Air‐water interfacial areas in unsaturated soils: Evaluation of interfacial domains

Abstract

A gas‐phase miscible‐displacement method, using decane as an interfacial tracer, was used to measure air‐water interfacial areas for a sand with water contents ranging from ∼2% to 20%. The expected trend of decreasing interfacial areas with increasing water contents was observed. The maximum estimated interfacial area of 19,500 cm−1 appears reasonable given it is smaller than the measured surface area of the porous medium (60,888 cm−1). Comparison of the experimental data presented herein with literature data provided further insight into the characterization of the air‐water interface in unsaturated porous media. Specifically, comparison of interfacial areas measured using gas‐phase versus aqueous‐phase methods indicates that the gas‐phase method generally yields larger interfacial areas than the aqueous‐phase methods, even when accounting for differences in water content and physical properties of the porous media. The observations are consistent with proposed differences in interfacial accessibility of the aqueous‐ and gas‐phase tracers. Evaluation of the data in light of functional interfacial domains, described herein, yields the hypothesis that aqueous interfacial tracers measure primarily air‐water interfaces formed by “capillary water,” while gas‐phase tracers measure air‐water interfaces formed by both capillary and surface‐adsorbed (film) water. The gas‐ and aqueous‐phase methods may each provide interfacial area information that is more relevant to specific problems of interest. For example, gas‐phase interfacial area measurements may be most relevant to contaminant transport in unsaturated systems, where retention at the air‐water interface may be significant. Conversely, the aqueous‐phase methods may yield information with direct bearing on multiphase flow processes that are dominated by capillary‐phase behavior.

Insights into the Relationships Among Capillary Pressure, Saturation, Interfacial Area and Relative Permeability Using Pore-Network Modeling

“…These techniques are mostly categorized under two main groups; tracers or imaging techniques. Aqueous tracers have been used by Karkare and Fort (1996), Brusseau et al (1997), Kim et al (1997), Saripalli et al (1998), Anwar (2000), Schaefer et al (2000), and Chen and Kibbey (2006), and gas tracers have been used in some other studies such as Kim et al (1999), Costanza-Robinson and Brusseau (2002), and Brusseau et al (2006). Imaging techniques have been used by Montemagno and Gray (1995), Wildenschild et al (2002), Cheng et al (2004), Culligan et al (2004), Al-Raoush and Willson (2005a,b), and Brusseau (2005, 2006).…”

Transport of colloids in unsaturated porous media: Explaining large‐scale behavior based on pore‐scale mechanisms

“…The transport of colloids in the unsaturated zone is thus greatly influenced by water content [Vilker and Burge, 1980;Lance and Gerba, 1984;Powelson et al, 1990;Poletika et al, 1995]. The AWI has high affinity for both hydrophobic and hydrophilic colloids and the sorption of colloids onto the AWI is often considered almost irreversible under unsaturated conditions, due to the capillary forces that hold the colloids at the interface [Ducker et al, 1994;Wan et al, 1994;Wan and Wilson, 1994b;Wan and Tokunaga, 2002;Costanza-Robinson and Brusseau, 2002;Sirivithayapakorn and Keller, 2003]. Therefore the AWI is believed to play an important role in retaining colloids in the vadose zone [Powelson et al, 1990;Poletika et al, 1995].…”

Simulation of liquid–liquid interfaces in porous media

“…It is important to note that γL1L2 is a constant factor in Equation 10. Therefore the choice of γL1L2…”

“…The change in the total interfacial free energy is calculated using equation (10). If ΔG ≤ 0 the new configuration is accepted.…”

“…It is also important to describe the flow of liquids in the porous material. [6,7] The interfacial area of two immiscible fluids inside a porous media can be measured by two main methods: 1) the interfacial tracer technique [8][9][10][11][11][12][13][14][15] and 2) high-resolution microtomography [16][17][18]. On the other hand, numerical simulations and theoretical models are useful to gain a better understanding of the shape and extent of the interface and its relationship with parameters as porosity, grain size and contact angle.…”