Measurement of air-water interfacial area for multiple hysteretic drainage curves in an unsaturated fine sand

A new method has been developed to measure fluid-fluid interfacial area during multiple drainages, along with the measurement of hysteretic capillary pressure-saturation (P-c-S) relationships in unsaturated porous media. The method makes use of an automated device which has been successfully used for rapid measurement of hysteretic P-c-S relationships, in combination with a novel technique for interfacial area measurement. A pure anionic surfactant, sodium octylbenzene sulfonate (SOBS), is used as a surface-active tracer, and a flow-through UV spectrometer is used to monitor the real-time concentration change of SOBS solution due to adsorption to the fluid-fluid interface during drainage. The Gibbs and Langmuir adsorption equations are applied in combination with a continuous mole balance to calculate interfacial areas. Using this method, air-water interfacial area of a fine sand was measured as a function of capillary pressure and saturation during primary, secondary, and one scanning drainages to explore the influence of drying/wetting history on interfacial area. Results show that 8-20 and 12-22 cm(2)/g more air-water interface was generated in secondary and scanning drainages, respectively, than in primary drainage, with the magnitude of the difference varying as a function of saturation. An advantage of the method is that interfacial tension variations from the method itself are relatively small, typically on the order of 5 mN/m, so measured areas are not skewed by surface-tension-induced changes in interfacial area. In a measurement specifically designed to study the influence of surfactant-induced interfacial tension variations, approximately two times more interfacial area was observed for a 25 mN/m interfacial tension change, in comparison with a system with relatively constant interfacial tension. Implications of results of interfacial area measurements for hysteresis in the three-dimensional relationship between capillary pressure, saturation, and interfacial area are discussed.