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Answers of Wounter Henrish Questions ofPharmaceutical Technology and Biopharmacy 2
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Exercizes Physical Chemistry PTB-2Rheology
1. What is the dimension of dynamic viscosity, ?
• De dimensie van dynamische viscositeit is: kg * m-1*s-1
2. Deduce the dimension of dynamic viscosity, .
3. The dynamic viscosity ( 2) 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 ( 1) is determined (t1). Next, the flow time (t2) is
determined for the unknown 𝑡liquid.
2
When can the formula 𝜂2 = 𝑡1 𝜂1 be used?
- This formula can be used when there is a dilute solution to the solvent.
- When the diluted solution and the solvent have the same (p) (density)
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 (Polymer solutions non-gelled). As the shear stress will
increase, the flow will become easier. This will decrease the viscosity
5. What is the difference between plastic and pseudo-plastic flow? What causes this
difference?
- By plastic flow behavior we are dealing mostly with cross-linked polymers
the polymers themselves have a yield stress. When you want to induce flow
you have to apply a shear stress higher than the yield stress. Here, you can see
flow due to smaller resistance against flow (e.g. tomato ketchup).
- By pseudo-plastic flow is a behavior seen by polymer solutions (non-gelled).
The solutions / compounds themselves will have no yield stress. The more
shear stress you apply, the less the viscosity is and the thinner the material
becomes. E.g. non-gelled polymer solutions.
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, The liquids have some links between them, which make this yield
stress. You have to apply higher shear stress in order to first break these
linkages and secondly to create a flow. It says something about above which
shear stress do we have deformation (mostly permanent). Here the material will
exhibit 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?
- When the shear stress is smaller than the yield stress molecules are not able to
move and the viscosity will be infinitely high.
8. Name three methods to determine the dynamic viscosity of a Newtonian
fluid. Can these methods also be used to determine non-Newtonian liquids?
1
, Exercizes Physical Chemistry PTB-2
- Capillary viscometer, falling sphere viscometer, rotation viscometer
- just the rotation viscometer can be used for non-Newtonian liquids.
9. Which of the two rheograms shown below concerns dilatant flow? Explain.
a)
b)
Figure B, At a very low shear stress, the viscosity is infinitely low. By
increasing the shear stress, the viscosity increases: it can be explained by
the graph. E.g. wet sand behavior.
2
, Exercizes Physical Chemistry PTB-2
10. Can a capillary viscometer be used to determine the viscosity of a non-Newtonian
liquid? Explain.
- No, because in non-Newtonian liquids when we apply a shear stress the viscosity
will be changed over the time → Viscosity is not constant
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).
- If the density difference was very big ➔ the measurement will be not
representative because then the particles will move either too fast or too slow along
the liquid. The column has not to be short, otherwise the spherical velocity is not
very well determined.
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.
- pseudoplastic-flow and dilatant flow
- plastic-flow and dilatant flow
- newtonian and dilatant flow
- newtonian and pseudo-plastic
!Volgorde anders! Dia 298
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 high, because there is no flow, the resistance here against flow is
infinitely high.
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 small shear stress
infinitely small, very small, very great or infinitely great? Explain.
- VERY GREAT, because we have plastic flow behavior and yieldstress,
viscosity is infinity high nothing happens there is no flow same as 14.
17. A liquid exhibits plastic flow behavior. Is de dynamic viscosity at a very small
shear stress infinitely small, very small, very great or infinitely great? Explain.
3
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