Formation and evolution of water menisci in unsaturated granular media

Loose particles in moist granular materials, such as unsaturated soils, are held together by capillary forces acting at the interparticle contacts. The magnitude of these capillary forces depends on the surface tension and on the radius of curvature of the menisci, which in turn depends on the contact angle of the air/water interface against the surface of the solid particles. Menisci are usually assumed to be predominantly concave on the side of the air with negative water pressure (relative to the atmosphere). Evidence for this comes from direct observations at the millimetre scale or from theoretical assumptions. This note presents data from environmental scanning electron microscopy of particles at the micrometre scale that contradict this assumption and show for the first time that, for a given water content, the contact angle between air/water interfaces and grains can give rise to a variety of meniscus shapes, with curvatures not all concave on the side of air. It was found that the curvature of water menisci, in both idealised and natural granular materials with variable particle sizes, shapes and nature, could vary along the border of a single meniscus or differ from one point to another separated at the micrometre scale, and is also dependent on the nature of materials and wetting history. Menisci can have both predominantly convex shapes (corresponding to compressive capillary pressure) and predominantly concave shapes (corresponding to tensile capillary water pressure). These observations confirm the importance of surface tension in the air/water interfaces (often also referred to as 'contractile skin') in holding particles together.