PROFESSOR CIMA: So, the next level of this is, instead of linear contact,
its areal contact.
So let's say I had a solid material with some area contact here.
And so this is solid one and this is solid two, and I bring them together
like the PDMS I showed you last time.
I'll show it again.
Now, remember, this interaction on a molecule per molecule basis goes as
one over r to the 6-th.
So you think you don't have to get very far away from one another before
the energy of interaction goes away.
Remember, r is of the dimensions of the molecule.
But now I got a hunk of PDMS here.
PDMS is CH3CH3O.
This is silicone, the same stuff you caulk your windows with.
You get the idea.
And on... it's a polymer.
So that's the silicone that we're going to show you in a moment, but
this is some area A. And it turns out that, in order to sum up this energy
of interaction, I have to take a little few molecules here and
integrate the energy of interactions over all the molecules here.
And then I do the same thing over here and integrate it all over here, and
guess what happens.
Instead of r^6, the energy of interaction over this area is
proportional to the area times divided by r^2, and it's a negative.
So then all of a sudden, the distance dependence went from r^6 to r^2,
because I'm not looking at one molecule, but I'm adding it up over
all these molecules.
And that's why--
in fact, I don't even need to go to this demonstration.
Have you ever wondered why, when I grind off pieces of this chalk on the
chalkboard that it stays there?
It's not covalently bonded.
I mean, I just take an eraser and wash it away.
What it is is little pieces of dust that are adhering to this board
because of what?
That.
Van der Waals forces.
London dispersion forces.
When they get to these macroscopic objects or even large molecules, they
start calling them van der Waals forces, I should say van der Waals.
And your text will go back and forth on this.
Most of them do.
But in the-- historically how it developed, it started with London at
the atomic level.
He was actually solving Schrodinger's equation.
And then the guy that actually did these behavior of macroscopic objects
was this guy van der Waals, and they at one point realized, hey, it's the
same thing.
And so the name goes back and forth.
Now, here's the PDMS example.
I only did it briefly, but there's two pieces of PDMS, this stuff, cured in
the bottom of a beaker or something like that.
One of my students has got a PDMS project.
And if you just touch them together, they stick.
And this is not because there's any covalent bonds there at all.
It's just this integration of these dispersion forces between the two
bodies that makes them stick.
Now, interestingly enough, if I put some dust on it, I don't know
if that will work.
Maybe this stuff is too--
If I put some dust on it, it doesn't stick.
Now, why is that?
No, what happens is I decreased area, right?
Because now the interface has got little tiny particles on it, dust in
between them, and now it may be just as strong of interactions, but the
area of contact between the two bodies is much, much smaller.
So this stuff is real.
Now we're starting to get into the area of the sort of materials.
This chemistry does impact the behavior of materials.
In fact, it's everything about the behavior of materials.