Convective gas flow in plant aeration and thermo-osmosis: A model experiment

The stationary thermo-osmotic pressure difference (Delta P)(stat) between a gaseous phase formed by air of constant volume and the outer atmosphere (P approximate to 1 bar) across a porous cellulose nitrate membrane is measured as a function of the effective temperature difference Delta T across the membranes. In the stationary state (i.e. vanishing gas flow across the membrane) the pressure of the phase with the higher temperature is higher than that of the phase with the lower temperature. The effective radius of the pore structure of the membrane (0.14 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 of the mean free path length of the molecules in the gaseous phase lambda at atmospheric pressure and r(eff) has a value of about (lambda/r(eff)) 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 of the (Delta P)(stat) vs. Delta T curves for the different types of membranes are compared with theoretically calculated values. The agreement between the two data sets is satisfactory. It is concluded that at atmospheric pressure under the condition (lambda/r(eff)) = 1, thermo-osmotically generated pressure differences can contribute significantly to the pressure difference needed as a driving force of the observed convective gas flow in plant aeration.