Micro-drops of water will jump from a wetting-resistant surface to a water-loving surface and only in that direction. Each drop transfers heat. Since the heat transfers only in the one direction, the gap acts as an insulator in the other direction. Each drop is so small, the transfer can disregard gravity. (Likewise the tiny but dense particles of cigarette smoke will stay aloft, jostled by the Brownian motion of other molecules in the air.) Chilling the hydrophobic source but not the hydrophilic destination surface boosts the effect.
One way flow has been achieved before with a 100:1 advantage in the desired direction but was limited to a certain shape of device or had to be held in one orientation to deal with gravity. Such devices are used on the Alaska pipeline to siphon heat away from the permafrost below. Chuan-Hua Chen's research group at Duke University came up with the breakthrough.
Think of a building skin that gives you air-conditioning by insulating in one direction and rapidly transferring heat out of the building in the other direction.