Quantification of the surface roughness of quartz sand using optical interferometry

In comparison to the description of particle size and shape, the surface roughness, which mainly affects the inter-particle friction, is more difficult to measure and quantify. One difficulty arises from the variability between particles and the heterogeneity of roughness within one particle. In this study, optical interferometry, which has the advantage of non-contact measurements of the particle surface, was adopted to measure the surface roughness of a quartzitic sand (Leighton Buzzard sand - LBS). The roughness was determined as the root mean square deviation (RMSf) of the surface from the mean plane over a field of view of 106.6*106.6 μm². This size of field of view is limited compared to the whole surface area of one particle. Three fractions of LBS particles were used to study the effect of particle size on the surface roughness and the roughness was measured at different points across the surface of coarse particles to assess the number of measurement points required for surface roughness quantification. The measurements revealed the followings. (1) The roughness of LBS can be measured by optical interferometry, mainly due to the high reflectivity of the quartz and the rounded particle shape. (2) RMSf of LBS with different particle sizes increases with the size of field of view first and tends to converge at larger sizes. (3) Surfaces of medium size (1.18-2.36 mm) particles are the smoothest. (4) Roughness of one particle varies at different measurement points, with no correlation between the mean value of RMSf and the number of points measured, while the standard deviation reaches a constant value only after a specific number of measurement points, 3 for 1.18-2.36 mm particles and 5 for 2.36-5mm particles

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

3D fractal analysis of multi–scale morphology of sand particles with μCT and interferometer

The particle morphology of granular materials comprises different characteristic scales, including particle shape and surface texture. Different methods have been proposed to characterise the morphology using three-dimensional parameters, among which is the fractal method. These methods, however, are applied either at the scale of particle shape or surface texture. A framework unifying the multi-scale morphology obtained from different measuring instruments could advance the current understanding to this topic, but is still lacking. This paper proposes a novel methodology to characterise the morphology of sand particles across different scales based on results from two previously adopted instruments with different measuring capabilities – an X-ray micro-computed tomography (μCT) and a high-resolution optical microscope equipped with an interferometer. The methodology is applied to sand-sized particles of a crushed granitic rock and a natural quarzitic sand (Fujian sand). By using spectrum analysis on data from both μCT and interferometer measurements, a single fractal dimension is found linking the spectrum of the two measurements for the crushed granitic rock. For Fujian sand, two self-affine patterns are observed, which serves as a separation between particle shape and surface texture, and also indicates that the fractal dimension obtained at larger scale may not be simply extended to small scales. The translation of surface measurements into numerically reconstructed particle morphology at particle shape and surface texture scale is demonstrated by using spherical harmonic expansion and power spectral density functions

Experimental insight into the particle morphology changes associated with landslide movement

Rainfall-induced landslides are major natural hazards that are inherently inter-disciplinary crossing various fields (from geology to hydrology) and scales (particle level to catchment scale and beyond). Comparatively, very few studies on landslides have been conducted at the particle level with most research focusing on particle breakage or the effect of particle size and shape on the macro-scale and particle contact behavior. Limited evidence suggests that soils under shear undergo changes not only of size but also of shape and surface roughness. However, the particle morphology descriptors used are frequently qualitative so that information on how the soil particles are damaged during landsliding remains incomplete. This paper uses quantitative particle morphology descriptors, namely particle size, shape, and particle surface roughness to investigate particle damage during landslide-induced shearing. A series of ring shear tests were conducted to simulate landslide movement in completely decomposed volcanic rocks (CDV), a typical soil in Hong Kong that was retrieved from a debris flow. Particle size, shape, and surface roughness were analyzed on original CDV particles and on samples subjected to ring shear testing. Owing to the crushable nature of the soil, particle breakage was the key factor controlling particle morphology, with the results revealing an intricate dependency of shape and surface roughness on particle size. Shearing enhanced the bimodal gradation of the soil, with the larger grains more rounded and smoother and the resulting fines with a more irregular shape. This may be attributed to a combination of chipping and abrasion of the coarser particles. Further research is needed to ascertain the effect of such particle morphology changes to landslide movement

Wettability assessment of an oil coated soil

Soil wettability depends on the nature of the grain’s surface, existence of hydrophobic substances and can vary both spatially and temporarily. Low wettability conditions occur in both the natural and the built environment, including waste and contaminated soils. Low wettability in soils influences water infiltration, evaporation, and soil water retention. This contribution presents an assessment of wettability of oil contaminated samples recovered from South Quay, Barry Docks, UK. The Water Drop Penetration Test and the Molarity of an Ethanol Droplet tests were used to assess soil wettability. Measurements were conducted on statically consolidated samples at decreasing water contents. The results are discussed within the context of unsaturated soil mechanics and have applications in geo-environmental engineering.link_to_subscribed_fulltex

3D Analysis of gravel surface texture

Surface texture plays an important role in understanding the mechanical and hydraulic behavior of a soil but has drawn little attention, especially for three dimensional parameters. Currently, the few studies on measurements and quantification of surface texture are limited to sand sized particles. To date, the surface texture of sand has been successfully obtained by using a high-resolution optical microscope for a given measurement area that is small and comparable to the sand particle size. However, for larger particles, such as gravel, the larger surface area may create difficulties. For instance, as the particle size increases, the surface texture could be influenced by other factors such as the mineral composition, scale-dependent fabric and others, hindering the simple extension of a method that quantifies the surface texture of sand to gravel. This paper attempts to address this challenge by proposing a method to measure and quantify the surface texture of gravel which explicitly takes into account the different texture scales. Crushed granite is taken as the testing material. To explore the surface texture at different scales, a 3D laser scanner (at millimeter scale) and a high resolution optical microscope equipped with interferometry (at micro-meter scale) have been employed. The novel method defines and quantifies the surface texture of gravel with the aid of the power spectral density function (PSD) and a fractal method. It was found that the fractal dimension at the two different scales do not coincide, which might be interpreted as two different self-affine patterns of the surface. For example, the features of single minerals (e.g. fractures, cleavage, hardness) dominate at the small scale and become less prominent at the large scale where grain size and shape prevail. When a single fractal dimension from PSD at the large scale is assumed for both scales, the associated error can be quantified using the square root of the surface height to a mean plane and is found to be within 15%. This suggests that the surface texture examined at a large scale by using the 3D laser scanner could be representative of the gravel, though cautions should be taken in individual cases for any quantitative studies of the effect of the surface texture

Formation and evolution of water menisci in unsaturated granular media

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