Interfacial interactions and colloid retention under steady flows in a capillary channel

Transport of Nanomaterials in Unsaturated Porous Media

“…Attachment to the air-water interface has been observed for both hydrophobic and hydrophilic colloids with either positive or negative surface charges (24,25,(29)(30)(31)(32), but hydrophobic colloids have been reported to have a greater affinity for the air-water interface than hydrophilic colloids (24,(29)(30)(31)(33)(34)(35). Major forces controlling colloid attachment to the air-water interface include electrostatic, van der Waals, hydrophobic, and capillary interactions (25,27,31,32). As has been observed for solid-water interface attachment, electrostatic interaction has also been found to be significant in air-water interface attachment, especially for hydrophilic colloids (31,36,37).…”

“…As such, positively charged colloids usually show greater adsorption to the air-water interface than negatively charged colloids (29,37,40). Hydrophobic interactions between colloids and the air-water interface are believed to be much stronger than DLVO forces (i.e., electrostatic and van der Waals forces), so are able to overcome the energy barrier for the air-water interface attachment, allowing even negatively charged colloids to attach to the air-water interface in some cases (31,32,41,42). As is the case with attachment to the solidwater interface, attachment to the air-water interface also strongly depends on surface properties of colloids (hydrophobicity, zeta potential), solution chemistry (pH, ionic strength), and pore water flow rate.…”

“…However, some researchers have suggested that decreased solid-water interface attachment in unsaturated transport is likely, partly due to the decreased amount of solid-water interface that is accessible to colloids as water content decreases (36). In fact, many studies have found higher affinity of colloids for the air-water interface than the solid-water interface (24)(25)(26)32). It is believed that colloid attachment to the air-water interface is irreversible due to the attractive capillary forces that hold colloids at the air-water interface once attachment occurs (29,42,44).…”

“…Water films surrounding solid grains may be as thin as tens of nanometers (36,45), close to or smaller than the size of most of colloids and nanomaterials. As such, film straining is believed to be at least as important as air-water interface attachment in unsaturated transport (27,32,34,35,(45)(46)(47)(48)(49)(50). Studies with bacteria (34,35), silica colloids (27), and latex microspheres (47)(48)(49)(50)(51) have suggested that film straining may be the dominant mechanism in immobilizing colloids during unsaturated transport in many systems.…”

Geological Disposal of Nuclear Waste: Fate and Transport of Radioactive Materials

“…Several mechanisms are responsible for colloid transport in unsaturated zone in addition to that of saturated zone, such as, liquid-gas interface capture, solid-liquid-gas interface capture, liquid-film straining, and storage in immobile liquid zones [13,18,27,44,51,68,70,79,85,87]. The strong force (capillary force) associated with the moving liquid-gas interfaces led to particle mobilization in the natural subsurface environment.…”