Optimising the hydrophobicity of sands by silanisation and powder coating

Sands are naturally hydrophilic granular materials, yet, rendering them hydrophobic could lend them to a wide range of geotechnical applications. This study describes a powder-coating procedure performed after chemically modifying the surfaces of coarse, medium and fine sands and examines its effect on their hydrophobicity. The purpose is to render these granular materials more hydrophobic than what is conventionally achieved by chemical methods using a simple technique. The procedure consists of first silanising both the sands and silica powder at a similar concentration by means of an organosilane to modify their surface chemistry, then the silica powder is adhered to the sands at a mass mixing ratio to alter their hydrophobicity. Irrespective of the concentrations and mixing ratios, the powder-coating procedure enhances the hydrophobicity of sands in comparison to the sole use of the chemical method. Changes in the morphology of the sand grains, such as their particle size, particle shape and surface roughness, resulting from the powder-coating procedure are examined by means of dynamic image analysis, profilometry and scanning electron microscopy. The effects of surface chemistry, surface roughness and air on the hydrophobicity of the sands are discussed based on theoretical wetting models to analyse the experimental results

Wettability of crushed air-dried minerals

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A comparison of wettability measurements on a synthesised water repellent sand

Controlling the wettability of granular materials such as soil offers the opportunity to generate new materials. Such materials can completely prevent or partially restrict infiltration depending on their wettability. In this study, the wettability of a synthesised water repellent sand, isolated into four different sieve fractions was investigated by means of 2 different methods: the sessile drop method (SDM) and the Wilhelmy plate method (WPM). Both methods were shown to be effective in the measurement of contact angles (CAs) despite considerable differences in their absolute values. These differences were primarily attributed to the different methodologies which relied on different principles to measure CAs. The CAs measured with both the SDM and WPM showed a decrease in magnitude as particle size increases. The maximum differences in CAs recorded with the SDM and WPM between the particle sizes were respectively 13.3° and 26.1°. In addition to adequately describing the methodology adopted for the measurement of CAs, it is recommended to use the SDM over the WPM for soil samples with considerable clay content

Engineering water repellency in granular solids

The use of water repellent granular solids such as soils is an innovative technology for use in applications such as water tight barriers. Synthesising such solids generally necessitate the exclusive use of chemical treatments with little consideration given to the physical characteristics of the solids. This paper summarises the theoretical framework of surface wettability and contact angle by illustrating the classic models developed. The wettability of 3 isolated sieve fractions of a sand was investigated after treatment with dimethyldichlorosilane (DMDCS). The largest contact angle (measured by the sessile drop method) was achieved with the finest fraction (63-212 μm). Comparison between a flat microscope slide treated with DMDCS and the 63-212 μm fraction showed that the sand had a significantly larger contact angle (a maximum difference of 20°). This difference was attributed to the particle characteristics which includes particle size, particle shape and surface roughness. The results of the study hint at the possible usage of the physical characteristics of soils in an engineering context to control water repellency

Effect of particle size on the measurement of the apparent contact angle in sand of varying wettability under air-dried conditions

Session: Advances in Experimental Methods: Mechanical PropertiesChanges in the wettability of soil are known to affect several processes such as infiltration and the shear strength of soil. In this study, the wettability of a medium to fine sand was chemically modified by using different concentrations of dimethyldichlorosilane (DMDCS). The sessile drop method (SDM) was used for the assessment of wettability of hydrophobised Leighton Buzzard Sand (LBS). The results demonstrate that beyond a concentration of 2 g per kg of LBS, the finer fraction had its apparent contact angle (ACA) increased up to 115° while the maximum ACA attained by the coarser fractions was 100°. At such high concentration of DMDCS, the effect of trapped air, which is known to increase the ACA, was found to be either small or insignificant. The standard deviations of the ACAs agreed well with past studies. The most important factors contributing to the water-repellent behaviour of chemically synthesised sand were attributed to the characteristics of the particles; these include surface area and particle shape.published_or_final_versio

A Semi-Automated Technique for Repeatable and Reproducible Contact Angle Measurements in Granular Materials using the Sessile Drop Method

Contact angles (CAs) are used to measure the extent to which a material is wettable. Granular materials such as natural soils and crushed minerals, which are commonly assumed wettable, can exhibit non-wetting characteristics. The sessile drop method (SDM) is a direct method widely used to generate and measure CAs, however, the procedure involved in their determination is often overlooked leading to very large standard deviations in their measurements. In this study, a close examination of the steps involved in extracting the CAs on granular materials shows that two factors, the image exposure and the position of the baseline, can affect CAs measurements significantly. Seven methods of fitting CAs were compared. It was found that the discrepancy between the methods became more and more significant as CAs increase in magnitude. A semi-automated technique has therefore been proposed through this study to improve the standard deviations of CAs measurements. The new technique uses five steps and involves an adjustment of the image exposure and manual movement of the baseline. The proposed method was tested on flat surfaces as well as granular materials (chemically treated sand and a naturally occurring hydrophobic mineral). The results have shown that the method can be applied for both flat and granular materials with a wide range of CAs. In particular, the standard deviations of flat surfaces (e.g., hydrophobized microscope slides) with CA in the range of 90 to 135° recorded improvements of 37%. For granular materials (e.g., fluorspar) with CA in the range of 105 to 120°, improvements of 33% in standard deviations have been observed

Physical properties controlling water repellency in synthesized granular solids

The wettability of granular solids such as soil is known to depend primarily on two factors: their inherent chemistry and their physical properties, such as their particle size, particle shape and surface roughness. Nevertheless, the distinctive physical properties of such materials have not been fully explored to gauge their wettability. In this study, the difference in wettability between a flat solid (microscope slide) and three granular solids, namely glass beads (GB), Leighton Buzzard Sand (LBS) and crushed Glass (CG), which have different physical properties, were examined. The effect of chemistry was isolated by strongly hydrophobizing the above materials by treatment with dimethyldichlorosilane. Wettability measurements were made by measuring the water–solid contact angle (CA) by the sessile drop method after adhering one layer of uniformly oriented granular solids on to double‐sided adhesive tape initially attached to a microscope slide. Techniques for particle characterization included sieving for particle size, dynamic image analysis for particle shape and confocal laser microscopy to determine surface roughness. Results show that all CAs of the granular solids exceeded that of the hydrophobized microscope slide (103°). The crushed glass had the largest CA (125°). With all three granular solids, there was an increase in CAs as particle size decreased. In addition, as particles became more angular, CAs increased. The influence of shape on wettability became more predominant as particle size decreased. The surface roughness parameter, Ra, was investigated and shown to be sensitive to both the size and shape of the particles. A decrease in Ra from 95.4 to 34.1 μm increased CAs from 107 to 125°. A similar change in CA was shown to correspond to an increase in void fraction from 40.7 to 77.4%. Our results have practical implications for the optimum use of soil by enhancing or suppressing water repellency