Mechanical properties of wet granular materials are due to random network of capillary bridges which form between adjacent grains, exerting attractive forces by surface tension of the liquid. This effect governs the statics and dynamics of hill slopes (land slides), the physics of mixing and agglomeration in food processing and pharmaceutical industry, etc. A direct relation between the mechanical properties, which are almost independent of the liquid content in a wide range, and the internal liquid morphology was not yet established. Using X-ray micro-tomography M. Scheel et al. measured the geometry of the liquid distribution. It was found that the open filling morphology is preferred to the compact one. As a consequence the Laplace pressure is almost constant which explain the mechanical characteristics.
M Scheel, R. Seemann, M. Brinkmann, M. Di Michiel, A. Sheppard, B. Breidenbach, S. Herminghaus, Morphological clues to wet granular mechanics, Nature Materials 7, 165 (2008).
Tomographic sections of glass beads (upper panel) and sand (lower panel) as a function of the liquid content.