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.
See also in The book of science:
Readings on wikipedia: