
Life at Low Reynolds Number (1976) [pdf] - abrie
http://www.biotec.tu-dresden.de/fileadmin/groups/guck/Seminar/1977_Purcell_life_at_low_reynolds_number.pdf
======
drostie
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](https://en.wikipedia.org/wiki/Brownian_ratchet)

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

Or screw through it ;)

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pdxgene
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.

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Swizec
I wrote a summary of this paper a while ago:
[http://swizec.com/blog/week-9-life-at-low-reynolds-
number/sw...](http://swizec.com/blog/week-9-life-at-low-reynolds-
number/swizec/6456)

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

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rrock
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.

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howeman
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...](https://www.youtube.com/watch?v=B9XL_OOwJ14&index=7&list=PLCdzC81EFrS-m5cVdmY05-ec9gBDMIkaT)

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

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

~~~
scentoni
> A Symposium in Honor of Victor F. Weiskopf

[https://en.wikipedia.org/wiki/Victor_Weisskopf](https://en.wikipedia.org/wiki/Victor_Weisskopf)

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karcass
[http://www.hrwiki.org/wiki/Reynold](http://www.hrwiki.org/wiki/Reynold)

------
JohnDeHope
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...](http://www.theguardian.com/lifeandstyle/2016/feb/19/how-to-pick-and-
stop-an-argument-oliver-burkeman). 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.

~~~
myself248
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.

