Do Atoms Ever Touch?

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 zom quite a lot on the internet and in the physics forums and the chemistry forums and in reddit and a range of other forum 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-contact and contact atomic force microscopy what is that distinction so there is such thing as contact atomic force microscopy there is indeed there's contact mode and there's non-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 so we'll actually you know particularly in the first couple of weeks when they come in just you are aware you're holding a cup of tea sorry just let me just take one more slug of it now put it over here stupid it's even closer to my heart because at the moment i've been working on this paper all over the last couple of weeks where we do what we 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 of the artist 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 well 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 van der waals 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 a particular location so they're fluctuating here and they're fluctuating here that means that on and very small fractions of time you're getting dipoles you're getting a charge a slight charge imbalance and that's slight charge and balance over here and that slight charge imbalance over here gives rise to an attractive force and that's really important if 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 interior are quite correct there is a repulsive force and it's ultimately due to something called the pauli exclusion principle which basically well let's not go into the whole details of the pauli 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 van der waals force which is everywhere around us the typical example is how does the gecko stick everywhere and it's due to the fan ultimately due to this founder of old's force so let me point out two things get those balls where you had them 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'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 when we think about simple laws like f is equal to m a okay etcetera 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 apart you didn't make the balls touch each other but the important thing yeah but the the the issue here is what we mean by contact okay so right so what do you science or is this are we just talking about words and definitions here i guess you know those two things are always 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 demonstrating 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 impulses once you reach this point that you tell me is contact yep 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 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 do you get atomic positions which are changing if you're crushing one molecule the simple fact that you say once you reach the contact point if we move them even closer they start repelling the simple fact that they can even be moved closer but how do we just says to me that they weren't encouraging no but how do we define contact in this case on the macroscopic world right how do 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 everywhere right okay well that's fantastic 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 this stuff about van der waals force you showed me this this magic point where the two forces reach an equilibrium and do you know what i took from that my hands don't touch when i clap but they get to this point where these forces but what okay and they never quite well let's no because the repulsive then let's do this 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 completely out of reach we're not going to have if we could have g's 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 so let's make a definition on the balance of forces so you're much more natural 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 atoms that's footballs are they in contact yes all right if i do that and 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 will 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-contact and contact 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 normal what all these analogies are trying to get at i think true contact between two particles say two electrons is the point at which the space occupied by the electron could have no planck lengths between it and the space occupied by another electron but a point where you could no longer fit like the point of the electron has zero planck lengths between the electron to the right that fundament in quantum mechanics you can never have that but that's that's an extra 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 mac but you're arguing that if two electrons occupy the same space not the same space there's just no longer well the problem 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 i think that's why things can't come yeah so therefore the analogy and 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 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 purpose 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 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 to get the physics right let me finish okay sorry sorry 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 agree i agree but you have to be careful to the extent to which you overplay that new hybrid this is the important thing and it's 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 over hype 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

Channel: Sixty Symbols

Views: 649,752

Keywords: sixtysymbols, Atom (Literature Subject), pauli exclusion principle

Id: P0TNJrTlbBQ

Length: 12min 5sec (725 seconds)

Published: Tue Aug 05 2014