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

Evolution of surface roughness of single sand grains with normal loading

The surfaces of soil grains are not perfectly smooth, especially examined at small scale. In geotechnical engineering, surface roughness has been found to be able to influence the inter-particle friction angle at micro scale and small-strain stiffness at macro scale. However, the quantity and quality of the studies on surface roughness of natural soils are still limited. In this study, the evolution of surface roughness of natural sand grains with increasing normal load was investigated by a single-particle compression apparatus. Thirty Leighton Buzzard sand (LBS) grains coarser than 2·36 mm were tested, and the surface roughness was measured before and after compression by an optical interferometer. The deformations of the asperities and of the bulk of the sand grains in the vicinity of the contact were mapped. Three stages were identified as the normal load increased: (a) plastic deformation of the asperities; (b) asperities and bulk plastic deformation; and (c) bulk only plastic deformation. At very small normal load, only the asperities were found to deform plastically, and the surface roughness of the sand grains decreases due to the flattening of the asperities. Within this regime, the load–displacement relationship of LBS grains under compression could be simulated by the modified Hertz model, which takes surface roughness into consideration. With increasing normal load, the bulk of the sand grains began to yield near the contact. The geometry of the surfaces of LBS grains in contact with the loading platen is the main factor that influences the plastic deformation of the bulk. Differently from the plastic deformation of the asperities, the plastic deformation of the bulk could both smoothen and roughen the surfaces. When plastic deformation of the bulk occurred, both Hertz and modified Hertz theory could not predict the load and displacement relationship of sand grains. Through analysing the cumulative distributions of surface roughness of 30 LBS grains at different normal loads by the Weibull function, the surface roughness was found to decrease dramatically with increasing normal load at first and then tended to be constant

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

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

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

Wettability decay in an oil-contaminated waste-mineral mixture with dry-wet cycles

The dependency of soil particle wettability on soil water content implies that soils subjected to drying-wetting cycles become wettable with wetting and water repellent with drying. While this has been demonstrated widely, the results are contradictory when water repellent soils are subjected to a sequence of cycles. Added to this, past wettability measurements were seldom done in batches of samples collected from the field at natural or dry water contents, with little appreciation that slight particle size variations, different drying-wetting histories and fabric (as required by different wettability measurement methods) may alter the results. This note presents soil particle wettability—soil water content relations by means of an index test following staged drying and wetting paths over a period of 8 months for an untreated, oil-contaminated anthropogenic soil (a mixture of slag, coal particles, fly ash and mineral particles) from Barry Docks (UK), a site formally used for oil storage, which is to be remediated and redeveloped for housing. The results revealed a decrease in the water repellency and increasing mineralization and bacterial activity with the wetting and drying cycles.postprin

Cavitation in high-capacity tensiometers:effect of water reservoir surface roughness

High-capacity tensiometers (HCTs) are sensors made to measure negative pore water pressure (suction) directly. In this paper, a new approach is proposed to expand the range and duration of suction measurements for a newly designed HCT. A new technique is employed to reduce significantly the roughness of the diaphragm’s surface on the water reservoir side in order to minimise the possibility of gas nuclei development and the subsequent early cavitation at the water–diaphragm interface. The procedures employed for the design, fabrication, saturation and calibration of the new tensiometers are explained in detail. Furthermore, the performance of the developed HCTs is examined based on a series of experiments carried out on a number of unsaturated clay specimens. An improvement in maximum sustainable suction in the range of 120–150% of their nominal capacity was obtained from different surface treatment methods. Moreover, the results show an improvement of up to 177% for the long-term stability of measurements, compared to the developed ordinary HCTs with untreated diaphragms