(583r) Colloidal Transport in a Surface Charge Heterogeneous Microfluidic Porous Medium
Colloidal facilitated transport of low-solubility contaminants in ground water flow has become a major concern recently, since contaminants adsorbed onto natural colloidal particles can travel kilometers over years. Therefore, understanding the behavior of those colloidal particles would provide insights for environmental protection and remediation. In the past most lab-scale experiments focused on studying interactions between particles and homogeneous or geometrically heterogeneous porous media. Hardly any of them tried to decouple the different chemical properties of the porous media grain. In order to shed some light in this aspect, we have fabricated a microfluidic device by packing 10 micron polystyrene (PS) beads with opposite surface charges into a pseudo-three-dimensional porous medium. By injecting 0.5 microns PS particles through the fabricated device, we further study the effect of chemical heterogeneity on the transport and deposition of colloidal particles. The porous medium assembled contains more than 3000 beads and has a size of 500 µm in length, 600 µm in width and 15 µm in height. An on-chip cross channel linked with an off-chip 4-way valve is applied to generate a step input of concentrated colloidal suspension and minimizes the influence of colloidal dispersion before they reach the porous medium. The breakthrough of the particles is recorded at the downstream of the porous medium. The colloidal retention profile is mapped using confocal microscopy. Since beads with different surface charges can be distinguished by fluorescence, we obtain colloidal retention data at the individual grain. With our platform, we aim at developing a relation between the overall collector efficiency and its constitutive collector efficiency under different salt conditions.