Molecular-Sized Outward-Swinging Gate: Entropy Decrease in an Isolated System?

We investigate the concept of molecular-sized outward-swinging gate, which allows for entropy decrease in an isolated system. In one direction, the gate can be pushed open by individual gas molecules; in the other direction, it tends to be pushed close, so that the gas permeability is asymmetric. It is experimentally realized by a nanoporous membrane one-sidedly surface-grafted with bendable organic chains, across which gas spontaneously and repeatedly flows from the low-pressure side to the high-pressure side. The system follows the basic principle of the second law of thermodynamics, as entropy remains maximized. However, because of the local nonchaoticity and the path irreversibility of the gate, entropy reaches a nonequilibrium maximum, and may shift to and from the equilibrium maximum without an energetic penalty. This process is passive, fundamentally different from Maxwell’s demon and its variants.