Evapero-Osmotic Circulation

摘要

Consider a fluid flow contained inside a rigid nonporous pipe shaped into an almost closed loop that is driven by both evaporation and osmosis. The loop is a rectangle in the vertical plane, with the longer sides parallel to gravity. A dissolved substance (salt, sugar) increases the density of the fluid over that of pure water when the temperature is kept constant. At the top of the loop an opening in the pipe exposes the fluid to the air, and evaporation takes place there. On the bottom of the loop a second opening in the pipe contains a semipermeable membrane that allows fresh water to enter but no dissolved substance to leave the system. There is a reservoir of pure water into which the lower part of the loop is inserted. It is claimed that if the circulation is primed in the right way, it will continue on its own indefinitely (or until the water in the reservoir is used up). This is the evapero-osmotic circulation, which may be a novel concept, and an application of it is made to increase our understanding of certain aspects of the fluid circulation inside vascular land plants. For example, cavitation cannot take place in this type of circulation, which is an advantage over existing theories. Faster circulations occur for rectangular loops that are both long in the vertical and short in the horizontal directions, as well as for higher rates of evaporation and osmosis. A feedback mechanism maintains the stability of the circulation.