Retention and remobilization of colloids during steady-state and transient two-phase flow

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The goal of this research was to investigate the difference in behavior of hydrophilic and hydrophobic colloids during transport in two-phase flow, in general, and their attachment and remobilization characters, in particular. Experiments were performed in a hydrophobic polydimethylsiloxane (PDMS) micromodel. Water and fluorinert-FC43 were used as the two immiscible liquids. Given the fact that PDMS is a hydrophobic material, fluorinert was the wetting phase and water was the nonwetting phase in this micromodel. As model colloids, we used hydrophilic polystyrene carboxylate-modified microspheres (dispersible in water) and hydrophobic fluorous-modified silica microspheres (dispersible in fluorinert) in separate experiments. Using a confocal laser scanning microscope, we directly observed fluid distribution and colloid movement within pores of the micromodel. We also obtained concentration breakthrough curves by measuring the fluorescent intensities in the outlet of the micromodel. The breakthrough curves during steady-state flow showed that the colloid attachment rate is inversely related to the background saturation of the fluid in which the colloids were dispersed. Our visualization results showed that the enhanced attachment of hydrophilic colloids at lower water saturations was due to the retention at the fluorinert-water interface and fluorinert-water-solid contact lines. This effect was observed to be much less in the case of hydrophobic colloids (dispersed in fluorinert). In order to explain the colloids behavior, we calculated interaction potential energies of colloids with PDMS surfaces, fluid-fluid interfaces, and fluid-fluid-solid contact lines. Also, balance of forces that control colloid, including DLVO, hydrodynamic, and surface tension forces, were determined. Our calculations showed that there is a stronger repulsive energy barrier between hydrophobic colloids and fluorinert-water interface and solid-fluid interface, compared with the hydrophilic colloids. Moreover, hydrophobic colloids were seen to aggregate due to strong attractive forces among them. These aggregates had even less tendency to attach to various interfaces, due to an increase in the corresponding energy barrier. For the colloid retention at fluid-fluid-solid contact lines, we found that the role of DLVO interactions was less important than capillary forces. During transient events, we found that attached colloids become remobilized. The colloids deposited on the solid-fluid interface were detached by the moving fluid-fluid-solid contact lines. But, this happened only when the liquid containing colloids was being displaced by the other liquid. We simulated breakthrough curves using a model based on a coupled system of equations for two-phase flow, advection-dispersion of colloids, adsorption to and desorption from fluidfluid interfaces and fluid-solid interfaces. Very good agreements were obtained among measured breakthrough curves, visualization results, and numerical modeling.

Section: Resultsmentioning

confidence: 99%

“…Recently, Zhang et al . [] conducted experiments on transport of hydrophilic particles (dispersed in water) during two‐phase flow in order to investigate how saturation change affects colloids retention and remobilization. The experiments were performed in a micromodel made of polydimethylsiloxane (PDMS) filled by water and fluorinert.…”

Section: Introductionmentioning

confidence: 99%

Effect of hydrophobicity on colloid transport during two‐phase flow in a micromodel

Zhang

Hassanizadeh

Liu

et al. 2014

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“…Colloid transport in a fluorocarbon‐water system was studied by Zhang et al . [], who used a hydrophobic micromodel, thereby making the fluorocarbon the wetting phase, and hydrophilic colloids were dispersed in the water phase. The authors found that at lower water saturation, more colloids were retained at the fluorocarbon‐water interface and the fluorocarbon‐water‐solid contact lines.…”

Section: Colloid Transport In Single Pore and Pore Network Micromodelsmentioning

confidence: 99%

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

Flury

Aramrak

2017

103

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.

“…Micromodels have been mostly used in studying displacements of two immiscible fluid phases in porous media [ Avraam et al ., ; Baouab et al ., ; Chang et al ., ; Corapcioglu et al ., ; NagaSiva et al ., ; Blois et al ., ; Zhang et al ., ]. Processes of drainage and imbibition, as well as the mechanisms that dominate them, like viscous or capillary fingering and snap‐off, have been studied using micromodels [ Zhang et al ., ; Ferer et al ., ; Grate et al ., ; Gutiérrez et al ., ; Hug et al ., ; Rangel‐German and Kovscek , ; Huh et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Micromodel study of two‐phase flow under transient conditions: Quantifying effects of specific interfacial area

Karadimitriou

Hassanizadeh

Niasar

et al. 2014

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Recent computational studies of two-phase flow suggest that the role of fluid-fluid interfaces should be explicitly included in the capillarity equation as well as equations of motion of phases. The aim of this study has been to perform experiments where transient movement of interfaces can be monitored and to determine interfacial variables and quantities under transient conditions. We have performed two-phase flow experiments in a transparent micromodel. Specific interfacial area is defined, and calculated from experimental data, as the ratio of the total area of interfaces between two phases per unit volume of the porous medium. Recent studies have shown that all drainage and imbibition data points for capillary pressure, saturation, and specific interfacial area fall on a unique surface. But, up to now, almost all micromodel studies of two-phase flow have dealt with quasi-static or steady state flow conditions. Thus, only equilibrium properties have been studied. We present the first study of two-phase flow in an elongated PDMS micromodel under transient conditions with high temporal and spatial resolutions. We have established that different relationships between capillary pressure, saturation, and specific interfacial area are obtained under steady state and transient conditions. The difference between the surfaces depends on the capillary number. Furthermore, we use our experimental results to obtain average (macroscale) velocity of fluid-fluid interfaces and the rate of change of specific interfacial area as a function of time and space. Both terms depend on saturation nonlinearly but show a linear dependence on the rate of change of saturation. We also determine macroscale material coefficients that appear in the equation of motion of fluid-fluid interfaces. This is the first time that these parameters are determined experimentally.