I do think Molekule is best at breaking these concepts into terms we can understand, but I am interested in learning 1911's position if I can figure out what he's talking about. I have been googling all these terms he is using and they just create a bottomless pit of additional questions. [/QB][/QUOTE]
Hey guys,
I finally found a good website that may help to explain viscosity as related to shear rate with explanatory graphs as I discussed in my post to the Molecular Man. There is also another one about additive types which says "Viscosity modifiers are generally oil-soluble organic polymers. With a given polymer system, shear stability decreases with an increase in molecular weight. The loss due to shear is reflected in a loss in lubricant viscosity. On the other hand, the "thickening power" of the viscosity modifier increases with an increase in molecular weight for a given polymer type. A performance balance must then be established which takes into consideration shear stability and viscosity needs as well as thermal and oxidative stability in actual engine operation. (Lubrizol)"
Now Molakule is a great contributor here, perhaps the best, and probably has forgotten more chemistry than I ever learned. However, his statement about larger molecules having increased mechanical stability was absurd and I feel that the truth of this matter should actually be somewhat intuitive to all and thus should not really require reference.
This website also discusses the sulfonates which are showing up in XOM Clean 7500. All this is at "http://www.oilmedic.com/lubprops.html"
Now to see some of please go to the website of a company which makes viscometers.
http://www.brookfieldengineering.com/support/viscosity/index.cfm#4. You will see a basic definition of viscosity which is the shear stress divided by the shear rate that it produces. The shear rate is the velocity gradient within the fluid in the direction perpendicular to the shear stress vector. It is the difference in speed divided by the difference in distance perpendicular to the stress field (assuming a linear velocity gradient). Their graph shows two infinitesimally thick layers of fluid moving at different speeds under the shearing action.
Now with the equation they give there, you can see that the shear stress is equal to the coefficient of viscosity times the shear rate. So if you graph shear stress versus shear rate ,the viscosity is the slope of the curve. Now scroll down a little lower and they show you how the shear stress relates to the shear rate for Newtonian and non-Newtonian fluids. However, these jabronies decided to plot shear rate versus shear stress instead of the other way around such that the inverse of the slope is the viscosity (maybe they think Molakule is hip since that is remniscent of the socialisitc way he chose to look at this?). Please observe the linear Newtonian fluid with constant slope (viscosity). Now, scroll down a bit more and look what happens to the psuedoplastic fluid as the shear rate and shear stress go up. You can see that the shear rate increases more for a given incremental increase in shear stress and the curve gets steeper (remember viscosity is the inverse of this slope). Thus you can see the viscosity dropping with shear rate (or shear stress as Molakule chooses to say). I truly hope the graphs are more clear than my incessant rambling and propensity towards gambling (see below).
I'll be out of town for a few days and I'll try to answer any questions about the little I know when I get back. My expertise is with solids and even for those, it's generally in the static condition (no flow) but the stuff we are discussing here is very fundamental such that someone not that knowledgeable (like Moi) can easily discuss such basic properties.
I'll be going to Vegas on a psychedelic trip
I'm reading murder novels.....trying to stay hip.
1911