# The Significance of Hydrophobicity for the Water Retention Properties of Sand and Coal
## 3 Results and Discussion
### Drying Path of the Water Retention Curves for Crushed Rock and Crushed Coal  
The drying path of the WRCs for different size fractions of the crushed materials determined from the hanging column are presented in Figure 8 in terms of the degree of saturation.  
Note that much of the data in Figure 8 that are close to full saturation have degree of saturation values slightly larger than one, which has no physical meaning. This reflects the sensitivity associated with trying to measure small changes in water content in small, moisture-sensitive materials. However, the error bars on most of these data extend below a degree of saturation of one, making it reasonable to presume that these values are indicative of fully saturated sample conditions.  
The first observation to be made from Figure 8 is that, as expected, the finer the grain size for a given material, the higher the air entry value, and the higher the degree of saturation for a given suction was. This applied to both the crushed rock and the crushed coal.  
Figure 7: ESEM images obtained during wetting of the natural sand (**a**) and hydrophobized sand (**b**). The arrows indicate the menisci bridging between particles.  
Given the values of the contact angles reported in Table 2 and the results obtained for the various sand mixtures (Figure 6), it would reasonably be expected that the coal should hold less water than the crushed rock for a given size fraction and at a given suction. However, this is not what Figure 8 shows. The crushed coal appeared to have similar drying path WRCs to the crushed rock for the 75-150 \(\upmu\)m and 50-75 \(\upmu\)m fractions, while the coal in the 212-300 \(\upmu\)m fraction, and particularly the 150-212 \(\upmu\)m fraction, appeared to hold more water than the crushed rock.  
Overall, the WRCs measured for the coal appeared to be similar to the WRCs measured for the hydrophilic materials of similar grain sizes. No reasonable hypothesis can be put forward to explain the unexpected water retention behavior of the crushed coal. It would appear that there were other unknown phenomena associated with the interaction of coal and water that were inconsistent with the expected effect of the CA on water retention.  
An ESEM image of the crushed coal is shown in Figure 9 which, similar to Figure 7, shows numerous water droplets and menisci. Figure 9 suggests that the CA of water droplets can vary considerably over even a single coal particle, for example, from around 90\({}^{\circ}\), as indicated by the cyan arrow, to angles consistent with hydrophilic material particles, as indicated by the yellow arrow. Some bridging menisci observed within the coal appear to show little to no curvature (white arrows), while others do show some concave curvature (black arrows). Thus, it appears that coal showed similarities to both hydrophobic and hydrophilic material behavior, likely due to the inherent chemical heterogeneity of coal. The localized hydrophilic behavior observed for the crushed coal could explain the coal  
Figure 8: Drying path of the water retention curves of crushed coal and crushed rock expressed as the saturation degree vs. suction: (**a**) 212–300-\(\upmu\)m size fraction; (**b**) 150–212-\(\upmu\)m size fraction; (**c**) 75–150-\(\upmu\)m size fraction and (**d**) 50–75-\(\upmu\)m size fraction.  
WRCs if this tendency was somehow dominant over the localized hydrophobic behavior that was also evident.  
It is important to note that interpretation of the geometry in these ESEM images (Figures 7 and 9) is not strictly conclusive due to the image being a two-dimensional projection of a three-dimensional assemblage, and a projection at another angle may show different expressions of the behavior. However, when comparing the behavior presented in Figures 7 and 9, it seems unlikely vastly different behavior would be observed at another angle.