C3. On Wettability and Unstable Flow in Porous Media

Once downloaded, these high definition QuickTime videos may be played using a computer video player with H.264 codec, 1280x720 pixels, millions of colors, AAC audio at 44100Hz and 29.97 frames per second. The data rate is 5Mbps. File sizes are on the order of 600-900 MB. (Other formats may be added later.) Free QuickTime players for Macintosh and Window computers may be located using a Google search on QuickTime. The DVD was produced by J. Robert Cooke.The effect of contact angle (CA) between water and porous media on flow regime in water repellent and sub-critically repellent soils will be illustrated and discussed. Water flow pattern that was monitored in a thin transparent flow chamber that ensures a 2D flow for water repellent soils (initial contact angle θ > 900) and sub-critical repellent porous media (permanent contact angle 300 < θ < 750) was remarkably affected by the CA. Plumes that were initiated by a point source at the soil surface (local perturbation) had different shape, size and longitudinal and transversal water content distributions for different contact angles and water application rates. The contact angle and water application rate control in particular the development of sharp water content decrease along the peripheral wetting front and non-monotonous water content variation along vertical cross section directions that are associated with unstable flow. For the low water application rate, thin and long plumes with saturation overshot behind the wetting front were observed for higher contact angles and wide plumes with lower average water content flow for the lower contact angles. On the contrary, negligible differences among the plumes were observed in all media for the high water application rates. Differences among the plumes became apparent only during the following drainage stage. The plumes in the media of high contact angle barely changed during the drainage period while the water content gradient along vertical cross sections turned positive at the lower part and negative along the upper part of the plumes in the media of lower contact angle. The discussion will include simulations made by a model developed for a bundle of capillaries of constant and variable cross section area and different contact angles.1_7zezj16

Effect of surfactant surface and interfacial tension reduction on infiltration into hydrophobic porous media

Surfactant molecules increase the infiltration rate into hydrophobic porous media by lowering the infiltrating water's surface tension and the interfacial tension between hydrophobic surfaces and water molecules. We investigated the relative effect of these rate-limiting processes on the infiltration rate of aqueous surfactant solution into hydrophobic porous media. Two surfactants at various concentrations were applied at a constant pressure head to 1D columns filled with hydrophobic soil, and water was applied at the same pressure head to columns filled with surfactant-pretreated hydrophobic soil. Based on the measured contact angle, surfactant pretreatment significantly reduced the hydrophobic soil's interfacial surface tension, which increased the infiltration rate compared to the direct aqueous surfactant application to the hydrophobic soil. The latter's slower infiltration rate was attributed to the depletion of surfactant molecules due to its adsorption to the hydrophobic molecules near the advancing wetting front, yielding an increase in the surface tension of the infiltrating solution. Surfactant pretreatment increased the opportunity time for surfactant adsorption to the hydrophobic molecules, resulting in interfacial tension reduction and infiltration rate increase. Diffusion-limited surfactant adsorption on the hydrophobic surfaces, leading to reduced interfacial tension between the surface and infiltrating liquid, had a greater impact on limiting infiltration into hydrophobic porous media compared to the reduction in surface tension of the infiltrating liquid due to surfactant presence

Pepper Plants Leaf Spectral Reflectance Changes as a Result of Root Rot Damage

Symptoms of root stress are hard to detect using non-invasive tools. This study reveals proof of concept for vegetation indices’ ability, usually used to sense canopy status, to detect root stress, and performance status. Pepper plants were grown under controlled greenhouse conditions under different potassium and salinity treatments. The plants’ spectral reflectance was measured on the last day of the experiment when more than half of the plants were already naturally infected by root disease. Vegetation indices were calculated for testing the capability to distinguish between healthy and root-damaged plants using spectral measurements. While no visible symptoms were observed in the leaves, the vegetation indices and red-edge position showed clear differences between the healthy and the root-infected plants. These results were achieved after a growth period of 32 days, indicating the ability to monitor root damage at an early growing stage using leaf spectral reflectance

Capillary pressure overshoot for unstable wetting fronts is explained by Hoffman's velocity-dependent contact-angle relationship

Pore velocity-dependent dynamic contact angles provide a mechanism for explaining the formation of fingers/columns in porous media. To study those dynamic contact angles when gravity is present, rectangular capillary tubes were used to facilitate observation of the complete interface without geometric distortion. Results show that the Hoffman (1975) relationship between dynamic contact angle and water velocity applies to gravity-affected flow fields, and that it (when adjusted for nonzero static contact angles) can be used to model dynamic capillary pressures for unstable wettings fronts in porous media by assuming that (1) pressure at the wetting front is discontinuous, (2) the flow field behind the fingertip is highly heterogeneous, and (3) the front line advances one or a few pores at the time. We demonstrate the utility of the Hoffman relationship for porous media with a published infiltration experiment by calculating the capillary pressure successfully at the unstable wetting front as a function of the flux of water in the finger and the grain size diameter

Exploitation of the SoilPRO® (SP) apparatus to measure soil surface reflectance in the field: Five case studies

The SoilPRO® (SP) is an assembly designed to acquire soil reflectance information in the field without disturbing the soil surface, and regardless of atmospheric and solar radiation conditions. This paper summarizes five case studies in which the SP assembly was used for different applications. The case studies consisted of: (1) generating surface spectral measurements under any atmospheric condition; (2) comparing the performance of the SP to the traditional bare fiber method for vicarious calibration of hyperspectral satellite sensors; (3) assessing water repellency of a soil surface governed by organic matter hydrophobicity; (4) spatial prediction of the rate of water infiltration into the soil profile as governed by the soil surface seal; and (5) using the SP apparatus to measure soil surface reflectance in South Shetland Island, Antartica under severe weather conditions. The case studies included calculation of spectral quality, prediction accuracy and measurement stability. The paper discusses each of the cases in detail and concludes that the SP (or similar assembly) is the best way to measure the reflectance of the original soil surface in the field. In the first case study, the spectrum collected by the SP under daily changing illumination was shown to be stable relative to the traditional measurement methods of contact probe or bare fiber. The second case study indicated that use of the SP for vicarious calibration is much more efficient (in terms of time and stability) than ground-truth practice over a large area, and in the third case study, the SP was able to assess a soil surface property governed by organic matter hydrophobicity better than the contact probe, which destroys the soil surface organic seal. A similar achievement was gained in the fourth case study, providing a better assessment of the water-infiltration rate into the soil. In the fifth case study, the SP demonstrated impressive high-quality acquisition of soil surface reflectance with a very low sun angle over the South Pole. Based on these case studies and the high quality of the data generated by the SP in the field, we suggest building, in parallel to the classical soil spectral libraries generated in the laboratory, field soil spectral libraries that will preserve the soil surface properties scanned in the field. We anticipate the development of more applications for the SP assembly based on the capabilities shown in this paper