Re: What is cohesion and how does it affect lubricants?

The McGraw-Hill encyclopedia of physics defines cohesion as "the tendency
of atoms or molecules to coalesce into extended condensed states", which is
a fancy way of saying that atoms and molecules tend to attract each other
when far away, and repel each other when close together.

The attractive force can be thought of as coming about because the electrons
around an atom or molecule are jiggling around.  So, even though the total
electrical charge is zero, at any given instant in time, because of the
jiggling of the electrons, there can be a small electric field from an
atom or molecule.  Over time, this field averages out to zero, but it
still gives rise to an attractive force between atoms or molecules which
are well separated from each other.

If you get the atoms or molecules close enough, though, they begin to push
away.  This repulsion may have several sources.  The obvious
one is that if they get really close, the electrons clouds of the atoms
start to have a lot of overlap, and the nuclei begin to be able to "see"
each other through the electron cloud.  Since the nuclei are both positively
charged, they will repel.  Less obvious are effects due to the principles
of exclusion and uncertainty.  These are not "forces" in the way we usually
think of forces, but rather arise from the probabilistic nature of quantum
mechanics.  The precise explanation of this is a bit complicated, but
the end result is that the electrons around atoms or molecules resist
being squeezed together, or into a small volume.  Again, though, this 
resistance doesn't come about because of anything like the electric force, 
but rather because the probability of getting the electrons squeezed
together or into a small volume is small compared to that of them being
further apart.  Strange, but true.

The net effect of the attraction and repulsion can be summed up in the
following crude diagram.

      Force
	| |
	| |
	| |
	|  |
	|   |
	|    \
       0|-----*--------------------- Distance
	|      \         _________ 
	|       \     ___
	|        \   /
	|         ---
	|
	|
	
The horizontal axis is the distance between atoms or molecules, and
the vertical axis is the force between them.  A negative force means
attraction.  From this, we can see that two atoms or molecules will,
if left to themselves, come together until the force between them equals
zero.  At this point, any motion in one direction or the other will
tend to push them back to the zero point.

Unfortunately, I wasn't able to find a very good answer as to how this
relates to lubricants; but I think I can make a pretty good educated
guess.  Clearly, you'd like to have a lubricant have at least two properties
related to cohesion.  The first is that the molecules of lubricant have
fairly high attractive cohesive forces.  This ensures that the lubricant
"stays together".  For instance, the amount of cohesion in a liquid translates
pretty directly into its boiling point.  If motor oil had the same boiling
point as water, it would be useless as a lubricant for your car engine,
since the engine heat would boil it off quickly.  Thus, 
the cohesive forces between oil molecules must be fairly high.  The
second requirement is that the cohesive forces between the lubricant
molecules is simultaneously small, since they need to slide by each other
with ease; otherwise, the particular substance wouldn't be much use as
a lubricant.  

How can a substance simultaneously have both strong and weak cohesive 
attraction?  Here's one possibility: organic molecules often form as
chains.  In some cases, the ends of the chains have very strong cohesion,
but the sides do not.  So, the ends are responsible for keeping the
lubricant from getting torn apart by heat or mechanical forces, but
with most of the ends bound up, the molecular chains slide by each
other easily, since the cohesive forces between the sides are very small.

Note that this is a pretty specific case; there are no doubt many other
factors that go into making a good lubricant.

References
----------
The McGraw-Hill Encyclopedia of Physics, Second Edition, 
Sybil F. Parker, ed., 1993, ISBN 0-07-051400-3.