Thermo-Osmosis Through Compacted Saturated Clay Membranes
1969
Dirksen, C.
Thermo-osmotic permeability of saturated Na-kaolinite decreased less rapidly with increasing compaction pressure, Pc, than did hydraulic permeability, resulting in a moderate increase of thermo-osmotic pressure difference Γ₁. At Pc = 435 kg/cm², a hydraulic pressure gradient of 1 cm water/cm caused still twice as much flow as a thermal gradient of 1C/cm (Γ₁ = 0.54 cm water/C). For saturated Na-bentonite at Pc = 7.0 kg/cm², Γ₁ decreased, but kT increased with increasing average temperature. Γ₁ was about three times higher than that of Na-kaolinite at Pc = 435 kg/cm². A temperature gradient of 15C/cm was within the linear region for Na-bentonite, but 22C/cm was not for Na-kaolinite. All thermal flow of water occurred from the warm to the cold side and was generally less than that reported for liquid water in unsaturated soils at comparable water contents. The apparent activation energy of thermo-osmotic water transport in Na-bentonite was estimated to be about 29 cal/g water; the heat of transport was more than three orders of magnitude smaller. Identification of Γ₁ with the heat of transport requires application of irreversible thermodynamics, which has not been done rigorously for soil water transport.
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