chapter 2 Flashcards
Give a plausible explanation for shear thinning for emulsions
When you start shearing such a cluster, there is a high resistance. These particles need to move past each other. Which gives you a high viscosity. If you start increasing the shear rate; you start breaking down the cluster until you have individual particles which will start to align themselves in layers. A layered structure will have far less resistance to shear than a structure with clusters. This is why dispersions are extremely shear thinning.
Give a plausible explanation for shear thickening in an emulsion
Layers are disrupted → clusters are formed → viscosity goes up
The experimentalist repeats the experiment with more weight, and finds that the
viscosity has decreased. Explain this result
Shear thinning: cheese consists of a network of protein strands; deformation breaks down the network and therefore viscosity decreases with increasing shear rate
Up to a strain of about 0.09 the storage modulus is constant. At larger strains the
modulus first increases to a maximum of 1180 Pa, at a strain of 0.16, and then decreases. Which kind of behavior occurs at strains between 0.09 and 0.16? (The gel was prepared at a pH where covalent sulfur bonds can be formed between the protein molecules)
Strain hardening: the gel is covalently cross-linked (sulphur bonds) and displays rubber-like behavior.
What happens with the system at strains above 0.16? The storage modules decreases after 0.16.
Gel fractures: strain thinning
A cubic piece of an elastic polymer gel with sides of 5 cm is placed between two parallel plates. An experimentalist applies a horizontal force of 5 N to the upper plate,
and measures delta L.
The experiment is repeated with a new sample, but now more force is applied. The value of the modulus is now lower. Explain this phenomenon in terms of the structure of the gel
The higher stress has resulted in shear softening of the sample. The resulting larger deformation has disrupted the structure of the polymer gel. The chains in the polymer
have partially disentangled leading to a weaker network
The experiment is repeated a third time, again with a new sample, and again with a force of 10 N, but now the upper plate moves at twice the speed. Do you expect the
value of G to be higher or lower than your answer in b). Explain your answer.
The value of G should be higher. This is a viscoelastic material, and when such a material is deformed faster, there is less time for relaxation to occur. Hence more
energy is stored in the polymer network and G is thus higher.
