Water compresses under a high gradient electric field | Hacker News Reader

Water compresses under a high gradient electric field

phys.org

135 pointsneo4sure48 comments

"This is an unexpected phenomenon, contrary to what we thought we knew about nanopore transport. It took three years to work out what it was the simulations were showing us. After exploring many potential solutions, the breakthrough came when we realized that we should not assume water is incompressible. Now that we understand what's happening in the computer simulations, we are able to reproduce this phenomenon in theoretical calculations."

Pretty interesting that the rules of the simulation were such that it took three years to explain an emergent phenomenon. I probably would have written it off as floating point shenanigans.

> I probably would have written it off as floating point shenanigans.

Pretty sure anyone in actual scientific computing stays far, far away from floating point numbers and sticks to arbitrary precision ones instead.

Double precision floating point numbers are the most common in scientific computing. Sometimes software will mix in single precision for more speed. The molecular dynamics package GROMACS can use mixed single/double precision, for example:

http://manual.gromacs.org/documentation/2016/install-guide/i...

I'm quite curious what niche of scientific computing you've dealt with that gave you that impression.

Error analysis can tell you whether the roundoff error would grow exponentially with time steps for the algorithm you use. If you avoid this kind of conditions in scientific computing, then double precision floating point number would work perfectly fine.

And by the way, scientific computing is a diverse discipline, or a federation of disciplines. Say, there may be little overlap between the algorithms, tools, and environments that a physicist uses for scientific computing and those used by a bioinformatician. So I don't think it is universally true in scientific computing that arbitrary precision is preferable to double precision.

I found it counterintuitive that it was predicted in simulation and then confirmed with experiments, rather than the other way around.

This is how lots of new science is discovered. One example is the Theory of Relativity. Sure it was not a software simulation per se but it was still simulated on paper. It was not confirmed by experiment until several years latter.

If I'm not mistaken I think what OP is referring to is an apparent emergent phenomena that contradicted theory, whereas I believe GR was discovered through theory (ie Einstein's "happiest thought" was in an elevator shaft).

With nanopore experiments, it's not unusual to see unusual behavior that could possibly be explained by macromolecule interactions, contamination, or electrical interference. There are often more unknowns than knowns due to the scale of the system, novel materials, measurement precision, unique physics, etc etc... What usually happens is the undesired measurement gets filed away for 'later study'. Unless you have some theoretical framework or model that explains it, you may never go back. So probably the experimental part was previously dismissed as an error or contamination, but now it is interesting because there is an explanation

That is how it's supposed to work. You build a theory based on known phenomena, then you look for any new phenoma it predicts and test them in experiments. Without the new phenomena, your theory is more likely to be just "overfitting" the data. A famous example of overfitting the data - adding more cruft to account for unexplained aspects - is this: https://en.wikipedia.org/wiki/Deferent_and_epicycle

It’s nice way to do engineering as well - Fully modeling every aspect of a collision first is often cheaper than doing multiple crash tests.

Years ago, one of the HP people involved with early ink-jet printers told me that intuition about fluid behavior totally fails at that scale, and they had to use computational fluid dynamics to figure out how to get ink droplets to behave.

It's always surprised me how many people do not know that water is incompressible! For example, when someone is filling a scuba tank, it's common to put the tank in a large, often metal-sided container of water. When asked why, the shop staff member will typically say: "In the unlikely event of the tank blowing up, the water will absorb the force of the explosion." But in fact, water being incompressible, it will absorb approximately 0% of that force - which is then transmitted undiminished to the metal walls - which then explode like a giant hand grenade, promptly maiming or killing everyone standing nearby! The purpose of the water is instead, to absorb the heat from adiabatic compression, and keep the tank cool.

> "...the breakthrough came when we realized that we should not assume water is incompressible..."

The shop staff was right, water is not incompressible (under important conditions that don't apply to their scenario!).

That's just wrong. The the blast from the tank would have to displace water before of the energy of the blast can exit the side or top of the container. That spreads out the energy from the blast and the container would redirect much of that energy upwards both of which reduces the effective radius of the blast.

The shopkeeper is still somewhat wrong though: it's redirecting the energy, not absorbing it.

Regardless, Mark Rober and The Backyard Scientist have a pretty great video demonstrating exactly what is going on here: https://youtu.be/W4DnuQOtA8E

I'm no scientist and you may well be right. But I'm skeptical of your statement that "much" of the blast would be directed upwards. If you look at the pictures of tank explosions, they often show the entire room flattened right to floor level - not a big hole in the roof and minor damage everywhere else.

> It's always surprised me how many people do not know that water is incompressible!

Interestingly enough, they're right (though they may not know it) since the article is about its ability to compress under the right circumstances!

But only by 3% - so I still won't set up my chillout lounge right next to the tank fill station! :-)

It's definitely both the cooling (otherwise the cylinder would get hot, just like a can of duster gets cold), and the safety thing. The walls of the container would generally be more than enough to withstand the acoustic shock and send that energy upward as a burst of water, but your real enemy is shrapnel from the cylinder, which the water will promptly slow to non-lethal speeds. I'll agree that a guy doing it in a plastic tank or a smallish metal one would be asking for trouble.

On the flipside, water is used for the periodic pressure testing of scuba tanks, precisely because it is essentially incompressible. So if a tank does blow up during testing, you have a very small expansion and thus very small energy release.

If you draw a curve of gauge pressure as a function of volume, the energy release is the area under the curve, and as the curve steepness goes to infinity (i.e. compressibility goes to zero) the energy released starting at a given pressure goes to zero.

I did know that, but I'm not sure how that applies. In your scenario, the tank ruptures, there's virtually no water expansion, so nothing happens. In my scenario, the tank ruptures, the internal gas content instantly expands by (say) 250 times, creating a massive force which I assume is transmitted virtually undiminished, through the water, to the sides of the container, which promptly explode outwards. I do accept that some of the force will go upwards, but I believe the container will still explode. Not really trying to disagree, just trying to get my head around it :-)

I'd have to do the math (and know the material properties and thickness of the tank walls) before saying one way or the other, whether that scheme is effective.

What it could also be doing, is keep the tank from going off like a missile through the building (added inertia of the water plus it takes much longer for the tank to tip sideways).

That is a really neat effect. Basically if you can put a water molecule inside a flat field it lines itself up with that field and can be compressed. I would bet this has an impact on Graphene based desalination efforts as well.

This sounds akin to electrons forming Cooper pairs in atoms.

https://en.wikipedia.org/wiki/Cooper_pair

How so?

That type of filtering was also something that I had in mind.

It would additionally be interesting if the water would react to EM fields and confined spaces in a way that allowed for a 'micro' pump. Such a system might be able to operate in zero-g and thus be useful on extended space missions.

That is a really good observation. Microgravity environments have an issue with the lack of sediment settling, and while the current centrifuge system are effective, it would be really interesting if you could separate the precipitates without spinning everything.

> Aksimentiev adds, "All of this only works because the membrane is so thin, and the electric field is focused where the membrane is, compressing the water molecule from both sides.

It's not _pushing_ the molecules together, rather _pulling_ them together. This happens because the electric field is smaller than the water molecule, and pulls from inside.

Does this also mean compressing/decompressing water can generate an electric field?

Probably, but it would take a lot of mechanical force. From the article:

> The compression is only 3 percent, but that pressurizes the water—it's equivalent to 100 atmospheres

Although, that's not uncommon for ice giants, so it might be causing part of Ouranos (aka Uranus) magnetic field.

> The troposphere is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar[1]

[1] https://en.wikipedia.org/wiki/Uranus

Implication: Depending on the energy input, couldn’t you create an engine from this? Sounds like a perpetual motion machine.

This effect does convert energy from mechanical to electrical, and so could be used in a type of engine. But AFAIK there would still be inefficiencies causing this conversion to be less than 100%, and so sadly not a perpetual motion machine.

Coincidentally, I just watched a great video by Ben Krasnow yesterday that suggests that the answer to fooker's question is indeed "Yes." There are other well-known principles along these lines, ranging from piezoelectricity to the Seebeck and Peltier effects, and it's almost more of a rule than an exception for such effects to behave symmetrically.

Glowing plasma created by a high speed jet of water: https://www.youtube.com/watch?v=_vTq8oGpqwM&list=LLaIdy5TkkQ...

He definitely has the "whole lot of mechanical force" thing going for him, considering that he's about 2 psi away from fragging himself with a hydraulic cylinder.

Nah, probably not.from how i read it, is not that the field is strong, but that it's so incredibly localised. The molecules arrange along the field lines (H being more positive and O more negative). Since the field lines are so close in this case, and the field's gradient (more or less: inverse of width between field lines) is so high, the O parts snuggle together a bit more, as do the H parts.

Anyway, that's how i read the article. (Please correct me if I'm wrong)

Compressing water, however, does not force the molecules to align (as far as I know). So no field generation by compression - at least not from my understanding of this experiment.

Well, maybe compressing water does not force molecules to align this way, bu enough pressure will cause water to align in a crystal lattice (not normal ice but something exotic like ice III, ice V, or ice VI).

Could this be used for a new kind of pump with no moving parts? Maybe a worm drive for submarines?

”Wilson found that a high electric field applied to a tiny hole in a graphene membrane would compress the water molecules travelling through the pore by 3 percent”

So, it changes the distances between the atoms of a water molecule? If so, would that mean this can affect chemical reactions involving dipoles, too?

Who said water does not compress? It doesnt compress much, but at the depths of the ocean it is a tiny bit denser than at the surface. I was suprised to see phys.org so cassually state that it doesnt.

> "Physics Professor Aleksei Aksimentiev and his post doctoral researcher James Wilson found that a high electric field applied to a tiny hole in a graphene membrane would compress the water molecules travelling through the pore by 3 percent."

Ahh, "3 percent" means that from a fluid dynamics point of view, the incompressibility assumption is still a good assumption valid for most macroscopic applications. :) Still very impressive to achieve that under ambient pressure with an electric field gradient.

What would happen if you compress water using this method and then seal the container it's in and turn off the electricity? Anything interesting? Does the water stay compressed? Does the container explode? Does it depend on the strength of the container?

Intuition says that the water would then exert more pressure on the container. Like if you emptied and sealed a plastic bottle while at sea level, and then ascended to a high altitude in an airplane (or climb a mountain). The bottle would be bloated. Whether or not it would explode depends entirely on the properties of the container..

Would not the bottle be squished now that it is under more pressure?

Yea, you're absolutely right, I swizzled the analogy. I fixed my previous comment. Thanks :)

I wonder if this mechanism can be used as a molecular solenoid.

I'm endlessly fascinated by the concept of molecular-scale machines.

This page, or something it loads, is causing Firefox to use 100% CPU. Cryptocurrency malware, perhaps?

Same here, it was lagging out my browser and I just updated it the other day too.

Even with ublock origin+umatrix, so not many scripts were run. I was wondering whether that causes a script to fail and loop infinitely. IDK what conditions you guys were running your browser as.

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