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https://doi.org/10.1016/b978-044452188-0.50005-7
Copy DOIPublication Date: Jan 1, 2008 |
This chapter deals with theoretical and experimental studies of thermo-osmosis of liquids and gases along with thermo-osmotic concentration differences. Correlation with kinetic theory has also been attempted. Thermo-osmosis is a phenomenon in which matter is driven through a membrane or an orifice from one chamber to another on account of the temperature difference between the two chambers. This can occur for a single fluid or a mixture of fluids. In a system without a membrane or a barrier, temperature gradient can give rise to concentration gradient and this phenomenon is called thermal diffusion. Conversely, a concentration gradient can give rise to temperature gradient. A membrane is a heterogeneous barrier between two homogeneous systems. It consists of a complicated network of pores, which may be connected to each other in a complex manner. The mechanism of transport depends on the size and shape of the pores, and nature of the permeant. The phenomenon of thermo-osmosis can occur only when the diameter of pores is comparable to the mean free path of the permeating species. For gaseous systems, the mean free path can be controlled by controlling the mean pressure. Theories based on non-equilibrium thermodynamics have been applied extensively to elucidate the phenomenon of thermo-osmosis. The methodology of nonequilibrium thermodynamics involves the evaluation of entropy production by the application of the laws of conservation of mass and energy and Gibbs equation. Appropriate fluxes and forces are chosen by suitably splitting the expression for entropy production, and, subsequently, thermodynamic transport equations are written.
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