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The impact of heat treatment and wetting–drying cycles on the wetting properties of sandy forest soils was explored. Topsoil and upper subsoil were sampled at three beech forest sites in northern Germany. The air‐dried soils were treated at 20, 40 and 80°C for 24 h, with materials treated at 20°C serving as reference. After wetting, materials were air‐dried or shock‐frozen in liquid N2 and freeze‐dried. Interfacial properties were monitored by sessile drop contact angles (CAs) and X‐ray photoelectron spectroscopy (XPS), which provide physical and chemical information on the outermost particle interface layer. CAs of reference samples were around 90° and significantly increased after 80°C‐treatment to >90°, whereas 40°C‐treatment had in comparison to reference soils no distinct impact on CA. Depending on the initial temperature treatment, air‐drying after wetting decreased CA to 60–80% and shock‐freezing and freeze‐drying decreased CA to 10–50% of the reference value. Results suggest that shock‐freezing may preserve the organic matter molecular structure that prevails during contact with water at the solid–liquid interface, thus indicating the wettability of the wet surface. Generally, wetting–drying cycles had the least impact on 80°C‐treated material. XPS analysis confirmed dynamic interfacial molecular restructuring processes by changes in O and C content and the content of non‐polar C compounds. A second heat treatment after two wetting–drying cycles again proved the distinct and pronounced impact of 80°C‐treatment on CA, especially with prior shock‐freezing and freeze‐drying. In conclusion, the findings of our study indicate a sensitive and partly reversible reorganization of the solid interfacial wetting properties. Results may conceptually be used to develop dynamic wettability models, which are needed to simulate the sensitive interplay between wettability and dynamic soil hydraulic functions at sites that are exposed to intensive and periodic moisture fluctuations. Highlights: T > 40°C increased contact angle; after wetting contact angle depended on how the water was removedShock‐freezing and freeze‐drying preserves contact angle of the wetted stateContact angle changes confirm interface molecular restructuring due to heat treatment or wettingContact angle change goes along with changes in element content within XPS analysis depth [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of wettability on colloid transport and deposition was studied. [•] Colloid breakthrough could be described with attachment and detachment kinetics. [•] High colloid deposition rates were found in hydrophobic porous media. [•] Effects of wettability were most pronounced at higher ionic strength. [•] High deposition rates were related to strongly attractive acid–base interactions. [Copyright &y& Elsevier]

Summary: Aggregate stability is frequently shown to be enhanced by strong soil water repellency, however, there is limited systematic evidence on this effect for moderately (subcritically) water repellent soils. This study aimed to investigate the specific effects of interfacial properties on the liquid penetration kinetics in relation to the stability of subcritically water repellent aggregates (4–6.3mm) from various arable and forest soils against breakdown by slaking. In contrast to many other studies, where aggregate stability was determined by wet sieving, we here assessed the stability by immersion of air-dry aggregates in water–ethanol solutions with surface tensions ranging from 30 to 70mNm−1. This approach allowed a highly sensitive discrimination of different stability levels and the determination of breakdown kinetics also for less stable aggregates. Interfacial properties were characterized in terms of contact angle measured on crushed aggregates, θ c, and calculated for intact aggregates, θ i, based on infiltration measurements with water and ethanol. Aggregate stability turned out to be higher in forest soils compared to arable soils with topsoil aggregates generally found to be more stable than subsoil aggregates. For water repellent aggregates, characterized by contact angles >40° and low water infiltration rates (<0.2mm3 s−0.5), the fraction of disrupted aggregates after 30s of immersion was generally below 10%, whereas in case of the more wettable aggregates, characterized by contact angles <10° and higher infiltration rates (>0.25mm3 s−0.5) more than 80% of the aggregates were disrupted. In accordance, we found a close relationship between aggregate stability and wettability with differences between θ c and θ i being generally small. In addition, aggregate stability turned out to be related to organic carbon content. However, correlation analysis revealed that both persistence of aggregate stability and kinetics of aggregate breakdown were more strongly affected by the contact angle, θ c (r =0.90 and r =−0.83, respectively) and θ i (r =0.89 and r =−0.76, respectively) than the organic carbon content (r =0.62 and −0.52, respectively), suggesting that stability was primarily controlled by aggregate interfacial properties. Calculation of liquid penetrativity as a function of surface tension and contact angle clearly demonstrated the importance of both solid and liquid interfacial properties in determining the stability of subcritically water repellent aggregates against slaking. [Copyright &y& Elsevier]

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 wettability-dependent 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. [ABSTRACT FROM AUTHOR]

The effect of organic matter sorption and microbial activity on sand particle wettability was investigated. Wettability generally decreased after sorption of organic matter. Low C/N ratios suggest that reduction in wettability was also caused by microbial activity. Formation of biogeochemical interfaces can strongly change original particle wettability. Batch incubation and column percolation experiments with dissolved organic matter solution at different pH values were performed to investigate the effect of organic matter sorption and microbial activity on the wettability of clean quartz sand particles. Sodium azide was used as a biocide to control microbial activity. Changes in wettability were evaluated by contact angle measurements. Wettability of the quartz sand was found to significantly decrease after sorption of organic matter. An adsorbed C content of <0.1 g C kg-1 turned out to be effective in increasing the contact angle from initially 30 to >70°. Results of the percolation experiments revealed that the change in contact angle was depth dependent, with the greatest increase in the upper part of the columns. The low C/N ratio (<10) of the organic matter adsorbed in the upper part of the sand columns percolated without biocide addition suggested that the depth dependency was caused not only by sorption of organic matter but also was affected by microbial activity. This finding was supported by an increasing contact angle for sand columns percolated with a solution close to neutral pH (6.7), where more favorable living conditions for bacteria could be assumed. We also observed a trend of a linear inverse relationship between C/N ratio and contact angle, i.e., increasing contact angle with decreasing C/N ratio. Overall, the results indicated that the formation of biogeochemical interfaces can strongly change the original wetting properties of mineral particle surfaces. [ABSTRACT FROM AUTHOR]

[Display omitted] • Functionalization of silica by alkylsilane precursors modifies surface properties. • Surfaces were analyzed by experiment and molecular dynamics simulations (MDS). • XPS and MDS helped to explain composition of modified surface and its wettability. • Contact angle depends on type, density, and arrangement of coating compound. Complexity of organic coatings on soil mineral particles hampers identification of the contribution of individual compounds to soil wettability. This study focused on the impact of different organic coatings on surface wettability by using dimethyl(DMS)-, aminopropyl(APS)-, and octadecyl(ODS)-siloxane coated glass slides prepared by reaction with organosilanes. Surfaces were examined for experimental contact angle (CA exp) and coating thickness (t exp) determined by X-ray photoelectron spectroscopy. Molecular dynamics simulations (MDS) were performed on models with a water nanodroplet in contact with the (0 0 1) surface of tridymite coated by DMS, APS, and ODS monolayers. Chain length, polarity of terminal group, and coating density were found as the main factors determining surface wettability. Calculated CA MDS for DMS and APS coatings were distinctly < 90°, while CA MDS for ODS was > 90°. The main factor for small CA MDS was the polarity of the terminal amino group for APS, and the short alkyl chain for DMS. Measured CA exp was ≥ 90° with t exp ≥ 1 nm for all coatings, except for the sample prepared with dimethyldiethoxysilane. This hinted a more complex surface coating with a multilayer arrangement. The results showed that combined MDS and experimental results from well-defined model systems will contribute to a better understanding of factors determining soil wettability. [ABSTRACT FROM AUTHOR]

Abstract: Soil wettability affects physical properties such as aggregate stability, infiltration rate, or erodibility. To describe the wetting properties of soil, the soil–air–water contact angle (CA) is often used. At present, a direct measurement of the CA determined on intact soil aggregates and a direct comparison with corresponding homogenized aggregates is still lacking, mainly because standard methods have not been defined to measure the wettability of soil aggregates. In this study, the Capillary Rise Method (CRM) was used to assess contact angles of single intact 1–2 mm soil aggregates, packings of intact aggregates and packings of crushed (homogenized) aggregates of 9 topsoils and 3 humus subsoils from 5 sites in Germany. In general, CAs of the homogenized aggregates were quite similar for all soils (65°±10°), while CAs for aggregate packings and single aggregates generally were larger for grassland and forest soils (60° to 80°) than for arable soils (0° to 20°). It was concluded that all soils contain potentially hydrophobic pore surfaces, but the effectiveness of these surfaces to create water repellency depends on the small-scale architecture of the pore space and the distribution/position of hydrophobic components inside the matrix. These findings suggest that it is misleading, particularly for agricultural soils, to quantify wetting properties by only analyzing homogenized soil. We propose that CRM-CA measurements should be extended to investigations of intact aggregates and the wetting coefficient k =cosθ should also be displayed to distinguish more clearly between completely wettable soils and soils with low subcritical repellency. Finally, it should be noted that the methods proposed are only applicable to non-hydrophobic soil exhibiting subcritical water repellency (CA >0°,…, CA ≤90°). [Copyright &y& Elsevier]