[FRIAM] The theory of everything

[thompnickson2 at gmail.com]

Speaking of Reynolds numbers?

A great many years ago I had an undergraduate honors student who wanted to work with slime molds.  These are social single celled organisms that, when things get tough, flow together to form a stem and a fruiting body.  From the fruiting body are distributed spores for the next generation. Only some small percentage of the original cells get into the fruiting body, so they pose a problem of the "group selection" type.  We were wondering whether we should be thinking of fruiting bodies as like dandelions or like burrs.  A little reflection about scale and viscosity suggested that dandelions was a stupid model.  The student devoted some time to mimicking with a probe what would happen if an ant brushed up against a fruiting body, and found that, indeed, they were extremely sticky.  We were overjoyed.  But then the student  fell in love, and I never saw him again.  

Let that be a lesson to you all.  

Nick  

Nicholas Thompson
Emeritus Professor of Ethology and Psychology
Clark University
ThompNickSon2 at gmail.com
https://wordpress.clarku.edu/nthompson/
 


-----Original Message-----
From: Friam <friam-bounces at redfish.com> On Behalf Of ? u?l?
Sent: Monday, July 6, 2020 2:18 PM
To: FriAM <friam at redfish.com>
Subject: Re: [FRIAM] The theory of everything

Sooooo... I'm familiar with both Reynolds and Mach from my days at Lockheed. But Mach, as I (probably don't) understand it, is tied to sound only because that's an indirect measure of the medium's compressibility, sound being compression waves. In thinking about these "low Reynolds number" organisms, I can't help but wonder what the analog for the "speed of sound" is for them. It strikes me that E. coli live way above "Mach 1", they live near Mach ∞, right? But if the medium is effectively incompressible, can there be pressure gradients across the organism's membrane? There must be, right? Since these things have internal architecture, including vacuoles, movement like the amoeba's "processes" seems more interesting than the poloidal rotation he mentions. Does the amoeba "push" against its medium? Or simply "grow through it"?

One of the coolest things about tissues to me is that they engineer their world, extruding their tools and infrastructure like so many dorks with 3D printers. I've only briefly skimmed the Purcell paper, but I didn't see anything about if/how microorganisms might do the same. If the amoeba-like ones "grow" through the medium, rather than pushing/pulling, then maybe the analog for the Mach number is related to diffusion and gradients, which makes Purcell's discussion of it on point, but doesn't go far enough.

On 7/6/20 8:02 AM, Jon Zingale wrote:
> The version of "Life at low Reynolds number" that I am familiar with 
> is this
> one:
> http://www.damtp.cam.ac.uk/user/gold/pdfs/purcell.pdf


--
☣ uǝlƃ

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