thermal transpiration

Thermal transpiration is the movement of gas through a capillary tube between enclosures that are initially at the same pressure but different temperatures. The gas moves from the cooler to the warmer chamber. This phenomenon is also known as thermal effusion.1

This occurs when two containers of the same gas at different temperatures are connected by a small opening. If the opening is small and the gas density is low, the pressure will be higher on the high-temperature side. If the initial pressures are equal, gas will flow from the low-temperature side to the high-temperature side, causing the pressure in the warmer chamber to increase. This steady-state result is called the thermomolecular pressure difference. This effect was first studied experimentally by Osborne Reynolds in 1879.2