Role of air‐water interfaces in colloid transport in porous media: A review

Abstract

Air‐water interfaces play an important role in unsaturated porous media, giving rise to phenomena like capillarity. Less recognized and understood are interactions of colloids with the air‐water interface in porous media and the implications of these interactions for fate and transport of colloids. In this review, we discuss how colloids, both suspended in the aqueous phase and attached at pore walls, interact with air‐water interfaces in porous media. We discuss the theory of colloid/air‐water interface interactions, based on the different forces acting between colloids and the air‐water interface (DLVO, hydrophobic, capillary forces) and based on thermodynamic considerations (Gibbs free energy). Subsurface colloids are usually electrostatically repelled from the air‐water interface because most subsurface colloids and the air‐water are negatively charged. However, hydrophobic interactions can lead to attraction to the air‐water interface. When colloids are at the air‐water interface, capillary forces are usually dominant over other forces. Moving air‐water interfaces are effective in mobilizing and transporting colloids from surfaces. Thermodynamic considerations show that, for a colloid, the air‐water interface is the favored state as compared with the suspension phase, except for hydrophilic colloids in the nanometer size range. Experimental evidence indicates that colloid mobilization in soils often occurs through macropores, although matrix transport is also prevalent in absence of macropores. Moving air‐water interfaces, e.g., occurring during infiltration, imbibition, or drainage, have been shown to scour colloids from surfaces and translocate colloids. Colloids can also be pinned to surfaces by thin water films and capillary menisci at the air‐water‐solid interface line, causing colloid retention and immobilization. Air‐water interfaces thus can both mobilize or immobilize colloids in porous media, depending on hydrodynamics and colloid and surface chemistry.

Inkjet technology for ceramic products. Influence of some process variables on ink penetration

“…Nanoparticle penetration into unsaturated porous media by capillary suction has drawn little attention , unlike nanoparticle transport in saturated porous media, as compiled by Babakhani et al They reviewed the experimental data obtained by numerous researchers (compiling more than 160 references), highlighting the influence that the properties of the liquid and porous medium, including the interactions of these media, had on nanoparticle retention. Despite the notable importance of having a nanoparticle concentration profile inside a porous medium in which a suspension has penetrated by capillary suction, no paper was found on this topic in inkjet printing technology (on paper, fabric, or ceramic substrate).…”

Dynamic Effects on the Mobilization of a Deposited Nanoparticle by a Moving Liquid-Liquid Interface

“…In the absence of confinement, there is no Laplace contribution to the capillary forces. On the other hand, in a channel of height H we can estimate the curvature (away from the particle) as / 2 cos and the capillary pressure force can be estimated as Δ 2 cos ⁄ , where we have used the Laplace equation Δ / [7]. Comparing the two forces, we find that the capillary force is negligible in our system (F P ) and should only be considered in highly confined situations where and F P ~ .…”

“…Continuous progress has been made in our understanding and modeling since the introduction of colloid filtration theory in saturated systems [4], but predicting the mobilization and transport of nanoparticles remains challenging [5,6]. Unsaturated and multiphase flows in geological porous media present additional difficulties to predict nanoparticle transport, due to their dynamic interaction with moving interfaces between fluids phases [7]. A critical aspect that this works aims to understand is the remobilization of deposited colloidal particles by dynamic liquid-liquid interfaces and the moving (three-phase) contact line.…”

“…Moreover, these experimental studies were performed at a mesoscopic level (i.e., pore-level) and thus lack details of the physics at play for individual particles during remobilization events. It is necessary to investigate the different forces acting at the single-particle level to understand how dynamic processes affect colloidal detachment and remobilization [7]. In this work, we consider the motion of liquid-liquid interfaces past a single nanoparticle deposited on a flat wall and document the presence of different dynamic regimes, including interface pinning, stick-slip motion, and particle rolling and detachment, depending on the magnitude of the driving force and the contact angle that the interface forms with the particle.…”

A proximity‐based image‐processing algorithm for colloid assignment in segmented multiphase flow datasets

“…Understanding the mechanisms associated with these phenomena can improve risk assessment after a contaminant spill or subsurface pathogen outbreak. Recently published review articles covering both saturated (Molnar et al ., 2015) and unsaturated (Flury & Aramrak, 2017) colloid transport describe modelling and experimental efforts aimed at more accurate prediction of the fate of colloids in the environment. However, these models and calculations are constructed under highly idealized conditions, therefore additional experimental evidence will be required for further model validation and verification.…”