Dear Dr. Scherge-

I'm not a tribologist, but I've done some studies involving friction on
silicon substrates (particularly with patterned chemical derivatization)
with a contact mode AFM.  I've seen some strong electrostatic forces as
well - even active charging of the tip and substrate, which I and a coworker
think results from the laser light creating electron- hole pairs in the
silicon.

I know this is kind of a vague question, but what is the mental model that
you use to explain friction to yourself?  In other words, how do you think
friction works on the nano/micro scale?

I ask the question because I have yet to come up with an explanation of
frictional force microscopy that I believe.  For instance, one can create
patterns of different chemistry on a silicon surface, and then observe that
(in contact mode under ambient conditions) the probe experiences higher
friction over amine groups than over hydroxyl groups (i.e. the cantilever
twists more).  If you think about this in terms of hydrogen bonds, you can
imagine (for instance) that the amine regions will form more hydrogen bonds
with the probe, so the cantilever should twist when moving from OH to NH2.
But why should it _stay_ twisted while in the NH2 region?  There is no change
in energy from point to point while in a region of defined chemistry, so there
should be no force, so there should be no twist, and therefore there should
be no friction signal.  But experimentally, there _is_ a friction signal,
so there must be a force.

Do you understand how this works?  Any insight would be appreciated.

Thanks very much,

Marc Unger (a puzzled grad student)
Caltech Chemistry




> We have been working on silicon and related materials concerning friction
> and adhesion for a couple of years. Since there was no commercial friction
> tester available, we built one (see bottom for URL).
> I assume that you are dealing with forces in the micro newton range.
> If this is so you have to expect electrostatic forces and after proper
> grounding stong capillary forces.
> It is therefore hard to say, which value friction has. We measured the
> friction force in air as well as in UHV and learned that the value drops
> as the pressure in the chamber decreases. This is caused by the desorption
> of water.
> Less water results in lower capillary forces. If the water has a thickness of
> just a couple of monolayers ordering phenomena set in, resulting in strong
> stick slips.
> As usual in tribology, it is too hard to forecast what friction will be like.
> Thus, it is always necessary to measure.
>
> Best Regards,
> Matt
>
>
> Dr. Matthias Scherge
> Microtribology Group
> Technische Universitaet Ilmenau
> PF 100565
> D-98684 Ilmenau
> Tel +49 3677 3782
> Fax +49 3677 3771
> ttp://imath.mathematik.tu-ilmenau.de/phys/personal/tribo.html
>
>
>
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