COLLOID INTERFACIAL REACTIONS IN OPEN MICROCHANNEL REPRESENTING UNSATURATED SOIL CAPILLARIES
|Anticipated Total Funding||$96,001.00|
|Anticipated End Year||2008|
NON-TECHNICAL SUMMARY: Understanding colloid retention and transport mechanisms in unsaturated porous media is crucial to the management of groundwater contamination by contaminants that could be strongly sorbed to and migrate with mobile colloids, by pathogenic microorganism, or manufactured nanoparticles. To date, only a few mechanistic investigations at the pore scale have been carried out and as a result, the mechanisms controlling colloid interfacial retention in unsaturated media are not fully understood and are currently being debated in the literature. Especially lacking is our understanding on the effect of flow dynamics on colloid retention at the various interfaces in unsaturated media. This project will experimentally examine colloid interfacial interactions at the pore scale using an innovative micromodel setup and how flow rate and pattern affect those interactions. We will also develop a simulation tool using a non-traditional LB approach to describe flow in the micromodel and to provide physically-based explanations for the hydrodynamic effect on colloid retention. Through experimentation and simulations, we will advance the fundamental understanding on the role surface tension and other electrochemical forces in microscale flow dynamics and how the resulting flow fields in turn control colloid retention at the various interfaces. OBJECTIVES: The purpose of the proposed project is to apply an innovative micromodel setup developed in our laboratory and an advanced flow simulation tool (i.e., the lattice Boltzmann method) to investigate mechanisms of colloid interfacial retention, with an emphasis on the effect of microscale fluid behavior. The proposed study includes both experimentation and theoretical/numerical analysis, with the following specific objectives: (1) experimentally elucidate the mechanisms of colloid interfacial retention in the open-channel micromodel; (2) using colloids as tracers to measure flow velocity and pattern in the micromodel; and (3) develop a simulation tool for the flow in the micromodel, using a non-traditional lattice Boltzmann (LB) approach, to provide physically-based explanations for the hydrodynamic effect on colloid retention. This research will provide pore-scale understanding of colloid interfacial interactions in partially saturated media and help sort out the current confusion and disagreement on the mechanisms involved in colloid (including viruses) retention and transport in the vadose zone. APPROACH: The proposed experiments will be conducted with capillary channels via visualization with a laser scanning confocal microscope (Carl Zeiss Axiovert 200M equipped with LSM 510, Germany). Our preliminary study on colloid interfacial retention in the micromodel channel has been restricted to one type of colloids (e.g, 1.0 micrometer sulfate yellow-green latex particles that are hydrophobic and negatively charged). To obtain a more complete understanding of colloid interfacial interactions, new experiments will include other types of colloids with different surface properties: (1) carboxylate-modified yellow-green fluorescent microspheres, which are also negatively charged but much less hydrophobic than the sulfate particles; (2) amidine-modified yellow-green fluorescent microspheres, which have similar hydrophobicity with the sulfate particles but are positively charged. The selected particles will be carefully characterized: (1) determination of size and zeta-potential with Zetasizer Nano ZS (Malvern Instruments, Southborough, MA); (2) measurement of contact angle by the sessile drop method with a goniometer (Kruss, Hamburg, Germany). Static experiments will be conducted to investigate the effect of ionic strength on colloid accumulation at the air-water interface (AWI) and the air-water-solid three phase contact line. For these experiments, the capillary channel will be filled with a known amount of colloidal suspension at a pre-determined concentration and monitored with the microscope through the confocal imaging software over time. Flow experiments will be conducted to evaluate the effect of hydrodynamic conditions on colloid retention at both AWI and the contact line. The movement and retention of colloids in the capillary channel will be visualized and imaged with the laser scanning confocal microscope. The aquired confocal images will be analyzed to: (1) estimate the average concentration of colloids per image area as well as process the images; (2) sort the particles according to their fluorescent intensities and eliminated the out of focus microspheres; and (3) measure particle velocities, which are used to build velocity profiles. Visualization and time-resolved image capturing in two and three dimensions will be performed for both static and dynamic micromodel experiments. Theoretical and numerical analysis include the following specific tasks: a) modify the preliminary 2D LB code used in our preliminary analysis to test the methods of Ginzburg & Steiner (2003) and Korner et al. (2005) for free-surface treatment; b) develop a physical description for surface tension variations and other electrochemical forces at AWI; c) develop a 3D LB code based on the first two tasks and existing 3D codes from Wang’s group (Wang and Afsharpoya, 2006a,b) and test the effects of channel sidewalls; and d) compare with quantitative experimental observations to provide physical explanations for the observed velocity profiles and flow patterns within the open channel. PROGRESS: 2006/09 TO 2007/08 This project involves investigation of colloid retention behavior in unsaturated porous media through pore-scale experimentation and numerical modeling. On the experimental front, Jin’s group has conducted experiments using open capillary tubes and with a laser scanning confocal microscope, which allow pore-scale observations of colloid retention at the air-water interface (AWI) as well as on the contact line. So far, the following experiments have been completed to examine the effects of various factors on colloid accumulation at AWI and contact line: (1) solution ionic strength; (2) hydrodynamic conditions; and (3) solution surface tension. Both static and flow experiments were performed. Main findings include: (1) changing solution ionic strength from 1 to 100 mM had minimal effect on colloid retention, indicating that other forces than electrostatic (e.g., hydrophobic) are involved; (2) flow through the open capillary channel resembled Poiseuille flow and AWI acted as a non-stress-free boundary, which was almost stagnant, and promoted colloid accumulation; (3) retention on contact line was dominated by film-straining and was more significant in the flow relative to static experiments; and (40 lower surface tension increased colloid retention at AWI and the contact line due to increased hydrophobic interaction. On the numerical modeling front, Wang’s group has made progress in two areas. The first area is the development of two-phase flow models based on the lattice Boltzmann equation (LBE) approach. So far, we have successfully reproduced the multiphase flow results of Kang et al. 2005 (J. Fluid Mech. 545: 41-66). This code was run with MPI (Message passing Interface) on an IBM cluster at the National Center for Atmospheric Research. In parallel, we are also working to develop another LBM code based on a single-phase LBE plus a finite-volume interface model to treat air-water flow. In this second approach, only the water flow is explicitly modeled and it is assumed that the air flow has negligible effect on the dynamics of the interface. A second area of numerical simulation deals with complex single-phase flows through porous medium, using both LBE approach and a Navier-Stokes based approach (i.e., Physalis). Our focus has been on flow and transport of colloids in two-dimensional porous medium. We have demonstrated that the LBE approach and the Physalis approach can produce identical fluid flow, confirming the accuracy of the flow simulation. Numerical modeling on the transport of colloids has revealed that the deposition rate or surface coverage depends on both the mean flow speed and fluid ionic strength, in qualitative agreement with experimental observations. The result from this project to date has been presented at three conferences and three journal manuscripts are currently under preparation.