.5 Steady Shear Oscillation -6 Angular Velocity Range (rad/s) 1x 10 to 300 Maximum Displacement (μm) 50 Angular Frequency Range (rad/s) 1x 10-7 to 628 Oscillation Step Change, Velocity (ms) 5 Displacement Resolution (nm) 10 Step Change, Strain (ms) 10 Axial Frequency Range (Hz) 1 x 10-5 to 16
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Concentric Cone and Parallel Torsion Cylinders Plate Plate Rectangular
Very Low Very Low Very Low Solids to Medium to High Viscosity Viscosity Viscosity to Soft Solids Water to Steel
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Assess the ‘viscosity’ of your sample When a variety of cones and plates are available, select diameter appropriate for viscosity of sample Low viscosity (milk) - 60mm geometry Medium viscosity (honey) - 40mm geometry High viscosity (caramel) – 20 or 25mm geometry Examine data in terms of absolute instrument variables torque/displacement/speed and modify geometry choice to move into optimum working range You may need to reconsider your selection after the first run!
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Strain Constant: ܭఊ ൌ (to convert angular velocity, rad/sec, to shear rate, 1/sec, at the edge or angular displacement, radians, to shear strain (unitless) at the edge. The radius, r, and the gap, h, are expressed in meters)
'ĂƉ;ŚͿ Stress Constant: ܭ ൌ ଶ ఙ గయ (to convert torque, N⋅m, to shear stress at the ŝĂŵĞƚĞƌ;Ϯ⋅⋅⋅ƌͿ edge, Pa, for Newtonian fluids. The radius, r, is expressed in meters)
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Low/Medium/High Viscosity Samples with long relaxation time Liquids Temperature Ramps/ Sweeps Soft Solids/Gels Materials that may slip Thermosetting materials Crosshatched or Sandblasted plates Samples with large particles Small sample volume
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Shear Stress
20 mm
40 mm
60 mm
ʹ As diameter decreases, shear stress increases ߪൌܯ ߨݎଶ
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Shear Rate 2 mm Increases
1 mm
0.5 mm
ݎ As gap height decreases, shear rate increases ߛሶ ൌȳ ݄
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For a given angle of deformation, there is a greater arc of deformation at the edge of the plate than at the center
,ݔ dx increases further from the center݀ ߛൌ ݄ h stays constant
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The cone shape produces a smaller gap height closer to inside, so the shear on the sample is constant
ݔ݀ ߛൌ h increases proportionally to dx, γ is uniform ݄
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ଵ Strain Constant: ܭఊ ൌఉ (to convert angular velocity, rad/sec, to shear rate. 1/sec, or angular displacement, radians, to shear strain, which is unit less. The angle, β, is expressed in radians)
ܭ ൌ ଷ Stress Constant: ఙ ଶగయ
(to convert torque, N⋅m, to shear stress, Pa. The radius, r, is expressed in meters)
Diameter (2⋅⋅⋅r) Truncation (gap)
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Very Low to High Viscosity Liquids High Shear Rate measurements Normal Stress Growth Unfilled Samples Isothermal Tests Small Sample Volume
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Shear Stress
20 mm
40 mm
60 mm
͵ As diameter decreases, shear stress increases ߪൌܯ ʹߨݎଷ
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Shear Rate Increases
2 °
1 °
0.5 °
ͳ As cone angle decreases, shear rate increases ߛሶ ൌȳ ߚ
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Typical Truncation Heights: 1° degree ~ 20 - 30 microns 2° degrees ~ 60 microns 4° degrees ~ 120 microns
Cone & Plate Truncation Height = Gap
Gap must be > or = 10 [particle size]!!
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× Under Filled sample: Lower torque contribution
× Over Filled sample: Additional stress from drag along the edges
9 Correct Filling
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ݎ ଶ ݎ ଶ Strain Constant: ͳ ʹ ܭఊ ൌ ଶ ଶ ݎʹ ݎͳ (to convert angular velocity, rad/sec, to shear rate, 1/sec, or angular displacement, radians, to shear
strain (unit less). The radii, r1 (inner) and r2 (outer), are expressed in meters)
* ݎ ଶ ݎ ଶ Stress Constant: ଵ ͳ ʹ ܭఙ ൌସగ ଶ ଶ ݎʹ ݎͳ
(to convert torque, N⋅m, to shear stress, Pa. The bob length, l, and the radius, r, are expressed in meters)
*Note including end correction factor. See TRIOS Help TAINSTRUMENTS.COM 'RXEOH:DOO
Use for very low viscosity systems (<1 mPas)
r1
r2 h r3
r4
ଶ ଶ ݎͳ ݎʹ Strain Constant: ܭఊ ൌ ଶ ଶ ݎʹ െݎͳ మ మ ଵ ାଶ Stress Constant: ܭఙ ൌ మ మ మ ସగήଶ ଵ ାଷ ARES Gap Settings: standard operating gap DW = 3.4 mm narrow operating gap DW = 2.0 mm
Use equation Gap > 3 × (R2 –R1) TAINSTRUMENTS.COM :KHQWR8VH&RQFHQWULF&\OLQGHUV
Low to Medium Viscosity Liquids Unstable Dispersions and Slurries Minimize Effects of Evaporation Weakly Structured Samples (Vane) High Shear Rates
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ݐ ܭ ൌ ఊ ௧ ଶ ݈ ͳ െ ͲǤ͵ͺ ௪ w = Width l = Length ͵ଵǤ଼ t = Thickness ܭ ൌ ௪ ఛ ݓήݐଶ
Advantages: Disadvantages:
High modulus samples No pure Torsion mode for Small temperature high strains gradient Simple to prepare
Torsion cylindrical also available
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Torsion and DMA geometries allow solid samples to be characterized in a temperature controlled environment
Torsion measures G’, G”, and Tan δ DMA measures E’, E”, and Tan δ ARES G2 DMA is standard function (50 μm amplitude) DMA is an optional DHR function (100 μm amplitude)
Rectangular and DMA 3-point bending and tension cylindrical torsion (cantilever not shown)
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Geometry Application Advantage Disadvantage
Cone/plate fluids, melts true viscosities temperature ramp difficult viscosity > 10mPas
Parallel Plate fluids, melts easy handling, shear gradient across viscosity > 10mPas temperature ramp sample
Couette low viscosity samples high shear rate large sample volume < 10 mPas
Double Wall Couette very low viscosity high shear rate cleaning difficult samples < 1mPas
Torsion Rectangular solid polymers, glassy to rubbery Limited by sample stiffness composites state
Limited by sample stiffness Solid polymers, films, Glassy to rubbery DMA (Oscillation and Composites state stress/strain)
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Instrument Calibrations
Inertia (Service)
Rotational Mapping
Geometry Calibrations:
Inertia
Friction
Gap Temperature Compensation
Rotational Mapping
Details in Appendix #4
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Instrument Calibrations Transducer Temperature Offsets Phase Angle (Service) Measure Gap Temperature Compensation
Geometry Calibrations: Compliance and Inertia (from table) Gap Temperature Compensation
Details in Appendix #4
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Rheometers are calibrated from the factory and again at installation. TA recommends routine validation or confidence checks using standard oils or Polydimethylsiloxane (PDMS).
PDMS is verified using a 25 mm parallel plate. Oscillation - Frequency Sweep: 1 to 100 rad/s with 5% strain at 30°C Verify modulus and frequency values at crossover
Standard silicone oils can be verified using cone, plate or concentric cylinder configurations. Flow – Ramp: 0 to 88 Pa at 25°C using a 60 mm 2° cone
Service performs this test at installation
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Set Peltier temperature to 25°C and equilibrate. Zero the geometry gap
Load sample Be careful not to introduce air bubbles!
Set the gap to the trim gap
Lock the head and trim with non-absorbent tool Important to allow time for temperature equilibration.
Go to geometry gap and initiate the experiment.
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Viscosity is… “lack of slipperiness” synonymous with internal friction resistance to flow
The Units of Viscosity are … SI unit is the Pascal.second (Pa.s) cgs unit is the Poise 10 Poise = 1 Pa.s 1 cP (centipoise) = 1 mPa.s (millipascal second)
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Describe In Spec Correctly σ ⋅ η = = M Kσ γ Ω ⋅ Kγ
Geometric Rheological Constitutive Raw rheometer Shape Parameter Equation Specifications Constants
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6 Asphalt Binder ------100,000 6 Polymer Melt ------1,000 6 Molasses ------100 6 Liquid Honey ------10 Need for 6 Glycerol ------1 Log scale 6 Olive Oil ------0.01 6 Water ------0.001 6 Air ------0.00001
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Newtonian Fluids - constant proportionality between shear stress and shear-rate
Non-Newtonian Fluids - Viscosity is time or shear rate dependent Time:
At constant shear-rate, if viscosity
Decreases with time – Thixotropy
Increases with time - Rheopexy Shear-rate:
Shear - thinning
Shear - thickening
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Ideal Yield Stress (Bingham Yield) Pa.s , Pa η, σ
γ ,1/s or σ, Pa γ ,1/s
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105 105
103 103 Pa.s 101 Pa.s 101 η, η, 10-1 10-1 10-6 10-4 10-2 100 102 104 10-1 10-0 10-1 102 103 ,1/s σ, Pa
105 ,GHDO
σ 101
10-1 10-6 10-4 10-2 100 102 104 γ ,1/s
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Dilatant material resists deformation more than in proportion to the applied
Pa.s force (shear-thickening) η, Cornstarch in water or sand Log on the beach are actually dilatant fluids, since they do not show the time- dependent, shear-induced change required in order to Log ߛሶ,1/s be labeled rheopectic
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Rheopectic materials become more viscous Rheopectic with increasing time of applied force Higher concentration latex dispersions and Shear Rate = Constant plastisol paste materials exhibit rheopectic Viscosity behavior Thixotropic Thixotropic materials become more fluid with increasing time of applied time force Coatings and inks can display thixotropy when sheared due to structure breakdown
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Flow Experiments Constant shear rate/stress (or Peak hold) Continuous stress/rate ramp and down Stepped flow (or Flow sweep) Steady state flow Flow temperature ramp
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Constant rate vs. time
Constant stress vs. time Stress /Rate
USES Single point testing Scope the time for steady state under certain rate
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3.500 4.000 Hand Wash Rate 1/s
3.500 3.000
3.000 2.500
2.500 viscosity (Pa.s) viscosity 2.000
2.000
1.500 shear stress (Pa) Viscosity at 1 1/s is 2.8 Pa⋅s 1.500
1.000 1.000
0.5000 0.5000
0 0 0 5.0000 10.000 15.000 20.000 25.000 30.000 time (s)
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Deformation Stress is applied to material at a constant m =Stress rate rate. Resultant strain (Pa/min) is monitored with time.
time (min)
USES Yield stress Scouting Viscosity Run
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Deformation Stress is first increased, σ then decreased, at a constant rate. Resultant strain is monitored with time. time
USES “Pseudo-thixotropy” from Hysteresis loop
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Run in Stress Control TA Instruments 500.0
400.0
300.0 ZĞĚ͗&ŝƌƐƚĐLJĐůĞ ůƵĞ͗^ĞĐŽŶĚĐLJĐůĞ 200.0
shear stress (Pa) FORDB3.04F-Up step FORDB3.04F-Down step FORDB3.05F-Up step 100.0 FORDB3.05F-Down step
0 0 0.1000 0.2000 0.3000 0.4000 0.5000 shear rate (1/s)
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Deformation σ Data point saved Stress is applied to sample. Žƌ Viscosity measurement is taken ߛሶ when material has reached steady state flow. The stress is increased(logarithmically) and the time process is repeated yielding a viscosity flow curve.
Delay time σ USES Žƌ Steady State Flow ߛሶ Viscosity Flow Curves γ or σ = Constant Yield Stress Measurements
time
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A series of logarithmic stress steps allowed to reach steady state, each one giving a single viscosity data point:
Data at each Shear Rate
Shear Thinning Region
Time Viscosity
η = σ / γ/ Shear Rate, 1/s (d dt)
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Control variables: Shear rate Velocity Torque Shear stress
Steady state algorithm
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0.8000 ^ĂŵƉůĞϭͲ&ůŽǁƐƚĞƉ ^ĂŵƉůĞϮͲ&ůŽǁƐƚĞƉ 0.7000 Which material would do a better job 0.6000 coating your throat?
0.5000
0.4000 viscosity (Pa.s) viscosity 0.3000
0.2000
0.1000
0 0.1000 1.000 10.00 100.0 1000 shear rate (1/s)
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10000 A.01F-Flow step Low shear rates B>A B.01F-Flow step
1000
100.0
viscosity (Pa.s) 10.00 Medium shear rates A>B
1.000
High shear rates B>A
0.1000 1.000E-4 1.000E-3 0.01000 0.1000 1.000 10.00 100.0 1000 shear rate (1/s)
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Hold the rate or stress yyyy constant whilst ramping the temperature.
time (min)
USES Measure the viscosity change vs. temperature
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Notice a nearly 2 decade decrease in viscosity. This displays the importance of thermal equilibration of the sample prior to testing. i.e. Conditioning Step or equilibration time for 3 to 5 min
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Small gaps give high shear rates Be careful with small gaps:
Gap errors (gap temperature compensation) and shear heating can cause large errors in data.
Recommended gap is between 0.5 to 2.0 mm.
Secondary flows can cause increase in viscosity curve Be careful with data interpretation at low shear rates Surface tension can affect measured viscosity, especially with aqueous materials
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Surface tension at low torques
Secondary flows at high shear rates
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Wall slip can manifest as “apparent double yielding” Can be tested by running the same test at different gaps For samples that don’t slip, the results will be independent of the gap
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When Stress Decreases with Shear Rate, it indicates that sample is leaving the gap
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Edge Failure – Sample leaves gap because of normal forces Look at stress vs. shear rate curve – stress should not decrease with increasing shear rate – this indicates sample is leaving gap Possible Solutions: use a smaller gap or smaller angle so that you get the same shear rate at a lower angular velocity if appropriate (i.e. Polymer melts) make use of Cox Merz Rule η γ η* () ≡ ω ( )
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Hooke’s Law of Elasticity: Stress = Modulus ⋅ Strain
γ γ γ 1 2 3
3
2
^ƚƌĞƐƐ;ʍͿ 1
1 2 3 ^ƚƌĂŝŶ;࠹Ϳ
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Hooke’s Law of Elasticity: Stress = Modulus ⋅ Strain E > E >E
ߪ ܧൌ γ γ γ ߛ 1 2 3
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Newton’s Law: stress = coefficient of viscosity ⋅ shear rate Ș ߪൌKήߛሶ ^ƚƌĞƐƐ;ʍͿ
^ŚĞĂƌZĂƚĞ;࠹Ϳ
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Range of Material Behavior Liquid Like------Solid Like Ideal Fluid ----- Most Materials -----Ideal Solid Purely Viscous ----- Viscoelastic ----- Purely Elastic
Viscoelasticity: Having both viscous and elastic properties
Materials behave in the linear manner, as described by Hooke and Newton, only on a small scale in stress or deformation.
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Long deformation time: pitch behaves like a highly viscous liquid th 9 drop fell July 2013 Short deformation time: pitch behaves like a solid
Started in 1927 by Thomas Parnell in Queensland, Australia ŚƚƚƉ͗ͬͬǁǁǁ͘ƚŚĞĂƚůĂŶƚŝĐ͘ĐŽŵͬƚĞĐŚŶŽůŽŐLJͬĂƌĐŚŝǀĞͬϮϬϭϯͬϬϳͬƚŚĞͲϯͲŵŽƐƚͲĞdžĐŝƚŝŶŐͲǁŽƌĚƐͲŝŶͲƐĐŝĞŶĐĞͲƌŝŐŚƚͲŶŽǁͲƚŚĞͲƉŝƚĐŚͲĚƌŽƉƉĞĚͬϮϳϳϵϭϵͬ
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T is short [< 1s] T is long [24 hours]
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Silly Putties have different characteristic relaxation times Dynamic (oscillatory) testing can measure time-dependent viscoelastic properties more efficiently by varying frequency (deformation time)
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Old Testament Prophetess who said (Judges 5:5): "The Mountains ‘Flowed’ before the Lord"
Everything Flows if you wait long enough!
Deborah Number, De - The ratio of a characteristic relaxation time of a material (τ) to a characteristic time of the relevant deformation process (T).
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Hookean elastic solid - IJ is infinite Newtonian Viscous Liquid - IJ is zero Polymer melts processing - IJ may be a few seconds
High De Solid-like behavior Low De Liquid-like behavior
IMPLICATION: Material can appear solid-like because 1) it has a very long characteristic relaxation time or 2) the relevant deformation process is very fast
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log Frequency Temperature
log Time log Time
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"If the deformation is small, or applied sufficiently slowly, the molecular arrangements are never far from equilibrium.
The mechanical response is then just a reflection of dynamic processes at the molecular level which go on constantly, even for a system at equilibrium.
This is the domain of LINEAR VISCOELASTICITY.
The magnitudes of stress and strain are related linearly, and the behavior for any liquid is completely described by a single function of time."
Mark, J., et. al., Physical Properties of Polymers, American Chemical Society, 1984, p. 102.
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Measuring linear viscoelastic properties helps us bridge the gap between molecular structure and product performance
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Define Oscillation Testing
Approach to Oscillation Experimentation Stress and Strain Sweep Time Sweep Frequency Sweep Temperature Ramp Temperature Sweep (TTS)
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Phase angle δ Strain, ε
Stress, σΎ
Dynamic stress applied sinusoidally User-defined Stress or Strain amplitude and frequency
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Time to complete one oscillation Frequency is the inverse of time Units Angular Frequency = radians/second Frequency = cycles/second (Hz) Rheologist must think in terms of rad/s. 1 Hz = 6.28 rad/s
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ω = 6.28 rad/s
ω = 12.560rad/s
0 0 0 0
dŝŵĞсϭƐĞĐ ω = 50 rad/s
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Strain and stress are calculated from peak amplitude in the displacement and torque waves, respectively.
0 0 0 0
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Deformation An oscillatory (sinusoidal) deformation (stress or strain) is applied to a sample. Response The material response (strain or stress) is measured.
The phase angle δ, or phase shift, between the deformation Phase angle δ and response is measured.
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Purely Elastic Response Purely Viscous Response (Hookean Solid) (Newtonian Liquid) δ сϬ° δ сϵϬ°
Strain Strain
Stress Stress
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Phase angle 0°< δ < 90°
Strain
Stress
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The stress in a dynamic experiment is referred to as the complex stress σ* The complex stress can be separated into two components: 1) An elastic stress in phase with the strain. σ' = σ*cosδ σ' is the degree to which material behaves like an elastic solid. 2) A viscous stress in phase with the strain rate. σ" = σ*sinδ σ" is the degree to which material behaves like an ideal liquid.
Phase angle δ Complex Stress, σ* σ* = σ' + iσ"
Complex number: ݔ݅ݕ ൌ ݔଶ ݕଶ The material functions can be described in Strain, ε terms of complex variables having both real and imaginary parts. Thus, using the relationship:
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כୗ୲୰ୣୱୱ כ The Modulus: Measure of materials overall resistance to ൌ ୗ୲୰ୟ୧୬ deformation. The Elastic (Storage) Modulus: כୗ୲୰ୣୱୱ Measure of elasticity of material. ᇱ ൌ Ɂ The ability of the material to store ୗ୲୰ୟ୧୬ energy. The Viscous (loss) Modulus: כ The ability of the material to ̶ ൌ ୗ୲୰ୣୱୱ Ɂ dissipate energy. Energy lost as ୗ୲୰ୟ୧୬ heat.
Tan Delta: ̶ Measure of material damping - ୋ Ɂ ൌ ᇱ such as vibration or sound ୋ damping.
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RUBBER BALL y LOSS (G”) TENNIS BALL y
STORAGE (G’)
G* Dynamic measurement G" represented as a vector Phase angle δ G'
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The viscosity measured in an oscillatory experiment is a Complex Viscosity much the way the modulus can be expressed as the complex modulus. The complex viscosity contains an elastic component and a term similar to the steady state viscosity.
The Complex viscosity is defined as: Ș* = Ș’ + i Ș” or Ș* = G*/Ȧ
Note: frequency must be in rad/sec!
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Parameter Shear Elongation Units
Strain ɶсɶϬƐŝŶ;ʘƚͿ ɸ сɸϬ ƐŝŶ;ʘƚͿ ---
σ σ τ τ Stress с ϬƐŝŶ;ʘƚнɷͿ с ϬƐŝŶ;ʘƚнɷͿ Pa
Storage Modulus '͛с;σ ͬɶ ͿĐŽƐɷ ͛с;τ ͬɸ ͿĐŽƐɷ Pa (Elasticity) Ϭ Ϭ Ϭ Ϭ
Loss Modulus '͟с;σ ͬɶ ͿƐŝŶɷ ͟с;τ ͬɸ ͿƐŝŶɷ Pa (Viscous Nature) Ϭ Ϭ Ϭ Ϭ
Tan į 'ͬ͟'͛ ͬ͛͟ ---
Complex Modulus 'Ύ с;'͛Ϯн'͟ϮͿϬ͘ϱ Ύ с;͛Ϯн͟ϮͿϬ͘ϱ Pa
ηΎ η Ύ Ύ Complex Viscosity с'Ύͬʘ с ͬʘ Pa·sec
Cox-Merz Rule for Linear Polymers: η*(Ȧ) = η(ߛሶ) @ ߛሶ = Ȧ
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Define Oscillation Testing
Approach to Oscillation Experimentation 1. Stress and Strain Sweep 2. Time Sweep 3. Frequency Sweep 4. Temperature Ramp 5. Temperature Sweep (TTS)
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The material response to increasing deformation amplitude (strain or stress) is monitored at a constant frequency and Stress or strain Time temperature.
Main use is to determine LVR All subsequent tests require an amplitude found in the LVR Tests assumes sample is stable If not stable use Time Sweep to determine stability
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Linear Region: Modulus Non-linear Region: independent Modulus is a function of strain of strain
G'
Stress
Constant γ сCritical Strain Slope Đ
Strain (amplitude)
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In general, the LVR is shortest when the sample is in its most solid form.
Solid G’
Liquid
% strain
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LVR decreases with increasing frequency Modulus increases with increasing frequency
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Linear response to a sinusoidal excitation is sinusoidal and represented by the fundamental in the frequency domain
Nonlinear response to a sinusoidal excitation is not sinusoidal and represented in the frequency domain by the fundamental and the harmonics
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^K^ γ γ ω (t) = 0 sin( t) τ τ ω δ (t) = 0 sin( t+ )
γ γ ω >K^ (t) = 0 sin( t)
Fourier Series expansion: τ τ ω ϕ τ ω ϕ (t) = 1 sin( 1t+ 1) + 3 sin(3 1t+ 3) + τ ω ϕ 5 sin(5 1t+ 5) + ... τ ω ϕ = Σ n sin(n 1+ n) n=1 odd Lissajous plot: Stress vs. Strain (shown) or stress vs. Shear rate
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The material response is monitored at a constant frequency, amplitude and
Stress or strain temperature. Time
USES Time dependent Thixotropy Cure Studies Stability against thermal degradation Solvent evaporation/drying
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Important, but often overlooked Visually observe the sample Determines if properties are changing over the time of testing Complex Fluids or Dispersions
Preshear or effects of loading
Drying or volatilization (use solvent trap)
Thixotropic or Rheopectic Polymers
Degradation (inert purge)
Crosslinking
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