Evolution of meniscus structures in hydrophobic granular systems

Hydrophobic soils, which form naturally in arid regions or after forest fires, can be problematic for land managers and engineers as they are often associated with impeded or preferential flow paths, increased surface runoff and soil erosion. However, the reduced rainwater infiltration capacity of water-repellent soils can also result in the improvement of the stability of slopes, landfills and capillary barrier cover systems, amongst others. Understanding the hydraulic conditions within these materials is critical if issues of stability and seepage are to become tractable. Traditional understanding of unsaturated hydrophobic soils suggests that convex water menisci, and so positive water pressures, should form between soil particles. However, the limited experimental results presented in the literature do not support this theory. In this work, the effect of particle shape on the formation and evolution of water meniscus structures is investigated at the macro (multiple particles) and particle scales, contrasting meniscus behaviours between spherical glass beads and angular sand grains. The spreading of a sessile drop in the macro-scale is examined and found that the angularity of the sand grains has a significant effect on the apparent contact angle of a sessile drop when deposited on a mono-layer of particles. At the particle scale, Environmental Scanning Electron Microscopy was used to investigate the formation and evolution of capillary bridges and the water retention hysteresis during two wetting and drying cycles. Again, it is shown that the shape and surface roughness of the particles are controlling factors in both the formation and evolution of liquid bridges and that stable convex and concave menisci can co-exist simultaneously between hydrophobic particle surfaces. Additionally, it was found that the hydrophobic nature of the particles allowed menisci to form across much larger separation distances than could be achieved through film coalescence between hydrophilic surfaces, with possible consequences for infiltration and imbibition modelling and, more broadly, manufacturing processes relying on hydrophobic substrates. Lastly, the hydrophobic soils qualitatively exhibited overall much less hysteresis of the water retention curve than their hydrophilic counterparts.