Best way to calculate how fluids alter friction coefficients on an inclined plane

[Spencerm396]

I want to test several different fluids on an inclined planes and figure out which one is the most slippery. What would be the most scientifically sound way to measure this?

 

[mikeph]

Have you studied much fluid dynamics? The issue with slowly flowing fluids isn't due to friction against the solid boundary (we usually assume there is infinite friction and that the fluid doesn't slip at all), it is all about the fluid's viscosity, which is basically a measure of its ability to slip over itself.

This can be measured with a viscometer.

 

[Spencerm396]

No, I have not. I'm interested in particular with what seems to be lubricated friction. Specifically motor oils. I was thinking about lubricating a surface and performing the standard inclined plane test to see which one causes the object to move at the smallest angle?

Dos this make sense or would you still suggest the viscometer.

 

[mikeph]

Ah, I didn't realise it was for lubrication. I'm not sure to be honest, there is probably some good literature out there which describes standard testing for oils. But what you suggested makes sense, as long as you can ensure the volume of oils and particulars of the apparatus are kept constant, it sounds like a reasonable test.

Perhaps you might also try to keep the incline constant and drag the upper plate with a rope/pulley/weight combination, then measure the terminal velocity? That would give you a dynamic friction measurement, rather than a static friction measurement (which I think might depend very sensitively on the initial conditions and not be particularly relevant for lubrication, which is mostly concerned with moving parts).

I'm certain viscosity will be directly related to any results, but your approach seems far more practical.

 

[Chestermiller]

Mikey W is certainly spot on. But might I recommend a slightly modified approach. Measure the viscosities of the oils in a standard viscometer, such as a capillary viscometer or a plate and cone device. Or better yet, see if you can look up the viscosities of the oils in the literature. Temperature will be a factor, since viscosity is often sensitive to temperature. After you know the viscosities, see how the result of the inclined plane experiments correlate with the viscosities. Better yet, develop a fluid mechanics lubrication model to calculate in advance what to expect from the inclined plane tests.

 

[AlephZero]

Better yet, develop a fluid mechanics lubrication model to calculate in advance what to expect from the inclined plane tests.

The "standard" model is the Reynolds lubrication equation. http://www2.imperial.ac.uk/~ajm8/M3A10/lub.pdf (or google for other references).

But you will need to learn quite a bit of fluid dynamics, and math, before it makes much sense.