Wetting of granular and porous materials

The wetting of a powder or porous material by a liquid is a crucial first step in processes such as dispersion, dissolution, granulation and reconstitution, which are important in fields such as mineral processing, pharmacy and food science. It is also essential for hydrating soil in agriculture. It is important for efficient oil recovery and, in the future, it may help to store CO₂ underground. The article provides a comprehensive review of the wetting of powders and granular materials, emphasizing the complexity introduced by surface heterogeneity, particle size distribution, and structural inhomogeneity. It begins by describing wetting phenomena on ideal, planar or regular surfaces. Fundamental concepts such as the Young equation and contact angle hysteresis, as well as the influence of surface roughness, heterogeneity, and dynamic effects, adaptation, and slide electrification. The review then analyzes how these theoretical frameworks extend to realistic powders, where a single contact angle is insufficient to characterize wetting behavior but whole distributions of contact angles are obtained. Several experimental methods for characterizing the wettability and surface energy of powders are discussed, including capillary rise, sessile drop and drop penetration test, X-ray tomography, inverse gas chromatography, secondary-ion mass spectrometry, sink and drop impact tests. The article highlights the limitations and suitability of each method in relation to specific applications and recommends careful selection based on application needs and powder properties.