Complex Fluids

Question 1

What is yield stress

Answer 1

the value of stress at the yield point of the fluid

Question 2

Explain the dynamic test

Answer 2

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)

Question 3

Explain the static test

Answer 3

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)

Question 4

What is slip?

Answer 4

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

Question 5

No slip example

Answer 5

Fluid sheared within concentric cylinder viscometer using Searle mode.

Question 6

Explain complex fluids

Answer 6

Composition : typically multiphase, polymeric

Microstructure: typically highly disordered, shear sensitive, transient

Question 7

Shear rate is equivalent to…

Answer 7

the velocity gradient

Question 8

What type of relationship do viscosity and temperature have?

Answer 8

Arrhenius type relationship

Question 9

Explain shear thinning

Answer 9

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

n < 1

Question 10

Explain shear thickening

Answer 10

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

n > 1

Question 11

What can you not do on a double logarithmic axis

Answer 11

Extrapolate the data

Question 12

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

Answer 12

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

Question 13

Explain the Searle system

Answer 13

Typical of CMT rheometer

Apply torque, M, the measure rotation

Question 14

Explain the Couette system

Answer 14

Typical of SMT rheometer

Apply rotation, then measure torque

Question 15

Assumptions for the concentric cylinder

Answer 15

Laminar flow

Incompressible flow

No radial flow

No axial flow

No slip at the surface of the cylinder

Question 16

Explain the concentric cylinder

Answer 16

Bob goes inside hollow cylinder (cup)

h is height of cylinder

R1 is bob radius

R2 is Cylinder radius

Question 17

Concentric cylinder, narrow gap

Answer 17

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

Question 18

Concentric cylinder, infinite cup

Answer 18

The stress at the cup becomes 0.

Question 19

Explain the cone and plate geometry

Answer 19

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

Shear rate is independent of radius

Question 20

Parallel plate geometry

Answer 20

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

Question 21

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

Answer 21

Laminar flow

Fully developed and steady flow

Incompressible fluid

Constant temperature

No pressure dependence on viscosity

No slip at the wall

Question 22

Darcy Weisbach equation

Answer 22

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

Question 23

How can you test slip

Answer 23

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.

Question 24

Yield stress measures are significantly prone to

Answer 24

slip errors

Question 25

Ways to mitigate slip

Answer 25

- 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

Question 26

Explain the slump test

Answer 26

- 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

Question 27

Slump test advantages + disadvantages

Answer 27

- Simple and cheap - Not accurate

Question 28

Sustainable operation

Answer 28

- 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)

Question 29

Explain thixotropy

Answer 29

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.

Question 30

Difference between thixotropic material vs shear thinning material

Answer 30

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.

Question 31

Explain Rheopexy

Answer 31

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.

Question 32

Explain the mechanical structure of the kelvin Voight model

Answer 32

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

Question 33

Kelvin Voight creep experiment

Answer 33

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.

Question 34

Explain the creep experiment

Answer 34

Apply constant stress and consider how strain will change in time

Question 35

Explain the mechanical structure of the Maxwell model

Answer 35

When the spring and the dashpot are connected in series

Question 36

Explain the step strain experiment

Answer 36

Apply step strain, consider how stress will change in time

Question 37

Result of performing step strain experiment on Maxwell model

Answer 37

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

Question 38

What happens when multiple Maxwell model elements are in parallel?

Answer 38

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

Question 39

Explain the Deborah Number

Answer 39

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

Question 40

Explain the mechanical structure of the burgers model

Answer 40

2 mode maxwell model or maxwell and voight in series

Question 41

Burgers response to creep/recovery experiment

Answer 41

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

Question 42

What is G''

Answer 42

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

Question 43

What is G'

Answer 43

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

Question 44

What is SMT

Answer 44

separate motor transducer angular displacement applied then torque measured

Question 45

What is CMT

Answer 45

combined motor transduce on a static plate torque applied then angular displacement measured

Question 46

Explain a frequency sweep

Answer 46

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.

Question 47

Explain Amplitude sweep

Answer 47

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

Question 48

Raw phase

Answer 48

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)