Force measurements between particles and the air‐water interface: Implications for particle mobilization in unsaturated porous media

Capillary forces acting at the air‐water interface play an important role in colloid fate and transport in subsurface porous media. We quantified capillary forces between different particles (sphere, cylinder, cube, disk, sheet, and natural mineral particles) and a moving air‐water interface. The particles had different sizes and contact angles (ranging from 14° to 121°). Theoretical calculations using the Young‐Laplace equation were used to support and generalize the experimental data. When the air‐water interface moved over the particles, there were strong capillary forces acting on the particles in the direction of the moving interface. The measured maximum capillary forces were similar to those calculated by the Young‐Laplace equation. The larger the contact angles and the larger the particle size, the stronger were the capillary forces. Particles with irregular shape and sharp edges experienced greater forces than smooth particles. Generalization of the results indicates that capillary forces exerted by a moving air‐water interface can readily exceed attractive Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) and gravity forces for typical subsurface particles, and a moving air‐water interface is therefore an effective mechanism for mobilization of particles in porous media. Particles in the colloidal size range are particularly susceptible for mobilization by a moving air‐water interface.

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