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EG-M01 Complex Fluid > Complex Fluids > Flashcards

Complex Fluids Flashcards

1
Q

What is yield stress

A

the value of stress at the yield point of the fluid

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2
Q

Explain the dynamic test

A

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)

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3
Q

Explain the static test

A

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)

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4
Q

What is slip?

A

When the displacement of the surface doesn’t correlate with the imposed/assumed strain angle

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5
Q

No slip example

A

Fluid sheared within concentric cylinder viscometer using Searle mode.

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6
Q

Explain complex fluids

A

Composition : typically multiphase, polymeric

Microstructure: typically highly disordered, shear sensitive, transient

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7
Q

Shear rate is equivalent to…

A

the velocity gradient

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8
Q

What type of relationship do viscosity and temperature have?

A

Arrhenius type relationship

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9
Q

Explain shear thinning

A

When the viscosity initially has an inverse relationship with the shear rate

n < 1

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10
Q

Explain shear thickening

A

When the viscosity initially has a positive relationship with the shear rate

n > 1

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11
Q

What can you not do on a double logarithmic axis

A

Extrapolate the data

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12
Q

What is the difference between the carreau model and the cross model?

A

The cross model has a less abrupt transition to the shear thinning region.

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13
Q

Explain the Searle system

A

Typical of CMT rheometer

Apply torque, M, the measure rotation

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14
Q

Explain the Couette system

A

Typical of SMT rheometer

Apply rotation, then measure torque

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15
Q

Assumptions for the concentric cylinder

A

Laminar flow

Incompressible flow

No radial flow

No axial flow

No slip at the surface of the cylinder

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16
Q

Explain the concentric cylinder

A

Bob goes inside hollow cylinder (cup)

h is height of cylinder

R1 is bob radius

R2 is Cylinder radius

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17
Q

Concentric cylinder, narrow gap

A

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

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18
Q

Concentric cylinder, infinite cup

A

The stress at the cup becomes 0.

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19
Q

Explain the cone and plate geometry

A

Requires less sample than CC system, can be operated in SMT or CMT

Shear rate is independent of radius

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20
Q

Parallel plate geometry

A

Can be used with grit unlike the C&G geometry. Can be used in SMT or CMT

21
Q

Pipe flow, considering the relationship between pressure drop, flow rate, and rheology, assumptions

A

Laminar flow

Fully developed and steady flow

Incompressible fluid

Constant temperature

No pressure dependence on viscosity

No slip at the wall

22
Q

Darcy Weisbach equation

A

change in pressure = f * L/D * density * velocity ^ 2

23
Q

How can you test slip

A

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.

24
Q

Yield stress measures are significantly prone to

A

slip errors

25
Q

Ways to mitigate slip

A
  • 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
26
Q

Explain the slump test

A
  • 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

27
Q

Slump test advantages + disadvantages

A
  • Simple and cheap
  • Not accurate
28
Q

Sustainable operation

A
  • 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)
29
Q

Explain thixotropy

A

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.

30
Q

Difference between thixotropic material vs shear thinning material

A

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.

31
Q

Explain Rheopexy

A

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.

32
Q

Explain the mechanical structure of the kelvin Voight model

A

Spring and dashpot connected in parallel. Top and bottom bars must remain parallel

33
Q

Kelvin Voight creep experiment

A
  1. system will begin to move at rate determined by viscosity of dashpot fluid
  2. limiting deformation will be achieved which will be determined by spring stiffness
  3. If stress removed, system will slowly return to original conformation because spring will want to return to original state.
34
Q

Explain the creep experiment

A

Apply constant stress and consider how strain will change in time

35
Q

Explain the mechanical structure of the Maxwell model

A

When the spring and the dashpot are connected in series

36
Q

Explain the step strain experiment

A

Apply step strain, consider how stress will change in time

37
Q

Result of performing step strain experiment on Maxwell model

A

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

38
Q

What happens when multiple Maxwell model elements are in parallel?

A

If there are N, modes, each with individual elasticity constant and viscosity, each will make an independent contribution to the overall stress.

39
Q

Explain the Deborah Number

A

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 &laquo_space;1 the process time is much longer than the relaxation time and elastic effects will be minimal.

40
Q

Explain the mechanical structure of the burgers model

A

2 mode maxwell model or maxwell and voight in series

41
Q

Burgers response to creep/recovery experiment

A

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

42
Q

What is G’’

A

The loss modulus
rheological parameter that characterizes the viscous or energy-dissipating behaviour of a material under deformation.

43
Q

What is G’

A

The storage modulus

rheological parameter that characterizes the elastic or energy-storing behaviour of a material under deformation.

44
Q

What is SMT

A

separate motor transducer

angular displacement applied then torque measured

45
Q

What is CMT

A

combined motor transduce

on a static plate torque applied then angular displacement measured

46
Q

Explain a frequency sweep

A

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.

47
Q

Explain Amplitude sweep

A

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

48
Q

Raw phase

A

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)

EG-M01 Complex Fluid (2 decks)

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