CONVECTIVE GAS-FLOW IN PLANT AERATION AND THERMO-OSMOSIS - A MODEL EXPERIMENT

The stationary thermo-osmotic pressure difference (Delta P)(stat) betw een a gaseous phase formed by air of constant volume and the outer atm osphere (P approximate to 1 bar) across a porous cellulose nitrate mem brane is measured as a function of the effective temperature differenc e Delta T across the membranes. In the stationary state (i.e. vanishin g gas flow across the membrane) the pressure of the phase with the hig her temperature is higher than that of the phase with the lower temper ature. The effective radius of the pore structure of the membrane (0.1 4 mu m < r(eff) < 1.27 mu m) and thickness of the membranes (105 mu m < delta(m) < 735 mu(m)) are parameters of the experiments. The ratio o f the mean free path length of the molecules in the gaseous phase lamb da at atmospheric pressure and r(eff) has a value of about (lambda/r(e ff)) approximate to 1. The relation between (Delta P)(stat) and Delta T are linear to a first approximation (e.g. r(eff) = 0.19 mu m: (Delta P)(stat) = A Delta T, where A = 59.4 Pa K-1; 0 < Delta T < 10 K). The values of the molar heat of transport Q(m) obtained from the slope o f the (Delta P)(stat) vs. Delta T curves for the different types of me mbranes are compared with theoretically calculated values. The agreeme nt between the two data sets is satisfactory. It is concluded that at atmospheric pressure under the condition (lambda/r(eff)) = 1, thermo-o smotically generated pressure differences can contribute significantly to the pressure difference needed as a driving force of the observed convective gas flow in plant aeration.