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July 6, 2018 11:51 PM   Subscribe

Swimming bacteria can reduce the viscosity of ordinary liquids like water.
By swimming together, [bacteria] can generate forces in the fluid that are large enough to counterbalance friction, such that the bacterial suspension behaves as a superfluid.

"While this phenomenon has been observed experimentally, a detailed theory about how it works has been missing," says Dr. Aurore Loisy, Research Associate in Applied Mathematics & Theoretical Physics at the University of Bristol.
posted by clew (10 comments total) 24 users marked this as a favorite
This is very cool. The kind of far-from-equilibrium physics associated with living systems can be very counterintuitive from the perspective of our conventional understanding of equilibrium physics, where things like negative viscosity should be impossible. I love condensed matter physicists for taking these problems seriously.
posted by biogeo at 7:24 AM on July 7, 2018 [3 favorites]

To me, this is the krazy implication: "This shows us that microscopic bacterial activity can be converted into macroscopic useful mechanical is possible to make bacteria work together and use that to power larger devices."

That's the most biopunk idea I've ever heard. Imagine a daily environment in which macroscale artifacts are powered by bacterial superflux. If nothing else, it beats Bacigalupi's kink-spring physics all to hell.
posted by adamgreenfield at 8:35 AM on July 7, 2018 [3 favorites]

Semen loses viscosity within a few minutes of ejaculation and also becomes transparent.

I'd assumed that was due to sperm swimming to the boundaries, but I ran into an apparently authoritative source a few years ago attributing that to some kind of enzymatic action. Maybe that will need to be revisited.
posted by jamjam at 12:11 PM on July 7, 2018 [2 favorites]

I appreciate that you posted this, because if you'd asked I would have bet that the more bacteria water had in it, the thicker it would be. But this is because I, as a rando, associate dangerous water sources with stagnant water, which is full of particulate matter and algae. It's important for people to learn not to judge water quality on appearance. I always have to remind myself that gleaming, rushing mountain brooks might be full of giardia.
posted by Countess Elena at 5:49 PM on July 7, 2018

Can somebody explain negative viscosity to me? It's breaking my brain.
posted by clawsoon at 3:45 AM on July 8, 2018

From your link, biogeo:
In equilibrium, minimization of a free energy determines the preferred state, and the system reaches this state independent of the initial conditions. Far from equilibrium, systems typically exhibit a very rich set of characteristic behaviors that are not generally described by a minimization principle.
This immediately made me think of protein structure determination. As I understand it, we generally figure out what a protein will look like after folding by looking for its minimum-energy state. Is that still a reasonable assumption?
posted by clawsoon at 4:47 AM on July 8, 2018 [1 favorite]

far-from-equilibrium physics

This is a key concept in my philosophy. Nothing interesting happens at equilibrium. Far from equilibrium, energy and mass can flow, permitting a sustained local decrease in entropy (while still having global entropy increase of course). A sustained local decrease in entropy might be called ... life?

I could go on if anyone is interested.
posted by M-x shell at 8:25 AM on July 8, 2018 [1 favorite]

M-x shell, I am indeed interested, because I, a non-physics-y sort of person, have occasionally thought that civilization is a localized negative entropy state -- at least by analogy.
posted by wires at 10:24 AM on July 8, 2018


The key concept is that when energy (or mass, which is itself energy and/or carries energy) flows, it increases the entropy of the system it flows into, according to:

S = Q / T

where S is the entropy increase, Q is the amount of energy that flowed, and T is the temperature of the source of the energy. When a given amount of energy moves, it brings with it less entropy if it comes from a place of higher temperature than if it comes from a place of lower temperature, because the denominator T is larger. So, if a system receives energy from a high temperature T1, and gives the same amount of energy away at a lower temperature T2 < T1, its local entropy decreases, while the entropy of its environment increases by the same amount (or more, due to dissipation). Mathematically,

(Q / T1) - (Q / T2) < 0

This can only happen far from equilibrium because at equilibrium everything is at the same temperature.

Some examples:

1. The earth receives energy from the Sun, which is millions of degrees. So, the entropy arriving is low. Energy makes its way through the physical and biological processes on earth and the leaves Earth through black body radiation at earth's temperature, much lower than the Sun's. So, Earth's overall entropy decreases, which allows for the tremendous complexity we see in the ecosystems and turbulent environment. This is one reason why global warming is so bad. If the earth's temperature rises, it won't lose as much entropy when it radiates. So, our ecosystems will get simpler. There are many indications of this happening already.

2. An analogue of temperature for some systems is chemical potential. Admittedly, I am out of my area of expertise here, but the idea is that food you eat has a higher chemical potential than the waste you expel. (Perhaps one of our chemists can clarify this point.) Assuming you don't gain or lose weight, your local entropy decreases. But always remember the waste leaving you is just as important to minimizing your entropy as the food coming in.

3. Your intuition about civilization is correct. Energy flows through our society, in part from the sun, but also, in recent centuries, from stored energy in the Earth. This energy flow through has resulted in tremendous complexity and specialization in our society. It is no coincidence that the rise of the middle class tracks closely with the rise of energy exploitation. If the amount of energy flowing through decreases, such as due to depletion of fossil fuels, the complexity of society can only decrease. A simpler society will have fewer rich and more numerous poor. Sound familiar?

I'm quite sure the above is true in the main, but if any of our MetaFilter scientists can improve the above discussion I'd welcome it.
posted by M-x shell at 3:03 PM on July 8, 2018

Can somebody explain negative viscosity to me? It's breaking my brain.

Well, there's no sense in which any of this is my field, but it is pretty cool so I took a look at the paper. I understood very little of it, but I think I got the gist of what negative viscosity means and how it works in this system.

Viscosity is the ratio of the stress in the fluid to the rate of change of strain (which is basically the relative speed of adjacent lines of laminar flow). If you imagine a bit of water on a fixed plate, with a free plate resting on top of the water, then as you tug horizontally on the free plate, stress builds up within the water as you pull the free plate, and the faster you pull the more stress there is. If you replace the water with molasses, the same rate of pull will produce more stress. This is what it means to have greater viscosity.

Negative viscosity means that at the macroscopic level, when you pull on the top plate, you get stress in the opposite direction. This is kind of hard for me to wrap my head around, too, but modifying the standard thought experiment above helps me a bit. Replace the fixed plate with another free plate, so you have a fluid sandwiched by two plates (maybe microscope slides or something), all resting on a frictionless table. For a normal, positive-viscosity fluid, if you pull on the top plate, the bottom plate will follow, due to the stress in the fluid. Because it's a fluid, the top plate will still be able to move relative to the bottom plate, but not as far as it does relative to the original fixed frame of reference (e.g., the table). Now suppose it's a negative-viscosity fluid. Now if you tug horizontally on the top plate, the bottom plate moves in the opposite direction.

I can't pretend to fully understand how this happens in the case of the bacteria, but the paper has some diagrams that helped me a bit. Imagine a single bacterium floating within water between two plates. When the top plate is pulled, the water undergoes shearing, meaning water closer to the top plate experiences greater stress than the water closer to the bottom plate. So on the top side of the bacterium the force is greater than the bottom, which means the bacterium experiences a torque. This causes it to rotate. Now, the fluid described here is dense with bacteria, meaning in every small but macroscopic subvolume of the fluid, there are many orientable bacteria. I don't fully follow the paper from this point (the math doesn't look too hard but would take me some time to work through, and I'm not invested enough to actually do that), but evidently the aggregate effect of these bacteria being subjected to torque in this way leads to a nonlinear stress function across the depth of the fluid. It's actually not uniformly negative, but the lower portion of the fluid is negative.

Anyway, I know that's pretty vague, but that's about the level of understanding I've got from it.
posted by biogeo at 9:56 PM on July 8, 2018 [1 favorite]

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