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> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecul
by bqmjjx0kac 2y ago
> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule.
How did he know that the film of oil was one molecule thick?
It feels like a huge assumption to me, but maybe this blog post left something out.
- deleted 2y ago[deleted]
- cb321 2y agoThe molecular scale was well estimated (at least for the gaseous phase of matter) by 1865 (25 years earlier not 5 as the incorrect HN title would suggest) https://physics.stackexchange.com/questions/13757/how-was-avogadros-number-first-determined https://physics.stackexchange.com/questions/13757/how-was-av... (and guesstimated by just "following your nose" in Gandalf-of-Lord-Of-The-Rings-ese in 1646!) Rayleigh's experiment is just accessible requiring very little training / background to describe. To interpret as a monolayer is honestly probably not so accessible at all, though, and a weakness of the atomsonly.news piece and seemingly not even done by Rayleigh himself. Modern retconning as zokier says elsewhere.
- munchler 2y agoAgreed. The experiment actually gives an upper limit on the size of a molecule in one particular dimension. Still a very useful result.
- layer8 2y agoIt isn’t necessarily an upper bound. The molecules might spread out more distant than their size.
- bagels 2y agoWouldn't that provide an upper bound then? If the real size is equal to or less than the calculated size?
- gus_massa 2y agoIn a very unlucky world, they can form a 2D net, with molecules instead of strings and a lot of tiny holes. If this seams impossible, remember that when water freeze into ice, it expands to a 3D "net" with empty holes.
- FpUser 2y ago[flagged]
- PhasmaFelis 2y ago> Cortesy of ChatGPT: Can we not, please? At the very least not unless you personally have the knowledge to confirm that ChatGPT didn't make any mistakes, as it did here.
- ajuc 2y agoThis should be a rule on hacker news.
- elevaet 2y agoAgreed. Everyone here knows how to use chatgpt. We come here for a different way to share, learn and interact.
- fii 2y agoAgreed, although it does make me wonder whether the number of mistakes chatgpt comments make would truly be greater (or at the least more harmful) than the outstanding number of folk who are confidently wrong in a way only obvious to domain experts. It’s easier to be skeptical of a bot.
- ummonk 2y agoSeems questionable that Rayleigh would have known that oil molecules are hydrophobic on one end and hydrophilic on the other.
- deleted 2y ago[deleted]
- happytoexplain 2y agoIt feels intuitive that a thin fluid on a low-friction surface (like water) would spread out "as much as possible" given enough time. There certainly may be confounding factors, but it seems like a reasonable thing to pin as an "assumption" in a hypothesis. I.e. he didn't have to "know" - assumptions are OK, and I don't feel like this one is huge.
- chmod775 2y ago> It feels intuitive that a thin fluid on a low-friction surface (like water) would spread out "as much as possible" given enough time. Most fluids do not behave this way in most circumstances, because of surface tension, so it's really not intuitive. This experiments is one of the few ways you can get an accurate measurement. Many other fluids will either mix or end up as bubbles/blobs many orders of magnitude thicker than a molecule.
- komali2 2y agoI'm confused, the blog wrote "known amount of water," so was it a closed little area like a bathtub? If you added a ton of oil wouldn't it spread out as much as possible aka 600 molecules thick or whatever? Or did he pour it into a huge lake or something?
- Cheer2171 2y agoOne drop in a soup bowl sized petri dish, measure the area it covers.
- stolen_biscuit 2y ago> How did he know that the film of oil was one molecule thick? He didn't. It was an assumption
- tech_ken 2y agoBlog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_literature/Rayleigh1890.pdf https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite... Rayleigh's experiment was actually trying to solve for the minimum thickness of oil required to stop some camphor shavings from moving around on the water. He never states it explicitly, but I think the assumption is that the minimum thickness required to stop the shavings' movement would be such that the oil volume 'just' covers the surface, ie. is 1 molecule thick everywhere and hence the shavings never touch water. I think he's specifically making a slightly more clever point about surface tension, but that's a little beyond me.
- youainti 2y agoCamphor would release compounds that adjust the surface tension of water. So the oil would break that direct relationship.
- tech_ken 2y agoAhhh that's cool, thanks for clarifying
- zokier 2y agoReading the paper, there is no mention of sizes of molecules. Did Rayleigh actually make the connection between film thickness and molecular size at some point? Or is that just modern retconning?
- tech_ken 2y agoIt's really (really) quick but the first line of the second para is: > In view, however, of the great interest which attaches to the determination of molecular magnitudes, the matter seemed well worthy of investigation... So it seems like his main goal was to understand the size of molecules via his film-thickness measurements
- zokier 2y agoReplying to myself, I found 1899 paper which is more explicit on the matter, and shows how Rayleigh was not all that certain about the results: The comparison of the present with former results throws an interesting light upon molecular magnitudes. It has been shown (Proc. Roy. Soc. March 1890) that the thickness of the film of olive-oil calculated as if continuous, which corresponds to the camphor-point, is about 2.0 μμ while from the present curves it follows that the point at which the tension begins to fall is about half as much, or 1.0 μμ [...] If we accept this view as substantially true, we conclude that the first drop in tension corresponds to a com- plete layer one molecule thick, and that the diameter of a molecule of oil is about 1.0 μμ XXXVI. Investigations in Capillarity:—The size of drops.—The liberation of gas from supersaturated solutions.—Colliding jets.—The tension of contaminated water-surfaces If we assume that the "about 2.0 μμ" value is just the previously mentioned 1.63 nm value rounded up, then that throws a wrench into the story, in particular this bit from blog post > Rayleigh’s final result was 1.63 nanometers. Olive oil is mainly composed of fat molecules called triacylglycerols, and we now know that they measure about 1.67 nanometers in length, implying that Rayleigh’s “primitive” estimates were off by just 2 percent is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that!
- jjk166 2y agoIf there were multiple layers of molecules then the film would spread out over a wider area. With repeated experiments it would be clear that films are always an integer multiple of this thickness and never thinner.
- MereInterest 2y agoExcept that you could have part of the surface covered in 1 molecule, and another portion covered in 2 molecules. Since you never directly measure the thickness, this would produce the same apparent thickness as a uniform 1.5 molecule thick layer.
- jjk166 2y agoThat's not how fluids work. The molecules spread out to form an even surface everywhere, so you can't have local high spots. You'd have to put in enough oil to cover the entire surface, and then put in more.
- kccqzy 2y agoYeah and the point is that there is more embedded knowledge about surface tension here. For example if I put a small drop of water onto my desk, it does not spread out into a thin film of one water molecule thick. It remains a droplet due to surface tension.
- jjk166 2y agoAnd if you put an oil drop into that little bead of water it will spread out because it destroys the surface tension. That's why Rayleigh was playing around with it to begin with, and the reason why a thin film of oil calms the surface of water.
- tantalor 2y agoScientists frequently have to make assumptions in order to make progress. Famous example is Darwin figured out that traits are inheritable by natural selection, and this is the driving force of evolution, without having any concept of the physical nature of DNA, or how genes could change (eg. by DNA mutation) to develop adaptations and thus make an organism more fit.
- kccqzy 2y agoThis is why I guess I was never really interested in scientific experiments personally and decided to study mathematics. These assumptions don't seem justified to me. At least in mathematics you always state these assumptions or hypotheses very clearly, or make them into axioms.
- tantalor 2y agoI don't know if anybody challenged Darwin on that point. It's an interesting question. The simplest explanation is that he deduced such a physical mechanism must exist but the science and technology available at the time could not locate it.
- taberiand 2y agoPerhaps at the time it was sufficient to define "molecule of oil" as "the height of the amount when spread maximally across the surface of water", and it just so happens that height is only 1 actual molecule
- mda 2y agoI have also immediately thought the same question. This is probably the most crucial part of the whole estimation and indeed left out in the article.
- bloak 2y agoIf you try the experiment lots of times with drops of different sizes you find the oil layer always has roughly the same thickness. That's an interesting observation that calls for some kind of explanation, and the hypothesisis that the thickness of the oil layer is the length of one molecule is perhaps the most obvious and plausible explanation. Then one would look for confirmation, of course. (What was the next thing to confirm this, historically?)
- shultays 2y agoAn idea: if oil forms a two dimensional shape, ie a single layer of molecules, then adding 2x amount of oil would give you twice the area. If it is three dimensional, say oil makes a bubble, then it would look smaller. Of course this also fails if the oil formed a disc of X layers of molecules
- Jeff_Brown 2y agoAnd even assuming it's one molecule thick, how did he know how tightly the oil molecules in that layer pack together?