Application of confocal laser scanning microscopy (CLSM) to visualize the effect of porous media wettability on unsaturated pore water configuration

Abstract

Purpose Wettability affects water configuration and thereby transport processes and microbial activity in soil. Approaches to visualize the effect of porous media wettability on water films surrounding particles are rarely available in the literature. The aim of this study is therefore (1) to visualize the effect of wettability on area and connectivity of the water phase and (2) to develop a segmentation strategy to enable water films and bulk water to be differentiated. Materials and methods Wettability of silica sand was rendered by silanization using dichlorodimethylsilane. The resulting contact angle was measured using the sessile drop method. Furthermore, wettability was characterized by the water penetration time test in air-dry samples and at a volumetric water content of 8 vol.%. Sulforhodamine B was used to stain distilled water at a concentration of 40 mg/l. By means of the Wilhelmy plate method, the influence of the dye on the liquid surface tension was tested. Confocal laser scanning microscopy (CLSM) was used to visualize the area and connectivity of the fluorescent-dyed water phase and the thickness of water films in untreated and silanized samples at water contents of 8, 16, and 32 vol.%. Results and discussion The silanization significantly increased the contact angle of silica sand, whereas the surface tension of the dye solution did not differ significantly from that of undyed water. CLSM visualized the distribution of water, focused across the surface grain layer. Thresholding of fluorescence signal in two-dimensional intensity projections enables the discrimination between film and bulk water. It was shown that even subcritical water repellency (contact angle <90) leads to a decrease in area and connectivity of the water phase by affecting film instead of bulk water. CLSM detects decreasing effects of wettability with increasing water content, which were no longer significant at a water content of 32 vol.%. CLSM was found to be more sensitive for detecting the effects of wettability than the water drop penetration time test.Conclusions CLSM provides new insight into wettabilitydependent water configuration. One advantage over conventional microscopy arises from the capability to visualize water focused over the pore space. Compared to other three-dimensional imaging techniques, the advantage of CLSM is its simplicity. If only the optically accessible upper layer of the porous media is of interest, sample preparation, measurements, and image analysis can easily be carried out with a cost-and time-saving setup.

The Role of Matric Potential, Solid Interfacial Chemistry, and Wettability on Isotopic Equilibrium Fractionation

“…Sorption of non‐polar organic compounds as well as heat treatment has a strong effect on particle wettability (Dekker and Ritsema, 1994; Reszkowska et al, 2014; Diehl et al, 2014; Woche et al, 2017). This has corresponding effects on the spatial distribution of the water content, on the binding state of thin water films on particles surfaces (Churaev, 2000), as well as on the shape and the radius of single water menisci (Müehl et al, 2012). Consequently, this affects the energetic interfacial equilibrium state of the water vapor pressure in a solid–water–vapor system and changes the equilibrium fractionation factor for 18 O/ 16 O and 2 H/ 1 H. However, as could be shown here, the grain size distribution has the major control on the isotopic equilibrium fractionation factor if non‐polar organic compounds are camouflaged by surface hydroxyl groups of silane.…”

Development of a universal microinfiltrometer to estimate extent and persistence of soil water repellency as a function of capillary pressure and interface chemical composition

“…It was assumed that insufficient hydraulic contact between the tip and soil caused a disruption of water films at the soil-tip contact zone (data not shown). Corresponding disconnected water films in partly saturated water repellent media have been visualized by Muehl et al (2012). Capillary barrier effects were also evident at the interface between the tip and soil, a well-known effect that occurs in layered wettable media, e.g., a fine-textured layer above a coarse textured soil layer (Li et al, 2014).…”

A critical evaluation of the Miller and Miller similar media theory for application to natural soils

“…Muehl et al . [] reported that even a subcritical level of water repellency may lead to a decrease in the water‐wetted area and disconnection of the water phase. For such cases, equations and may not perfectly hold even if a hypothetical microscopic similarity exists.…”