Problem Gel Point
You have the following sketch for system the gel point:
for both lines
(a) (8) Please show qualitative plot of n* (complex viscosity) versus frequency. co. (plotted on log-log
scale) with as many details as possible.
(b)(5) What is the relaxation time of your system the gel point?
(c) (5) You forgot to save the data file from the rheometer, the only information you have are the above
sketch and that G`` at c==] rad/s is 2,000 Pa at the gel point Is there any way you can estimate G' for
this frequency? If you can, please provide the information Otherwise, explain.
(d) (6) Your coworker Carl also thinks that the fractal dimension of the gel can be calculated So he does
some back-of the envelope calculations and tells you that de 2.11 and therefore your system is
undergoing diffusion-limited aggregation (DLA. Please comment: is Carl correct in making the claim
that the fractal dimension can be alculated? If so, is his analysis correct? If not, why not? Explain
Problem 2 Self-Healing Behavior of Thixotropic Hydrogel
You have created fragile hydrogel that casily broken when exposed to shear stress. However, when left
undisturbed for period of time after being broken, it can self -heal and regain its original gel structure.
You want to further characterize this hydrogel.
(a) (5) Describe what experiment(s) you could perform to calculate the maximum amount of shear stress
the hydrogel can withstand before breaking
(b) (6) Describe what experiment(s) you could perform to calculate the time required for the hydroge to
self-heal after being broken.
Problem 5 True or False (with explanation/justification)
(a)(4)A quick way to check if sample is thixotropic is to do a hysteresis experiment. The larger the area
in the loop. the more thixotropic the material is
(b) (4) During chemical crosslinking of polymer, you can measure G' and G" as function of time, and
use the crossover point of G' and G' as the gel point for some specific cases
(c) (4) It is advantageous to use the multiwave technique when performing frequency sweep experiment
of: sample its gel point.
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