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A method is proposed for determining the area of the air−water interface in wet but unsaturated porous media. The method depends on determining the critical concentration of surfactant necessary to achieve water movement within the system. Effective surfactants are insoluble in water, are spread as monolayers at the air−water interface, and form solid monolayers which have little or no effect on the surface tension of water until surface concentration of surfactant molecules reaches a known critical value. At this critical concentration, capillary pressure gradients are established within the system, and water moves. Studies of the effects of different water concentrations on air−water interfacial areas show the correct trends and trend toward the correct limits. Advantages and limitations of the technique are discussed. The air−water interfacial area in wet unsaturated porous media has not previously been experimentally accessible.

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