right what I want to talk about today is
do things touch do atoms touch do
molecules touch when I bring my hands
together are they actually touching or
are they repelling there's um quite a
lot on the internet and in the physics
forums and the chemistry forums and in
Reddit and a range of other fora about
this question so it's pretty close to my
heart because what arrived in my inbox
yesterday my mailbox was this so this is
my field of research non contact Atomic
Force microscopy the thing that puzzles
me is if things never really contact
each other why do we draw a distinction
between non contct and contact Atomic
Force microscopy what is that
distinction so there is a such thing as
contact Atomic Force micros there is
indeed there's contact mode and there's
nonc contact mode and I tell the first
year students about this a lot and they
actually a number of them have referred
to this um discussion on the internet
soing well actually you know
particularly in the first couple of
weeks when they come in you are aware
you're holding a cup of tea
oh
sorry just let me take one more slug of
it and I'll put it over here stupid it's
even closer to my heart because at the
moment um I've been working on this
paper all over the last couple of weeks
where we do what we've called and others
before us have called Point contact
probe microscopy notice the word contact
we go back to 1986 the inventors of the
scanning tuning microscope and the
atomic Force microscope so these are two
tools that allow us to see in not only
individual atoms but the state-ofthe-art
is to see individual bonds and I've
waffled at length about these in
previous videos just because you use the
word doesn't automatically mean it's
happening the important thing is we can
have a very rigid definition of what
contact means and the problem is with
the discussions some of the discussions
on the
internet the the argument seems to be
here's one atom or let's call it a
molecule here's one molecule here's
another molecule the problem with a lot
of the discussions is that well you
bring these molecules together you bring
them together and then they start to
repel
that's
wrong that's that's that's fundamentally
wrong that's not what happens you bring
two molecules together even if they
don't form a chemical bond they attract
it's something called the Vander world's
force and the where it originates from
is that you've got electrons over here
you've got electrons over here quantum
mechanics tells us that we don't pin
those electrons down to this particular
location so they're fluctuating here and
they're fluctuating here that means that
on on very small fractions of time
you're getting dipoles you're getting a
charge a slight charge in balance and
that slight charge and balance over here
and that slight charge and balance over
here gives rise to an attractive force
and that's really important be long
before you get to the repulsive side you
have an attractive Force where we can
Define contact very easily is at the
point where that attractive force is
balanced by the repulsive force and
those discussions on the internet are
quite correct there is a repulsive force
and it's ultimately due to something
called the PO Exclusion Principle which
basically
well let's not go into the whole details
of the poly Exclusion Principle lots of
videos and 60 symbols about that we have
a very well defined position at which
the net force the balance of the
repulsive and the attractive
contributions is zero and that actually
defines the bond length or defines the
intermolecular separation if we don't
have a chemical bond but we do not need
a sharing of electrons we do not need an
exchange of an electrons this vand
deval's Force which is everywhere around
us the typical example is how does the
gecko stick everywhere and it's due to
the F ultimately due to this F of Worlds
Force so let me point out two things get
those bows where you had them yep let me
point out two things you've just done
there first of all you've made a big
point of this is the definition of
context So you you're saying this is
just a definition no but it's it's what
we need to know is what do we do in
science all the time we try to have
rigid definitions we try to have rigid
um criteria you know and we think about
simple laws like f is equal ma okay Etc
I'm willing to accept that's I'm even
willing to accept that's a scientific
definition of contact even when you were
doing that analogy for me you kept the
balls aart you didn't make the balls
touch each other but the important thing
yeah but the the issue here is what we
mean by contact okay so right so is this
science or is this are we just talking
about words and definitions here guess
you know those two things are always in
intrinsically entangled you know
definitions and what we mean and what
analogies we use and what models we use
how accurate are they my concern is this
idea of which is demonstrably wrong
demonstrably because we do this
measurement in the lab on a daily basis
we take one molecule up here we bring it
to another and we measure that Force
directly and we can see where that Force
turns around where that Force goes to
zero due to this balance of the
attractive impulsive once you reach this
point that you tell me is contact yeah
can they move any closer if they move
closer they get a repulsive force in the
opposite direction that's the key thing
if they move closer they start to
experience a repulsive force and if they
never came into contact we can measure
this directly hang on we can measure
this directly in the experiment we can
bring this one molecule in and we can
crush this one how how do you get Atomic
positions which are changing if you're
crushing one molecule the simple fact
that you say once you reach the Conta
point if we move them even closer they
start repelling the simple fact that
they can even be moved closer how do we
says to me that they weren't in contact
no but how do we Define contact in this
case on the macroscopic world right how
do we we bring this in we bring it there
right so I can push down on it but if at
the point of contact right if we try I
really need a Phil I don't think anyone
is arguing about the definition of
contact on the macroscopic level
everyone okay right so on the
microscopic level the thing that I find
concerning is the idea that it's only
electron
repulsion attraction is not mentioned
even when you've got what are called
physical interactions so not chemical
interactions this type of interaction
put my hand as right at the atomic and
the molecular level there is an
attractive Force there before that
repulsive Force comes in and it's the
balance of those two
forces exactly so you know you've got
something falling under Gravity
right okay so we've got a reaction force
from the
floor you know how do you define contact
in this case it's we're basically just
scaling down that type of idea down to
the atomic level because why else would
we have another definition let me tell
you how how I feel about what you've
just said at the very start you use the
analogy of clapping hands and you said
some people say these two hands have not
come into
contact you said that's not true let me
tell you why you then showed me the
stuff about bol's Force you showed me
this this magic point where the two
forces reach an
equilibrium and you know what I took
from
that my hands don't touch when I clap
they get to this point where these
forces out and they never quite got let
because the repulsive Force way well
then let's define what you mean by
touching do you mean that the nuclear
cores come together do you mean that the
nuclei come because if so that's you
know there is really no point in
discussion because those energy scales
are complet completely out of reach
we're not going to have if we could have
gez if we could have nuclear fusion by
doing that wonderful it's about a
question of how do you define that
contact point and my point is that that
point in science is pretty well defined
well what are the fundamental quantities
in physics energy Mass Force Etc and so
if we're talking about Force let's make
a definition on the balance of forces so
you're making the definition of contact
the point where these two forces balance
out and we can't move the atom closer
yeah no no I didn't say we can't move
the atom closer okay well then if we can
move the atom closer I argue they're not
in contact because that we could move
them closer right so let's go back to
this let's go back to this are they in
contact hang on are we are you talking
about them as footballs or atom as
footballs are they in contact yes all
right if I do that I squeeze down on
that one right so that's exactly what's
happening when you push these in you get
a repulsive Force but the important
thing is where do you you have to it's
not like we have a switch on it's not
like the forces switch on and switch off
instantaneously over very very small
ranges they they very smoothly I must
admit that's one definition you can have
other definitions of the contact Point
yeah right so if it's a chemical bond
for example you could say right at the
point at which the overlap of the
electron density is such and such we'll
Define that as the important thing is
that the physics is such we can dress it
up in whatever mathematical language you
like but the physics is such that at the
atomic level we have contact because
otherwise
why would I why would we call these
techniques non cont and contact here
here's what I think here's a scientific
definition of what I think a normal
person would think of as contact between
two
particles I think and what all these
person what all these analogies are
trying to get out I think true contact
between two particles say two electrons
is the point at which the space occupy
by the electron could have no plank
lengths between it and the space
occupied by another
electron a point where you could no
longer fit like the point of the
electron has zero plank lengths between
the electron to the right but that
fundament in quantum mechanics you can
never have that but that's that's an EXT
yeah but that's an extreme definition
where would you use that definition
that's like saying you're going to
Define contact as when I overlap this
football with the other one and they
occupy the same space that's exactly
what you're saying well when you touched
them they got to a point where I could
no longer put a piece of paper between
them and so they're in contact for like
on the map you're arguing that if two
electrons occupy the same space not the
same space there's just no longer yeah
but the problem is you know from you've
done these videos over the years we
cannot Define an electron like a
particle like that so there I think
that's why things can't cont so
therefore the analogy my point again is
the analogy breaks down and you've got
to come up you've got to use a
scientific definition that everybody can
agree with I AG agree it has clear
delineation points which is very helpful
for a definition and if that's the
definition you want that's fine but I
think your definition of contact your
scientific definition of contact the
point where force x and force Y come
into balance isn't what the normal
person but the problem is you can't
extend what the normal person thinks
about touching down to the quantum level
so you have to come up with an another
definition your analogy breaks down
analogy entirely breaks down because you
cannot extend it down to the quantum
level so when people make these videos
or have these discussions that upset you
where they say things don't contact what
I think they're doing is these people
have
realized The Wonder of what's going on
at this microscopic but they have let me
finish okay sorry Sor they've realized
The Wonder of what's happening at this
Nano level and a really good way to
convey The Wonder of that is to say do
you know what these things aren't even
touching I AG I agree but you have to be
careful to the extent to which you over
play that new hype it this is the
important thing and it's the it's true
not just in terms of public engagement
in terms of of YouTube videos and
education via VIA online uh channels Etc
it's true also in terms of the science
we do it's very easy for us to overhype
our results and and and give them a
connection to the real world that might
not be justified you've got to be very
careful when you put analogies across to
explain what the deficiencies might be
in those analogies
all right