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Temperature Fluctuations at a Wetting Front: III. Apparent Activation Energies for Water Movement in the Liquid and Vapor Phases¹

ABSTRACT

Mean flow rates for water movement in the liquid and vapor phases were measured at several temperatures in three representative porous media. Apparent activation energies, calculated by means of a form of the Arrhenius equation, were found to vary from 4.3 to 6.1 kcal. per mole. A comparison of these values with the magnitudes of the potential energy barriers encountered in several relevant phenomena led to a postulation of the rate-limiting processes for the two flow situations. The rate of advance of the liquid front was concluded to be limited partly by the viscosity of thin films of water at the leading edges of menisci and partly by the rate of extension of the air-water interface. The rate of vapor movement was concluded to be determined primarily by the evaporation rate at the wetting front.

¹ Technical Paper 780, Arizona Agr. Expt. Sta., Tucson. Contribution from the Department of Agricultural Chemistry and Soils. Presented before Div. I, Soil Science Society of America, Aug. 20, 1962, at Ithaca, N. Y.

² Soil Physicist, Research Associate, and Graduate Assistant, respectively.

Received for publication January 28, 1963. Accepted for publication April 24, 1963.