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Rheology1. What is the unit of dynamic viscosity?
Pa*S
2. Deduce the unit of dynamic viscosity.
3. The dynamic viscosity of a certain (Newtonian) fluid is determined by means of a capillary
viscometer. For this purpose, the flow time of a (Newtonian) liquid with a known dynamic viscosity is
determined. Next, the flow time is determined for the unknown liquid. When can the formula
be used?
4. The viscosity of a non-gelled polymer solution appears to decrease when the shear stress increases.
With which type of flow behavior are we dealing with. Give an explanation for the flow behavior.
Pseudo-plastic flow behavior. For plastic and pseudo-plastic flow behavior the viscosity decreases
when the applied shear stress increases. But it is a non-gelled polymer solution, so it is not plastic
because no cross-links need to be broken in order for the solution to flow. It cannot be Newtonian,
since that always has a constant viscosity and for dilation the viscosity would increase.
5. What is the difference between plastic and pseudo-plastic flow? What causes this difference?
The links that need to be broken
6. With which type of flow behavior are we dealing with when a liquid exhibits a yield stress? Explain
why a liquid can exhibit yield stress.
Plastic flow behavior.
7. Explain why a liquid can exhibit a yield stress. Which statement can be made about the viscosity
when the shear stress is smaller than the yield stress?
Because of crosslinks. Yield stress is the amount of stress to be applied to get the liquid to flow. The
viscosity is then infinitely high and the liquid shows solid state behavior.
8. Name three methods to determine the dynamic viscosity of Newtonian fluids. Can these methods
also be used to determine the dynamic viscosity of non-Newtonian liquids? Explain.
1. Capillary viscometer
,2. Falling sphere viscometer
3. Rotation viscometer
Only rotation viscometer can be used for non-newtonians liquids. The falling sphere and capillary
viscometer have changes in shear stress during the measurement and for non-newtonians this
means also a different viscosity. The rotation viscometer works because it the spindle rotates at
different speeds, adjustable shear rates and the resistance is then measured. The results are plotted
in a calibration curve which is used to calculate the viscosity.
9. Which of the two rheograms shown below concerns dilatant flow? Explain.
B. Because the viscosity increase when more shear stress is
applied.
11. Explain that with the falling sphere viscometer the difference in density between the sphere and
the liquid may not be very large and the column may not be very short. (Assume that the accuracy of
the measurement is not the problem).
The falling sphere viscometer is a device used to measure the viscosity of a fluid by measuring the
time it takes for a sphere to fall through the fluid in a vertical column. The accuracy of the
measurement is not the problem, but if the difference in density between the sphere and the liquid
is not very large and the column is not very short, then the viscosity of the fluid will be harder to
measure accurately. This is because the force of gravity acting on the sphere is proportional to the
difference in density between the sphere and the liquid. A small difference in density and a short
column will lead to a weaker gravitational force, which in turn will lead to a longer falling time and a
greater uncertainty in the measured viscosity.
12. The rheological behavior of two liquids, A and B, has been studied. It appears that at a low shear
stress fluid A, has a higher dynamic viscosity than liquid B and with high shear stress the other way
around. These observations can occur when liquid A exhibits plastic behavior and fluid B Newtonian.
Name the 4 other combinations of flow behavior that can lead to the same observations.
1. Liquid A exhibits thixotropic and fluid B exhibits dilatant behavior.
2. Liquid A exhibits pseudoplastic behavior and fluid B exhibits dilatant behavior.
3. Liquid A exhibits pseudoplastic behavior and fluid B exhibits Newtonian behavior.
4. Liquid A exhibits Newtonian behavior and fluid B exhibits dilatant behavior.
13. Explain why a gelled polymer solution exhibits a yield stress.
A gelled polymer solution exhibits a yield stress because of the physical interaction between the
molecules in the polymer solution. This interaction creates a network of molecules that resists flow.
This network of molecules acts as a barrier to the movement of the solution and causes the solution
to require a certain amount of force before it begins to flow. This force is known as the yield stress.
14. Is the viscosity of a gelled polymer solution at a very small shear stress infinitely small, very small,
, very great or infinitely great? Explain.
Infinitely great, because cross-links need to be broken
15. A liquid exhibits pseudo-plastic flow behavior. Is de dynamic viscosity at a very small shear stress
infinitely small, very small, very great or infinitely great? Explain.
It is great, because the viscosity decreases when shear stress is applied. But not infinitely great
because no yield stress is necessary for the solution to flow.
16. Is the viscosity of a non-gelled polymer solution at a very low shear stress infinitely small, very
small, very great or infinitely great? Explain.
A non-gelled polymer solution gives pseudo-plastic flow behavior. This means there are no cross-links
to be broken, so no yield stress is necessary. The viscosity is not infinitely great. But with pseudo-
plastic flow behavior, the viscosity decrease when the shear stress increases. That’s why the viscosity
of a non-gelled polymer solution is very great.
17. A liquid exhibits plastic flow behavior. Is de dynamic viscosity at a very low shear stress infinitely
small, very small, very great or infinitely great? Explain.
With plastic flow behavior, yield stress is necessary in order for the liquid to flow. And yield stress is
the amount of stress that needs to be applied before the material will flow. When shear stress<yield
stress, the liquid shows solid state behavior because the dynamic viscosity is infinitely high.
18. Why is pseudo-plastic flow behavior also referred to as “shear thinning”?
Because the viscosity the decreases when the applied shear stress increases. So when there is more
shear stress applied, the liquid gets ‘thinner’ having less resistance to flow than when a lower shield
stress is applied.
19. Why is dilatant flow behavior also referred to as “shear thickening”?
Because the viscosity of the liquid increase when the applied shear stress increases. When there is
more shear stress applied, the liquid gets ‘thicker’, showing more resistance to flow than when a
lower amount of shield stress is applied.
20. It has been found that a certain polymer solution exhibits pseudo-plastic flow behavior. Show the
shear rate as a function of the shear stress in a graph. Explain why at a constant shear stress, the
viscosity of the solution decreases in time. What is the name of this phenomenon?
At a constant shear stress, the viscosity of the solution decreases in time because of shear thinning.
Shear thinning is a phenomenon that occurs when the viscosity of a fluid decreases with increasing
shear rate. This is due to the disruption of the molecules in the fluid, which reduces the friction
between them and therefore decreases the viscosity.
21. Show the shear rate as a function of shear stress in a graph for a Newtonian, plastic, pseudo-
plastic and dilatant material.
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