Isothermal and non-isothermal water transport in porous membranes. II. The steady state

Abstract

Thermoosmotic behaviour was studied in simple systems constituted of grossly porous hydrophobic membranes permeated by distilled water. Attention was focused on steady-state conditions, characterized by absence of net transmembrane volume flow, obtained equilibrating thermoosmotic pressure with an external counterpressure. Comparison of hydraulic and thermoosmotic fluxes with steady-state pressure gives insight in the peculiar thermofluidodynamics of volume flow in non-isothermal membrane channels. This investigation was extended to volume transport caused by combination of the two thermodynamic forces constituted of the temperature and pressure gradients, synergic or antagonistic across the membrane. The experimental findings can be fruitfully compared with theoretical predictions of the system's behaviour derived from different approaches. Results obtained with six different membrane types, under a wide range of experimental conditions, lend support to the “thermal radiation pressure theory” which attributes the various effects of matter transport produced by a temperature gradient to the transfer of momentum from drifting thermal excitations to atoms and molecules in the material crossed by heat flow.