Colloid retention at air–water interface in a capillary channel

With the purpose of better understanding of interactions between colloids and air–water interface (AWI) as applied to natural unsaturated porous media, pore-scale experiments were conducted using confocal microscopy with sulfate (hydrophobic) and carboxylate-modified (hydrophilic) polystyrene microspheres as model colloids. Static experiments in an open capillary channel were performed at four solution chemistries to investigate the effects of electrostatic interactions and surface tension on colloid interactions with AWI. Additional experiments were conducted using a microfluidic channel to improve visualization and evaluate the interactions in a dynamic system. Electrostatic interactions, colloid contact angle, and surface tension (due to the influence of surface tension on hydrophobic attraction between a colloid and AWI) had an effect on colloid retention at AWI. The experimental behavior of colloids was quantitatively evaluated using the treatment of hydrophobic force available in the literature, which reproduced the observed experimental trend. The presence of an attractive force between a colloid and AWI, which depends on colloid and AWI hydrophobicity and scales in magnitude with electrostatic force, was demonstrated. Theoretical estimation of the colloid–AWI interactions (including electrostatic, van der Waals, and hydrophobic interactions) as well as experimental observations suggest that retention of colloids at AWI occurs mainly via secondary-minimum retention due to hydrophobic attraction. While the retained colloids can be transported with AWI, secondary-minimum retention also serves as a means to deliver colloids to the contact line (contact region of air, water, and solid phases), where more significant retention was observed.