Friction heating (Bowden & Hughes) + why water is a bad lubricant

11:37Okay. So now we've got a good way to get a layer of water >> in between whatever I'm standing on. Let's say skates. Let's use skates for this whole example. I'm standing on skates. Between my skates and the ice, there's going to be a tiny liquid of water because layer of water because I'm going to move with the skates. That's going to cause friction. and the friction is co going to cause heating and that heating is going to cause some melting and I'm going to get a layer of water. >> What it makes me think about is when you know you were in kindergarten or first grade, you're running around and you were in the classroom and the classroom had a rug and somehow you ended up sliding on your knees like Harry Maguire after scoring a header and you'd get rug burn and it would feel hot.

12:18>> Yes. because your knees were moving on the surface. That was friction and it was heat and the heat burned my flesh. But similarly, the >> but also if you were to measure the temperature of the carpet >> y >> it would be a tiny bit >> it would be warmer. And so this the point is you know this contact between the bottom of the skate >> and the top of the ice layer. >> We understand intuitively as just regular people Yeah. >> that when we rub our hands together or we slide our knees on rugs, there's heat there. There's heat there. >> And that's the entry point potentially for what they're trying to say here. >> Yep. Yep. Exactly. >> Bowden and Hughes. >> Yes. That was Bowden and Hughes in 1939 out of the Royal Society. >> Okay, that's all fine except the

13:01lubricant in this case is still liquid water. >> Yes. >> In between the two contact surfaces, right? There's ice. There's my skates that are metal. And the lubricant in between is liquid water. And that's what you're saying is what's causing slipperiness, right? Liquid water is a terrible lubricant. [laughter] Okay? Because it has very low viscosity. >> And that's kind of the point. The reason why life works, the reason why water can go up the phylm and and xylem of plants is because it's very low viscosity. So the leaf when the leaf pulls on some water, it'll go all the way up to the roots and that force will get relegated. Right. That makes sense. >> So it's it's it's actually a feature of water that it's very low viscosity.

13:43>> Yes. But that's not going to work in this case because whenever we want to lubricate metal on metal for example we use oil right like car oil and things like that >> those things have very high viscosity the point of high viscosity is if I have a contact between two pieces >> I don't want the liquid in between to give way [clears throat] >> right I want it I want there to be a film in between such that there's no metalonmetal contact or in this case solid on solid contact, you need high viscosity such that the the the liquid actually resists >> that tension, >> that the pressure >> the pressure. Yeah. >> Liquid water is not going to do that.

14:24>> Got it. >> Liquid water is just going to get the hell out of the way. >> And so the thickness of that film, if you were to actually the hydrodnamic thickness of that film >> is going to be nanometers or less compared to something like a highly viscous substance like oil. That's why we use oil in our engines and not water. If we used water, the engine parts would be rubbing up against each other. It would not be a good time. >> Right? So, that's the central discrepancy. Now, we're we're at 1939. We figured out why there's liquid water. >> Yes. >> But we haven't quite figured out how the liquid water becomes this lubricant. It really shouldn't, >> right? Because because what we're saying is just by the skate blade in our

15:05analogy contacting the ice >> Mhm. >> on its own based on what we know it should not create that nanometer scale layer of water. >> Yeah. Yeah. >> Like that how is that happening? >> Yeah. Like if it's a nanometer scale, no if we do the if we do the physics the the the scale should be nanometers. >> It should be nanometers. >> But it needs to be a lot thicker. Nanometers is like you know tens of atoms. 1 nanome is 10 hydrogen atoms. >> That's not enough to like keep me from contacting the ice, >> right? You know what I mean? >> Yes. >> So, so that's the idea. How do we get a >> how do we get a a layer of water and and we get that layer of water to be more

15:46viscous than bulk liquid water. >> Understood. >> Okay. And that's where there was this