Capillary retention of colloids in unsaturated porous media

Understanding colloid transport behavior in unsaturated porous media is important because mobile colloid-contaminant complexes and colloidal pathogens can degrade groundwater quality. Visual evidence suggests that colloids are retained at the air-water meniscus-solid (AW_mS) interface in unsaturated porous media, but there is no quantitative theoretical explanation of this retention mechanism to date. A theoretical study is presented here to quantify energy potentials represented by capillary forces on colloids at the AW_mS interface in unsaturated porous media. Our calculations indicate that the capillary energy potentials for colloids retained in films or at the AW_mS interface range from 10⁷ to 10⁸ kt, which are several orders greater than the Derjaguin-Landau-Verwey-Overbeek energy potentials. Capillary forces for colloids at the AW_mS interface can be decomposed into a force that pushes the colloid back in the bulk solution and one that pins the colloid against the surface of the grain. A friction force generated between the colloid and grain can prevent the colloids from moving back into solution. When drag forces can be ignored, colloids will remain at the AW_mS interface as long as the static friction coefficient is greater than the tangent of the water-grain contact angle. It is independent of the surface tension.