1879: Thermal transpiration - The book of science

Thermal transpiration
When molecules abhor
Leaking pores
Commentary

Osborne Reynolds, Martin Knudsen fluid dynamics

Thermal transpiration

Osborne Reynolds explained the behavior
of Crookes radiometer, having found
that heating a porous plate only on one side
moves gas molecules from the cooler to the hotter side.
A Knudsen pump moves gas by the same force
without moving parts. Gas molecules move
from the cooler to the hotter end of a narrow channel.

When molecules abhor

Strangely it seems
gas molecules creep
from cold to heat
through narrow pores
as though they abhor
crowd and cloud
and going alone
were allowed.

Leaking pores

Your gas leaks from tiny pores
you didn’t even know were there.
Helium from your balloon
is on its way to outer space.
Since entropy is a universal law,
you might wonder about
all your other leaking pores.
But not to worry.
Your existence suggests
life can yet sustain itself.

Thermal transpiration is enhanced when the average distance that a gas molecule moves between collisions is larger than the dimensions of the channel in which the molecules move, given that the walls of the channel at its entrance are cooler than the walls at its exit.

However, even after all these years, Osborne Reynold’s explanation of the Crookes radiometer is still controversial.