Colloid transport, retention, and remobilization during two-phase flow: Micro-model investigation and modeling

Abstract

In this study the transport of colloids in a two-phase fluid system is investigated. In particular, the effects on the interface of two immiscible fluids in steady-state and transient circumstances in a micro-porous network are investigated. The experimental setup is designed consisting of micro porous networks, a fluid dosing system and an optical detection system. For the detection we used a Confocal laser microscope. The micro porous networks are manufactured with an intermediate step of etched glass in a flexible plastic material (PDMS). They are covered by an extreme thin glass plate of 120μm with a PDMS coating of 20μm. This resulted in a network of about 90 pores of 30µm with a homogeneous hydrophobic surface. The focusing of the laser allowed us to image the process to 30µm inside the pores. The two fluids we used were Fluorinert-FC43 and DI-water. We made use of different colloidal particles (Polystyrene and Silica microspheres) with either hydrophobic or hydrophilic properties as model colloids. The fluid flow is controlled by an extreme accurate syringe pump with very low flow. A manifold allows us to choose without interruption for wetting, non-wetting or colloid containing fluids being injected in the network. In the outflow channel the concentration of colloids can be monitored. Series of experiments were carried out at different flow rates, various saturation, different transient flow stages and surface properties of the particles. The results are obtained as single images as well as series of images with a frame rate of less than 1 sec. Also, the concentration breakthrough curves were obtained at the outflow channel by measuring the fluorescence intensity. The results showed that more significant retention of colloids under lower water saturations. As observed, these mainly due to colloids attached to the fluid-fluid interfaces and fluid-fluid-solid contact lines. The surface properties of the particles appear to be of great importance. The hydrophobic ones do stick more at the interface than the hydrophilic ones. Regarding our computations this is obvious due to the strong energy barrier. Under transient conditions, the colloids that were strongly attached to the solid walls were remobilized with the moving interfaces and contact lines. Modeling of the breakthrough curves was done based on three approaches. (1) A constant detachment model; (2) a modified model from Cheng and Saiers (2009) in which attachment and detachment are separately modeled as a kinetic process; (3) modified model 2 in which the particle interaction with fluid-fluid interfaces is viewed as an equilibrium distribution process. Through fitting the column experiments results of Torkzaban et al. (2006a, b) and the measured breakthrough curves in the micro-model, we found model (3) fitted the results the best.