Your friend Hugo has been working with an unknown colloidal particle (named BQ)s
ID: 635320 • Letter: Y
Question
Your friend Hugo has been working with an unknown colloidal particle (named BQ)supplied to him from a company. Since very little is known about BQ except size, 0.8 microns in diameter, Hugo is trying to decipher its rheological behavior in various solvents (e.g, whether it acts as a hard sphere, has electrostatic, van der Waals, or static interactions, etc.) (a) (10) A high school intern working for Hugo mixes 0.15 volume fraction of BQ in a Newtonian solvent of viscosity 4 Pas. She then measures the viscosity and says that she found the suspension to have a viscosity of 55 Pas. However, when Hugo measures the same sample, he gets a viscosity of 4.75 Pa.s. Are either of these values possible or are both making mistakes in their measurements? If one is correct, who is correct, and if one of these measurements is accurate, can you say what is happening in terms of rheological behavior? Expain your answer (b) (10) Hugo's boss then walks into the lab with two more samples, BQ1 and BQ2. She says that one has charges on its surface while the other does not; however, she does not remember one is which. Hugo puts 0.15 volume fraction of each colloid in water. He finds the viscosity of B BQ2. However, when he puts 0.15 volume fraction of each colloid in nonpolar oil he finds the viscosity of BQ1 and BQ2 to be essentially same. Please explain what is going on in each case; is there any way you can determine from these experiments which colloid has electrostatic charges? Please justify your answers. [Note that in part b the experimental results are accurate, i.e., there were no errors in the measurements.]Explanation / Answer
a) by addition of colloid the solution becomes non-Newtonian (assumption)
At rest, micro-structured units are oriented randomly corresponding to their minimum energy state. At low levels of shearing, the system resists any deformation by offering a very high resistance either by exhibiting a very high value of viscosity or a yield stress. As the magnitude of external shear stress is gradually increased, the structural units (also known as ”flow units”) respond by aligning themselves with the direction of flow, or by deforming to orient along the streamlines, or by way of the disintegration of aggregates into small flow units or into primary particles. Polymer molecules which are coiled and entangled at low shear rates gradually become disentangled, and finally fully straighten out. All these changes in micro-structures facilitate flow, i.e., these lead to the lowering of their apparent viscosity with shear which leads to shear-thinning behaviour. so both correct
Here assuming that the particles are spheres and the solution thus is Newtonian the first measurement is more correct as the particles are dispersed more while in the second case the measurement is done after some time so this may have caused particles to settle thus accounting for lower viscosity.
b)
From the experiment, it has been seen that BQ2 has an electrostatic charge. when charged particles are immersed in an electrolyte solution its viscosity decrease because of ion movement.
In the experiment when water is added it acts as an electrolyte and facilitates ion movement.
while in oil the effect of the charged particle can not be observed because there is no electrolyte or any medium for charged particle movement.
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