The role of contact angle on unstable flow formation during infiltration and drainage in wettable porous media

Abstract

[1] The impact of contact angle on 2-D spatial and temporal water-content distribution during infiltration and drainage was experimentally studied. The 0.3-0.5 mm fraction of a quartz dune sand was treated and turned subcritically repellent (contact angle of 33 , 48 , 56 , and 75 for S33, S48, S56, and S75, respectively). The media were packed uniformly in transparent flow chambers and water was supplied to the surface as a point source at different rates (1-20 ml/min). A sequence of gray-value images was taken by CCD camera during infiltration and subsequent drainage; gray values were converted to volumetric water content by water volume balance. Narrow and long plumes with water accumulation behind the downward moving wetting front (tip) and negative water gradient above it (tail) developed in the S56 and S75 media during infiltration at lower water application rates. The plumes became bulbous with spatially uniform water-content distribution as water application rates increased. All plumes in these media propagated downward at a constant rate during infiltration and did not change their shape during drainage. In contrast, regular plume shapes were observed in the S33 and S48 media at all flow rates, and drainage profiles were nonmonotonic with a transition plane at the depth that water reached during infiltration. Given that the studied media have similar pore-size distributions, the conclusion is that imbibition hindered by the nonzero contact angle induced pressure buildup at the wetting front (dynamic water-entry value) that controlled the plume shape and internal water-content distribution during infiltration and drainage.Citation: Wallach, R., M. Margolis, and E. R. Graber (2013), The role of contact angle on unstable flow formation during infiltration and drainage in wettable porous media, Water Resour. Res., 49,[6508][6509][6510][6511][6512][6513][6514][6515][6516][6517][6518][6519][6520][6521]

Coupling of interfacial soil properties and bio‐hydrological processes: The flow cell concept

“…Studies using radiography were mostly conducted with Hele-Shaw cells or flow chambers with disturbed soil material (e.g., Bayer, Vogel, & Roth, 2004;Rezanezhad, Vogel, & Roth, 2006) or were characterized by image analyses (e.g., Wallach, Margolis, & Graber, 2013) when using thin rectangular experimental set-ups to conduct a 2D flow field. For our study with undisturbed soil, it was more challenging to quantify changes in volumetric water contents by calibrations as, for example, Bayer et al (2004) did in their study.…”

The moving‐boundary approach for modeling gravity‐driven stable and unstable flow in soils

“…Experimental studies performed under field conditions and in sufficiently large soil slabs have shown that water flow can occur from low to high water contents pressure [ Hill and Parlange , ; Starr et al ., ; Diment and Watson , ; Glass et al ., ; Selker et al ., ; Liu et al ., ; Glass and Nicholl , ; Yao and Hendrickx , ; Bauters et al ., ; Sililo and Tellam , ; Wang et al ., , to name only a few]. These spatial water content/pressure distributions, found in distinct finger‐like (preferential) flow paths, denoted as “gravity‐driven fingering.” Furthermore, there is a consensus among these researchers that such fingers are initially formed by instabilities at the wetting front, and once they are formed, the water pressure [ Stonestrom and Akstin , ; Geiger and Durnford , ] and water saturation [ DiCarlo , ; Shiozawa and Fujimaki , ; Wallach et al ., ] become higher at the wetting front than at shallower depths, close to the soil surface. Moreover, for these cases, the change between the wet and dry soil is abrupt, forming a sharp wetting front.…”

“…It is already known that partial wettability of soils induces gravity‐driven fingering that is characterized by a sharp wetting front and a nonmonotonic saturation distribution [ Wallach and Jortzick , ; Xiong et al ., ; Wallach et al ., ]. In the latter study, water was infiltrated with a point source into similar sands with different artificially induced contact angles (33, 48, 56, and 75°).…”

Understanding Preferential Flow in the Vadose Zone: Recent Advances and Future Prospects

“…Preferential flow results from significant spatial variations in water velocities due to heterogeneities in soil properties at both pore and Darcy scales. Preferential flow can also occur in highly uniform soils in the form of unstable “fingers,” especially in coarse, single‐grained, and water‐repellent soils (DiCarlo, 2013; Wallach et al, 2013). Although the underlying causes of preferential flow may differ, one common signature is a convergence of flow toward preferential pathways and nonequilibrium in terms of differences in water pressures and solute concentrations transverse to the flow direction, which means that no single value can describe the state of the system.…”