Life at Low Reynolds Number (1976) [pdf]

Life at Low Reynolds Number (1976) [pdf](science.curie.fr)

58 points by addcninblue 4 years ago | 9 comments

dandare 4 years ago |

So this sent me down the wikipedia rabbit hole :D

One thing I never realised is how dramatically the viscosity of water changes with temperature. This is probably why you can recognise hot water from a cold water in a video.

Temperature (C) / Viscosity (mPa*s)

0.01 1.7911

10 1.3059

20 1.0016

25 0.89002

30 0.79722

40 0.65273

50 0.54652

60 0.46603

70 0.40355

80 0.35405

90 0.31417

99.606 0.28275

HPsquared 4 years ago | |

Another interesting temperature dependence of water is the negative thermal expansion between 0°C and +4°C. The maximum density is at +4°C.

Therefore, between 0 and 4°C the usual law of natural convection (heat rises) is inverted and you get "heat falls" instead.

What this means is that in winter, the ice on top of water can be frozen but the water can be warmer further down and resist further heat transfer. Natural convection will not act to cool the water from above, and a stable stratified temperature gradient can form. This allows bodies of water to remain liquid in winter for longer than you'd expect in a "normal" liquid.

pfdietz 4 years ago | |

I wonder how pressure affects that, particularly near boiling.

EDIT: https://www.nature.com/articles/2151053a0

steerablesafe 4 years ago | |

I had the pleasure to measure that in a university lab, it's very apparent when measuring near 0°C takes roughly 2x longer than on room temp.

KineticLensman 4 years ago | |

I've sometimes thought that the sound of water in my shower changes as the water reaches max temperature. Perhaps that's why?

ThrustVectoring 4 years ago | | |

I was always able to tell as a kid in my childhood home, likely plumbing-system dependent.

pge 4 years ago |

The author of this paper is Ed Purcell - https://en.wikipedia.org/wiki/Edward_Mills_Purcell

If you studied physics in college, you may have used the textbook he wrote, Electricity & Magnetism.

mncharity 4 years ago |

Hmm, it looks like (TFA author) Purcell's "Back of the Envelope" AmJPhys column, paywalled for decades by AAPT, is currently available online.[1]

"a monthly feature [...] from January 1983 through July 1984. Three new "order-of-magnitude" problems were presented each month [...] there were 57 problems, the discussion of which fills something like 150 column-inches". Eg, the ratio of tide influence from Moon and Sun is 7/3, so what is the ratio of their mean density?

[1] https://www.aapt.org/Publications/AJP/Readers/back_of_the_en...

gtsnexp 4 years ago |

Beautiful illustrations!