Evaporation-induced evolution of the capillary force between two grains

• Published in: Granular matter 2014-10, Vol.16 (5), p.815-828
• Main Authors: Mielniczuk, Boleslaw, Hueckel, Tomasz, Youssoufi, Moulay Saïd El
• Format: Article
• Language: English
• Subjects: Bridges (structures); Capillarity; Civil Engineering; Complex Fluids and Microfluidics; Constants; Contact angle; Engineering Fluid Dynamics; Engineering Sciences; Engineering Thermodynamics; Evaporation; Evolution; Fluid mechanics; Foundations; Geoengineering; Grain boundaries; Heat and Mass Transfer; Hydraulics; Industrial Chemistry/Chemical Engineering; Liquid bridges; Materials Science; Original Paper; Physics; Physics and Astronomy; Separation; Soft and Granular Matter; Surface tension
• ISSN: 1434-5021; 1434-7636
• DOI: 10.1007/s10035-014-0512-6

The evolution of capillary forces during evaporation and the corresponding changes in the geometrical characteristics of liquid (water) bridges between two glass spheres with constant separation are examined experimentally. For comparison, the liquid bridges were also tested for mechanical extension (at constant volume). The obtained results reveal substantial differences between the evolution of capillary force due to evaporation and the evolution due to extension of the liquid bridges. During both evaporation and extension, the change of interparticle capillary forces consists in a force decrease to zero either gradually or via rupture of the bridge. At small separations between the grains (short & wide bridges) during evaporation and at large volumes during extension, there is a slight initial increase of force. During evaporation, the capillary force decreases slowly at the beginning of the process and quickly at the end of the process; during extension, the capillary force decreases quickly at the beginning and slowly at the end of the process. Rupture during evaporation of the bridges occurs most abruptly for bridges with wider separations (tall and thin), sometimes occurring after only 25 % of the water volume was evaporated. The evolution (pinning/depinning) of two geometrical characteristics of the bridge, the diameter of the three-phase contact line and the “apparent” contact angle at the solid/liquid/gas interface, seem to control the capillary force evolution. The findings are of relevance to the mechanics of unsaturated granular media in the final phase of drying.