Life at Low Reynolds Number (1976) [pdf](biotec.tu-dresden.de)

Life at Low Reynolds Number (1976) [pdf]

http://www.biotec.tu-dresden.de/fileadmin/groups/guck/Seminar/1977_Purcell_life_at_low_reynolds_number.pdf

9 comments

This was the subject of one of the lectures I took in Biophysics at the TU Delft in the Netherlands, great to see this talked about. A lot of our intuitions about motion is based on the old swimming idea of "I'll kick a vortex of water backwards and it will propel me forwards!" and that simply doesn't work if you can't exceed this critical force of the fluid, which is what you need to do to make vortices and transfer out of a laminar regime. Instead you have to essentially "crawl along" the fluid around you, rather than "swimming" in the normal sense.

Another lecture which was memorable dealt with the sheer variety of different situations and biomechanisms which you can rule out using the Second Law of Thermodynamics. The bottom line is that when you're at thermal equilibrium then everything must jitter thermally by the same amount, which is what excludes "Brownian ratchets" [1] from working: if you peek inside at the spring and asymmetrical "teeth" inside an actual ratchet, then you realize that the work extracted actually comes from heat-transfer to the spring, which is being implicitly assumed to be at 0 temperature in the naive "oh, you get energy out of nothing!" argument. It's a very simple 2nd-law argument to make, but really hard to poke around inside and figure out what's going on internally.

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

mirimir10 anni fa

> Instead you have to essentially "crawl along" the fluid around you, rather than "swimming" in the normal sense.

Or screw through it ;)

pdxgene10 anni fa

Thanks for this. Brings back wonderful memories of the biomechanics courses I took way back in college.

I recall with glee the shock of recognition that all creatures live in moving fluids -- and our experiences are delineated merely by relative sizes and viscosities.

Maybe it's nerdy to say so, but it was definitely one of the top ten epiphanies of my life, and definitely helped me feel more connected to the natural world.

Swizec10 anni fa

I wrote a summary of this paper a while ago: http://swizec.com/blog/week-9-life-at-low-reynolds-number/sw...

I think I made it more understandable but who knows. Was def fun to read and interpret. Highly recommend.

rrock10 anni fa

This one's a classic. It was one of the most memorable papers that I read as a grad student, so now I cover it every year in my course. Really enlightening.

howeman10 anni fa

There's a really nice video series on fluid mechanics if you're okay with old-style videos. The one on low Reynolds number is here https://www.youtube.com/watch?v=B9XL_OOwJ14&index=7&list=PLC...

In particular, there is a nice illustration of "inertia doesn't matter" at 15 minutes.

mchahn10 anni fa

It left me very curious as to who Viki is. He mentions her (him?) often. Anyone know?

scentoni10 anni fa

> A Symposium in Honor of Victor F. Weiskopf

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

karcass10 anni fa

http://www.hrwiki.org/wiki/Reynold

JohnDeHope10 anni fa

Tonight I'm wrestling with if and how to argue with someone. And then I read this. "In other words, this bug can't do anything by stirring its local surroundings. It might as well wait for things to diffuse, either in or out. ... You can thrash around a lot, but the fellow who just sits there quietly waiting for stuff to diffuse will collect just as much. ... At one time I thought that the reason the thing swims is that if it swims it can get more stuff, because the medium is full of molecules the bug would like to have. ... The increased intake varies like the square root of the bug's velocity so the swimming does no good at all in that respect. But what it can do is find places where the food is better or more abundant. That is, it does not move like a cow that is grazing a pasture - it moves to find greener pastures." My brain asplode. He's not talking about bugs swimming. He's talking about when and how to initiate conflict. Read this http://www.theguardian.com/lifeandstyle/2016/feb/19/how-to-p.... Stirring is initiating pointless conflict as Burkeman describes. It gets you nowhere. You're better off just sitting still and waiting for the problem to go away. You're literally and scientifically better off doing nothing. "Let it go" as they say. But we do argue, we do have conflict, and it does sometimes have a meaningful positive outcome. The bugs do swim, but not by the externally obvious way high Reynolds number swimmers do. Okay so pointless argument begging is akin to using a stirring motion in a high Reynolds number environment. And in such an environment bugs do swim, but not in a physically pointless and impossible way. And their motivation is not to "find more food to eat" but to be in a place where "more food is low-hanging fruit" they can absorb through diffusion. So arguing to try to get resources, or concessions, or approval, or power, is like thinking you're going to swim with a stirring motion in a low Reynolds number... you're not going to and you're going to fail if you try. You'd be better off just sitting there and waiting randomly for whatever it is you're seeking to come to you, as unlikely as that may seem. But we know the bugs do swim. We do argue. And some of us are able to argue and win, not in a simplistic "she won the debate" sense, but in a meaningful "the world is a better place because she won" sense. So what's the difference. If bugs in low Reynolds number environments don't swim to reach more resources, but swim to be in a place where there are more resources to absorb, what is the equivalent conflict strategy to that? Don't argue to force change on the environment, argue to make the environment richer. Using Burkeman's story from the beginning of his piece, don't start a pointless argument by saying "there's more resources over there" or "it'd be better if you did X". Yeah sure in a very simplistic and straightforward way there might be a resource over there, or an improvement if we did X, but do you realize that in a low Reynolds number environment it is literally impossible for use to move towards that and catch it? It is physically impossible to swim towards individual pieces of food, it is psychologically impossible to effect meaningful change by arguing that things should be different. Yes there is a piece of food over there, and yes things would be better if I had picked up the mess. But you can't reach that food efficiently and you can't argue somebody into changing. It's late and I have to go to bed. If anybody can square this circle for me please do so.

myself24810 anni fa

Whoah. This is breathtakingly off-topic but totally profound, as it encapsulates a lot of how and why I get involved in arguments and feel fruitless afterward. Thank you for writing it. It'll have to rattle around in my brain for a while, and I may post another reply later.

[deleted]10 anni fa