A PHYSICAL MODEL INVOLVING NUCLEPORE MEMBRANES TO INVESTIGATE THE MECHANISM OF HUMIDITY-INDUCED CONVECTION IN PHRAGMITES-AUSTRALIS

Nuclepore membranes of known pore size and porosity, attached to small glass chambers were used to investigate the mechanism of humidity-induced convection found in Phragmites and other species. The convective flow of gases and the development of static pressures were studied in relation to a variety of conditions imposed on opposite sides of the membrane. Data were obtained in relation to pore size (0.015 to 3 mu m diameter) and porosity, temperature and humidity gradients, membrane area and resistance to venting. A humidity gradient across the Nuclepore membranes was far more effective than a temperature gradient pe,. se for inducing flow; also pores of the order 0.2 mu m diameter were the most effective. With a steep enough humidity gradient, however, significant flows could also be induced across pores as large as 2 mu m diameter. The results accord with previous experiments and observations on Phragmites and with the mathematical treatment of the mechanism. It is concluded that humidity-induced convection should play a greater role than thermal transpiration (thermo-osmosis) in effecting convections in Phragmites and other non-floating-leaved species.