Fig. 2: Schematics of two gas flow mechanisms.
From: Knudsen pumps: a review

a Thermal creep flow, b inverted thermal creep flow. The sizes of the circular symbols depict the gas velocity, and their colors depict the gas temperature
From: Knudsen pumps: a review

a Thermal creep flow, b inverted thermal creep flow. The sizes of the circular symbols depict the gas velocity, and their colors depict the gas temperature