Complex Fluids Flashcards
What is yield stress
the value of stress at the yield point of the fluid
Explain the dynamic test
when the fluid is moving, finds the dynamic/lower yield point. The yield point is lower than the extrapolated data. The graph is stress (y) vs strain rate (x)
Explain the static test
When the fluid is static, the yield stress is greater than the dynamic/lower yield stress due to the internal resistance/microstructure. the graph is stress (y) vs strain rate (x)
What is slip?
When the displacement of the surface doesn’t correlate with the imposed/assumed strain angle
No slip example
Fluid sheared within concentric cylinder viscometer using Searle mode.
Explain complex fluids
Composition : typically multiphase, polymeric
Microstructure: typically highly disordered, shear sensitive, transient
Shear rate is equivalent to…
the velocity gradient
What type of relationship do viscosity and temperature have?
Arrhenius type relationship
Explain shear thinning
When the viscosity initially has an inverse relationship with the shear rate
n < 1
Explain shear thickening
When the viscosity initially has a positive relationship with the shear rate
n > 1
What can you not do on a double logarithmic axis
Extrapolate the data
What is the difference between the carreau model and the cross model?
The cross model has a less abrupt transition to the shear thinning region.
Explain the Searle system
Typical of CMT rheometer
Apply torque, M, the measure rotation
Explain the Couette system
Typical of SMT rheometer
Apply rotation, then measure torque
Assumptions for the concentric cylinder
Laminar flow
Incompressible flow
No radial flow
No axial flow
No slip at the surface of the cylinder
Explain the concentric cylinder
Bob goes inside hollow cylinder (cup)
h is height of cylinder
R1 is bob radius
R2 is Cylinder radius
Concentric cylinder, narrow gap
If distance between bob and cup becomes small, then the average stress can be used. As the gap becomes increasingly small, the flow becomes less influenced by the curvature of the geometry and simple shear conditions approached.
Narrow gap valid when R1/R2 >= 0.97
Concentric cylinder, infinite cup
The stress at the cup becomes 0.
Explain the cone and plate geometry
Requires less sample than CC system, can be operated in SMT or CMT
Shear rate is independent of radius
Parallel plate geometry
Can be used with grit unlike the C&G geometry. Can be used in SMT or CMT
Pipe flow, considering the relationship between pressure drop, flow rate, and rheology, assumptions
Laminar flow
Fully developed and steady flow
Incompressible fluid
Constant temperature
No pressure dependence on viscosity
No slip at the wall
Darcy Weisbach equation
change in pressure = f * L/D * density * velocity ^ 2
How can you test slip
Using parallel plate geometry, slip velocity is independent of gap size, whereas velocity difference across the sample varies as a function of the gap size and stress.
The slip velocity can be estimated after repeating tests varying the gap size.
Yield stress measures are significantly prone to
slip errors
Ways to mitigate slip
- Using a rough surface area on the geometry
- Use the vane system
where an absence of solid surface at the outer rotation means slip effects are avoided
Explain the slump test
- Single point measurement
- Open ended cylinder lifted off of fluid.
- Decrease in height measured
yield stress* = 1/2 + 1/2 * (S)^0.5
S = x / H
yield stress* = Yield stress / density x gravity x H
Slump test advantages + disadvantages
- Simple and cheap
- Not accurate
Sustainable operation
- Waste volume reduction by dewatering and recycle water
- Low solids conc => Newtonian
easy pipeline transport, large environmental impact - Increased solid conc => non-Newtonian
(shear thinning behaviour)
Transport behaviour modified
Improved environmental impact - Further increase conc yield stress materials, increased viscosities
pipeline transport now major concern
ideal environmental result
(dry stacking or concentrated waste)
Explain thixotropy
A decrease of the apparent viscosity under constant shear stress or shear rate, followed by gradual recovery when the stress or shear rate is removed. The effect is time dependent.
Thixotropic materials can recover their former properties when left undisturbed for sufficient time.
Difference between thixotropic material vs shear thinning material
Widley suggested that shear thinning materials are thixotropic
If recovery is very rapid, the phenomenon is observed as structural viscosity, if slow its observed as thixotropy.
Explain Rheopexy
An increase of the apparent viscosity under constant shear stress/rate followed by gradual recovery when the stress or shear rate is removed. Effect is time dependent.
Explain the mechanical structure of the kelvin Voight model
Spring and dashpot connected in parallel. Top and bottom bars must remain parallel
Kelvin Voight creep experiment
- system will begin to move at rate determined by viscosity of dashpot fluid
- limiting deformation will be achieved which will be determined by spring stiffness
- If stress removed, system will slowly return to original conformation because spring will want to return to original state.
Explain the creep experiment
Apply constant stress and consider how strain will change in time
Explain the mechanical structure of the Maxwell model
When the spring and the dashpot are connected in series
Explain the step strain experiment
Apply step strain, consider how stress will change in time
Result of performing step strain experiment on Maxwell model
1) apply step strain
2) spring will extend but dashpot has no time to
3) Dashpot moves to relax the stress in the spring
4) Spring eventually returns to original configuration
5) memory of the overall initial configuration is eventually lost
What happens when multiple Maxwell model elements are in parallel?
If there are N, modes, each with individual elasticity constant and viscosity, each will make an independent contribution to the overall stress.
Explain the Deborah Number
At long times material response dominated by viscous behaviour, at short times it is dominated by elastic behaviour.
De = relaxation time / process time scale
For De «_space;1 the process time is much longer than the relaxation time and elastic effects will be minimal.
Explain the mechanical structure of the burgers model
2 mode maxwell model or maxwell and voight in series
Burgers response to creep/recovery experiment
1) Maxwell spring reacts first …. step strain
2) Voight element also begins to react
delayed approach to limiting strain
3) Dashpot also reacts….constant strain
4) stress removed
5) maxwell spring reacts first and returns to original configuration
6) Voight also begins to react…. delayed approach to original configuration
7) dashpot stops reacting
ensure to know the graph to go with these steps
What is G’’
The loss modulus
rheological parameter that characterizes the viscous or energy-dissipating behaviour of a material under deformation.
What is G’
The storage modulus
rheological parameter that characterizes the elastic or energy-storing behaviour of a material under deformation.
What is SMT
separate motor transducer
angular displacement applied then torque measured
What is CMT
combined motor transduce
on a static plate torque applied then angular displacement measured
Explain a frequency sweep
Determines how a material’s viscoelastic properties change with varying deformation frequency.
Evaluates the material’s response to dynamic loading conditions at different rates.
Viscoelastic behaviour over range of time scales
resolution effects
internal effects using CMT
The amplitude of deformation is kept constant, and the frequency is systematically varied.
The material is subjected to sinusoidal deformation at different frequencies.
Explain Amplitude sweep
Examines how a material’s viscoelastic properties change with varying amplitude of deformation.
Investigates the material’s response to different levels of stress or strain.
The frequency of deformation is kept constant, and the amplitude is systematically varied.
The material is subjected to sinusoidal deformation at different amplitudes.
Always required ( determine LVR)
Often used to study yielding
Raw phase
Material behaviour limited to 0 <= phase <= 90
in the presence of inertia 0 <= raw phase <= 180
if raw phase => 90 the response is dominated by inertia therefore should consider using a lighter geometry or smaller gap (maintaining the strain)
