[curiouscat]

I always assumed implicitly that the coefficient of static friction was always more than the coefficient of dynamic friction.

But I was reading an article on lubricants and it mentioned that this may not always be true. I wonder if anyone knows the mechanism behind this.

This would mean it'd take that body less force to start it moving than to keep it moving. Sounds strange. The source was pretty trustworthy though; I'll try and post a link if I can.

[Corribus]

Never heard of this but apparently it's possible.

Maybe this will help:

http://scholar.lib.vt.edu/theses/available/etd-07282008-135807/unrestricted/LD5655.V855_1995.L42.pdf

Esp section 2.1.2.

[curiouscat]

Thanks. I still find it bizarre though. 

[curiouscat]

This was the citation (Kirk Othmer Encyl. of Chem. Tech.):

The static coefficient measured for a hard steel surface on another hard
steel surface is 0.78. The dynamic coefficient measured for hard steel on hard
steel is 0.42. When a thin film of light mineral oil is applied to these surfaces,
the static coefficient drops to 0.23. The dynamic coefficient with a light oil film
drops to
0.1. Adding a friction modifier to the oil can reduce or reverse the difference
between the two coefficients. Adding stearic acid to the lubricant, eg, for
hard steel on hard steel, reduces the static coefficient to 0.0052, which is lower
than the dynamic coefficient, 0.029

[Borek]

Makes me think of non-Newtonian liquids - dilatants (AKA shear thickening materials).

[curiouscat]

Makes me think of non-Newtonian liquids - dilatants (AKA shear thickening materials).

Yes. That was what I thought. Hard to think of why a surface would behave that way though.

[Borek]

Once you add a lubricant it is no longer just a surface/surface thing. Properties of the film start to play an important role, apparently it dominates the situation.

Edit: it makes me think of some optical devices I used in the past, like binoculars or lenses. Quite often they have some ring to change focus or zoom level, and in many cases the ring rotates easily when moved slowly, but definitely harder when you try to rotate it fast. It can be similar situation.

[curiouscat]

Once you add a lubricant it is no longer just a surface/surface thing. Properties of the film start to play an important role, apparently it dominates the situation.

Edit: it makes me think of some optical devices I used in the past, like binoculars or lenses. Quite often they have some ring to change focus or zoom level, and in many cases the ring rotates easily when moved slowly, but definitely harder when you try to rotate it fast. It can be similar situation.

Reminds me of drag.

[Enthalpy]

Non-Newtonian fluids... and maybe more explanations?

Like seizure, even if weak and local. It would appear only after some way, hence not at static friction.

Or heat. It does happen with PTFE, which has several transitions little above room temperature - call one vitrous if you like - that increase the friction coefficient a lot. This is one reason among many that make pure PTFE seldom usable as a friction material, creepage being one other.

Or a lubricating film destroyed by movement. Can be reversible. A monomolecular layer, or a badly wetting liquid film.