Effect of hydrophobicity on colloid transport during two‐phase flow in a micromodel
Authors: Zhang Qiulan , Hassanizadeh S. M. , Liu B. , Schijven J. F. , Karadimitriou N. K. 
Published: September 2014

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 fluid‐fluid interfaces and fluid‐solid interfaces. Very good agreements were obtained among measured breakthrough curves, visualization results, and numerical modeling.