Life at low Reynolds Number
Edward Purcell

Abstract:

Viscosity is a physical property of fluids that is familiarly thought of as
a sort of "stickiness" or "stiffness".
The viscosity is a factor in the Reynolds number, which estimates the balance 
between inertial and viscous forces.  
The Reynolds number is also influenced by a characteristic length.
Small scale fluid processes can equivalently be thought of as normal
scale with a greatly increased viscosity, as though swimming through honey.
In such cases, the Stokes equations become the appropriate model, and 
we can deduce several consequences of this fact.
For tiny swimmers, only certain types of propulsion will actually
result in forward motion.  
Moreover, a creature seeking a higher concentration of food must essentially 
swim against the diffusion current.
These surprising facts about the realities of swimming were considered
long ago by Osborne Reynolds.

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Suggested Sections:

INTRODUCTION

  This is a talk that I would not, I'm afraid, have the nerve to give
  under any other circumstances...

THE LOW REYNOLDS NUMBER WORLD

  Well, here we go.  In Fig 1. you see an object which is moving through
  the fluid with velocity v.

THEORY OF SWIMMING AT LOW REYNOLDS NUMBERS

  I want to talk about swimming at low Reynolds numbers in a very
  general way.

ACTUAL MICRO SWIMMERS

  Well, let's look at some real animals (Fig 10).

THEORY OF HELICAL PROPULSION

  I got interested in the way a rotating corkscrew can propel something.

SEEKING FOOD AGAINST DIFFUSIVE FLOW

  So the interesting question is not how they swim.

CONCLUSION: OSBORNE REYNOLDS

  I come back for a moment to Osborne Reynolds.