Hydrogen and helium are nice and floaty, but they are so little! They escape thru every tiny pore, and then stuff starts to sink. Is it necessary that buoyancy requires small size? Look at alkanes - some of them are huge molecules but still are buoyant in water.
Here at BUNGCO we are hammering away at a light-than-air macromolecule. This will be a molecule that via steric whatsits keeps other of its ilk (not to mention demon N2) at a distance, but is itself so large and poorly diffusing that it tends to stay in whatever shoddy balloon you put it in.
Initial candidates include a beryllium - boron polymer and megafullerenes incorporating extremely large aromatic carbon rings.-- bungston, Jun 22 2011 Evacuated Silica Aerogel http://en.wikipedia.org/wiki/AerogelA lighter than air solid [Twizz, Jun 23 2011] Is it possible to create a lighter-than-air aerogel? http://www.quora.co...er-than-air-aerogel [xaviergisz, May 15 2015] Graphene Vacuum Balls [xaviergisz, May 15 2015] This is an interesting possibility.
I don't know if it's doable, though. Given that your molecules will be heavier than helium atoms (at least), your only hope is to keep them further apart. Yet all gases have a nearly-identical molar volume (ie, almost identical average spacing between molecules, under given conditions), even though the molecular sizes are very different.
I don't think electrostatics would work. If the molecules have a net charge, there has to be a counterion in their with them, and you'll basically create a crystalline solid. If the molecules are polar (no net charge, but positive and negative parts), then the molecules will again join up like magnets and you'll have a solid (or a liquid, like water).
So, your only option is to make the molecules absolutely huge - comparable to the molecular spacing in a regular gas. But then you're really talking about making tiny vacuum blimps.
Still, [+]-- MaxwellBuchanan, Jun 22 2011 Lighter than air solids aren't impossible. [+]-- Voice, Jun 22 2011 The basic problem is that air is light because of the average distance of nothingness in-between air molecules. Liquid air, after all (with most of that empty space removed), is actually almost as dense as water. So, your spherical macromolecule, all the atoms of which are connected to very-nearby other atoms, needs to be volumous enough such that the total vacuum it encloses more-than compensates for the average lack of space between the atoms of that macromolecule. After which the usual problems regarding resisting external air pressure could apply...-- Vernon, Jun 22 2011 / the usual problems regarding resisting external air pressure could apply.../
I wonder about this. On the molecular scale, what is air pressure? I would think that intermolecular interactions would be much more important.-- bungston, Jun 22 2011 //extremely large aromatic carbon rings// which it would be extremely easy for a N2 molecule to blithely slip by and take up residence inside ?
Dunno... it might be possible to make a megamolecule; the equivalent of the vacuum-blimp city, but then you have to tie the molecules together to make a substance.-- FlyingToaster, Jun 22 2011 I am not sure it is possible for a molecule to traverse the center of an aromatic ring. I get the idea that electrons are hot and heavy in there and they don't appreciate any nonaromatic electrons trying to shoulder their way into the party.-- bungston, Jun 22 2011 ok, I misthought "aromatic": Organic Chemistry 101 was quite awhile ago.
The space between air molecules is an order of magnitude larger than an atom, that's gonna be one bigass soccer-ball in order to have a smaller mass:volume, and at best you only have 4 or 6 contact points to other soccer balls without doing something like filling the interstices.... hmm, assumedly with smaller soccer-balls which by themselves probably wouldn't be "LTA".
... sorta surprised there isn't already a PhD thesis giving the "blueprints" for an LTA solid.
[+]-- FlyingToaster, Jun 22 2011 / [+] / — MaxwellBuchanan
/ [+] / — Voice
/ [+] / — FlyingToaster
/ (+1) /
...At least two of you are stinkin' liars.-- ytk, Jun 23 2011 re vernon's comments on airpressure: a truly bigass molecule would be like a monolayer sheet forming a macroscopic balloon. Air pressure etc would apply to something that big.-- bungston, Jun 23 2011 isn't there a principle called Avocado's Law or Dalton's Law that says that a certain number of atoms or molecules occupy a certain volume at a certain temperature and pressure, regardless of the mass of each atom or molecule? Or am I being unnecessarily literal or failing to get the joke?-- afinehowdoyoudo, Jun 23 2011 //Liquid air, after all, is actually almost as dense as water.// By my calculations it's denser, in the same way that humid air is lighter than dry air.
I can't really see this being possible unless you get into quantum harmonics, which I know almost nothing about, therefore it's suitably advanced enough for me to consider it magic.-- marklar, Jun 23 2011 Bad science. Generally, the only thing that counts is molecular weight.-- ldischler, Jun 23 2011 I've thought about this before but in a somewhat different context. More specifically, i've long been curious about a Hoberman sphere molecule which starts off small and is initially kept small by another substance "hooking" it together which dissolves in acid or otherwise reacts. The sphere then springs open and leaves spaces too small for most gas molecules in air to enter, effectively maintaining a vacuum inside itself with the exception of gases consisting of lighter particles such as hydrogen and helium, which are in any case lighter than air.-- nineteenthly, Jun 23 2011 Baked. Aerogel.
See link.-- Twizz, Jun 23 2011 Baked.
Hot air.-- neelandan, Jun 23 2011 What's inside a C60 "BuckyBall" molecule?-- hippo, Jun 23 2011 R-12 is a rather famous example of an LTA complex molecular gas.-- RayfordSteele, Jun 23 2011 The Wikipedia page on aerogels links to one called SEAgel, which is made from agar. Allegedly, it's made by supercritical drying of an agar gel, and can also be made lighter than air.
Agar is readily available - I wonder what happens if you just freeze-dry agar gel? I feel a short experiment coming on...-- MaxwellBuchanan, Jun 23 2011 What [ldishler] said, except that long molecules can be less leaky than short ones. Doesn't helium leak out faster than hydrogen, for this reason?
As for evacuated aerogel, has anyone actually done it? I've only ever seen claims that it would *theoretically* be a lighter-than-air solid.-- spidermother, Jun 23 2011 // Doesn't helium leak out faster than hydrogen, for this reason? //
Helium is a Noble (inert) gas, so exists essentially as "free" atoms. Hydrogen tends to form H2 molecules so that both nucleii (protons) can "fill" their 1s electron orbitals.
Hydrogen isn't big, but the weak bonding induced by the electron sharing makes the molecule huge in nuclear terms.-- 8th of 7, Jun 23 2011 That's what I meant; in one dimension, a hydrogen molecule is huge compared to a helium atom, so it leaks out more slowly for a given pore size (or so I've heard).-- spidermother, Jun 23 2011 hmm, wouldn't an H ion and an He atom be the same size ? (delimited by the electron shell).
But what does the e-orbit look like in an H2 molecule. Is it circular 'round the nuclei and straight across between them or is it elliptical 'round both.-- FlyingToaster, Jun 23 2011 //H ion// Do you mean a single H atom? If so, it would be larger than a He atom. Monatomic hydrogen and helium have, respectively, 1 and 2 electrons in the 1s orbital. In general, as an orbital is filled, atomic radius gets smaller.-- spidermother, Jun 23 2011 //I wonder what happens if you just freeze-dry agar gel?//
The experiment is partially done. The result was a thin piece of translucent leathery goo.
Ars longa, vita brevis.-- MaxwellBuchanan, Jun 23 2011 Perhaps you should seal a piece of agar gel, and a quantity of desiccant, in an airtight jar and freeze the lot. I would expect it to take up to a week or more.-- spidermother, Jun 23 2011 Possibly so. I think part of the problem was that I didn't freeze the agar before I began - vacuum cooling froze it pretty fast, but perhaps not hard enough. Next time I'll freeze a block in liquid nitrogen and then pop it in the vacuum chamber.-- MaxwellBuchanan, Jun 23 2011 Ah. I was unaware that you had liquid nitrogen and vacuum to hand. As you were.
I'm still guessing that you might want to remove the water slowly, using a gentle vacuum (possibly with desiccant), to avoid damaging the structure.-- spidermother, Jun 23 2011 // an H ion and an He atom be the same size ? //
Which ion ?
If you can make H- ions, you have our respect and admiration.
H+ ions are, for all practical purposes, just a proton (yes, yes, deuterium, tritium, quadrium, not statistically significant) which is, by most standards, fairly small.
H2 looks pretty much like the "dumb-bell" you would imagine from two little bits of stuff loosely held together by electrostatics.-- 8th of 7, Jun 23 2011 //electrostatics// ah right then. (monatomic Hydrogen what I meant to say)
So an H2 molecule should be able to fit in lengthwise where an He atom can...-- FlyingToaster, Jun 23 2011 Yes, but most collisions with pores will not be precisely axially aligned. Diffusion will therefore be slower. The rate of diffusion across a membrane is related to the proportion of collisions resulting in a molecule crossing to the other side.
It's like throwing golf balls, versus rubber bones, at a wall with small holes in it.-- spidermother, Jun 23 2011 //you might want to remove the water slowly, using a gentle vacuum (possibly with desiccant)//
I think the dessicant shouldn't make any difference as long as it's a hard vacuum - either way, the water is leaving the gel into an effective well.
As for the speed of drying, I'm not sure. If the stuff is frozen, then I don't see how the speed of removal will matter (ie, the remaining agar strands will either support themselves or not). If it's not frozen, then the water will just pull the agar in and collapse it by surface tension.
I'm thinking about the freeze-dried raspberries in breakfast cereals - they're very dry and light, but have retained their shape admirably. I must find out how they do dat.-- MaxwellBuchanan, Jun 23 2011 You're probably right. I was just guessing.-- spidermother, Jun 23 2011 Shame on you ...-- 8th of 7, Jun 23 2011 Yes, I also read that the initial drying period takes several days, at least - the explanation being that trying to make it happen any faster would involve heating the gel to the point where damage occurs. So, I thought I was wrong, but it turns out I was mistaken.-- spidermother, Jun 25 2011 [+] If the cage is vacuum-filled, as in the design, the mass density could be less than that of air. Not bad science. Not baked. Original. Possible in principle. What's not to like?-- sqeaketh the wheel, Jun 26 2011 ummmm, the possibility of floating garbage?-- 2 fries shy of a happy meal, Jun 26 2011 For neutral-buoyancy, a graphene-based molecule of 2.87um diameter, also known as C982,707,908.
[8th of 7] probably wants to do his own math.-- FlyingToaster, Jun 26 2011 Apparently, the rate of effusion of a gas through very small pores is inversely proportional to the square root of the molecular mass, and is not affected by size or shape (Graham's law). (The supposed influence of the different shapes of hydrogen dimers vs helium monomers is something I heard ages ago, but never bothered to check.)-- spidermother, Jun 26 2011 Water vapor. H2O is lighter then O2. Big enough to not escape through Mylar.-- pashute, May 14 2015 to put this in perspective, the density of air varies a lot. might not work on earth doesn't in itself make the idea unworthy of exploration for use in other atmospheres.-- WcW, May 14 2015 Much as I like the idea, it occurs to me that, if implemented, eventually there would be atmospheric garbage patches, similar to ones found in Earth's oceans... or Earth orbit, filled with chunks of dirigible.-- FlyingToaster, May 14 2015 Soap bubbles can be filled with helium and are lighter than air down to the size of about 1mm diameter (from memory).
Instead of a film of soap (which is a type of liquid crystal), a bubble could be formed from a polymerisable liquid crystal. The polymerisation could be catalysed by UV light.-- xaviergisz, May 15 2015 random, halfbakery