# The Significance of Hydrophobicity for the Water Retention Properties of Sand and Coal
## 2 Materials and Methods  
### Materials  
The materials used in this study included washed Stockton beach sand [13], blocks of intact coal from a Permian black (bituminous) coal mine in New South Wales, Australia and fresh crushed aggregate from a dacite rock quarry. The sand was selected as a suitable basic granular material, for which the effects of a progressively increased proportion of hydrophobic particles could be studied. The crushed fresh dacite rock was selected to provide a source of clay-free soil with typical hydrophilic minerals that might be found in most soils and which could serve in a comparison with crushed coal, with its textural characteristics being otherwise equivalent. The sand was used in its natural condition; that is, its particle size and shape were not modified in any way. The rock and coal were first crushed into pieces of less than 10 mm in size using a ceramic pestle and then ground into a powder with an agate mill (Retsch RM200). The crushed material was then sorted into four particle size ranges by wet sieving (with deionized water) through appropriate sieve stacks. The process was accelerated by using an ultrasonic probe to agitate the particles, increasing the drainage rate of the water and particles through each sieve. Coal is known to spontaneously oxidize upon exposure to air, which could potentially alter its hydrophobicity [14; 15; 16]. To avoid measuring data on coal with evolving properties, the coal was allowed to oxidize for 6 months before being used. It was assumed that after this period of time, further oxidation at the coal surface would be slow enough such that the hydrophobicity would be relatively consistent over the testing period. The particle size ranges and corresponding specific gravity (measured using a Micromeritics Accupyc II) for all of the materials are reported in Table 1.  
From the observations of microscope images of each particle size fraction of coal and rock, the particles appeared to have a similar angular shape, as is evident from the microscope images of the crushed materials between 150 and 212 m in Figure 1.  
Similar shapes to those shown in Figure 1 were seen for all size fractions. The microscope images of the crushed materials were also used to construct an estimate of the PSD \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Particle Size Range (\(\mu\)m)** & **G\({}_{\text{s}}\) of Crushed Rock** & **G\({}_{\text{s}}\) of Crushed Coal** & **Sand** \\ \hline
212–300 & 2.62 & 1.38 & - \\
150–212 & 2.62 & 1.4 & - \\
75–150 & 2.62 & 1.4 & - \\
50–75 & 2.62 & 1.39 & - \\ \(\approx\)100–300 & - & - & 2.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specific gravity (G\({}_{\text{s}}\)) of the natural sand and the crushed rock and crushed coal at each size fraction. Figure 1: Microscope images of crushed materials: (**a**) rock, showing crystal fragments of quartz (clear) and feldspar (translucent) and (**b**) coal. within each sieve size to verify that, within the size fraction, the materials had a similar PSD. To construct the GSD for each material, the open-source image analysis software ImageJ was used, which provided the maximum and minimum Feret diameter of each particle in an image. Each particle was then simplified to a sphere, with the diameter equal to the average of the two Feret diameters. Note that the accuracy of the PSD constructed with this method was limited by a lack of information in the third dimension. Nevertheless, each particle in the microscope images had an associated volume and diameter, allowing the size distributions to be determined. The resulting PSDs are shown in Figure 2, which shows that the PSDs of the crushed rock and crushed coal were relatively consistent for all but one size fraction. Only the 75-150 \(\upmu\)m fraction displayed a small difference between rock and coal, which was still deemed acceptable given the lack of control of particles between two consecutive sieves.