MODELING OF SEPARATION OF METAL IONS IN A MICROFLUIDIC CHIP

Numerical simulation of the separation of metal ions in a microfluidic cross-chip is presented. The models of electroosmotic flow and electrophoretic migration of charged particles resulted from the applied electrical field are developed and implemented in the CFX-4 CFD software. The models are shown to be able to describe the involved electro-chemical and physical processes happening in complex microfluidic chip geometry. NOMENCLATURE c concentration, mol/m α degree of dissociation D diffusivity, m/s ε dielectric constant F Faraday constant, C/mol ζ zeta potential, V R gas constant, J/(mol.K) μ dynamic viscosity, Pa.s v velocity, m/s ρ density, kg/m i subscript for ith ionic components σ electrical conductivity, S/m φ electrical potential, V ω mobility, m/(V.s) INTRODUCTION Over the last decade, microfluidic devices have been intensively developed and used for sampling, mixing, separation, sensing and miniaturised chemical and biological reaction platforms. By demonstrating radical reductions in size and weight, and better heat transfer, mass transfer and other performance advantages compared to conventional systems, microfluidic devices have the potential to revolutionize chemical analysis, chemical synthesis, industrial automation, plant safety, medical and biomedical applications. Separation is one of the many important functions that a microfluidic chip may perform. One way of separating molecules is electrophoresis. The electrophoretic separation technique is based on the principle that, under the influence of an applied potential field, different species in solution will migrate at different velocities from one another. When an external electrical field is applied to a solution of charged species, each ion moves toward the electrode of opposite charge. Electrophoresis has been performed on a support medium that acts as physical support and mechanical stability for the fluidic buffer system. Capillary electrophoresis has emerged as an alternative format for medium-based ones and, with the advent of microtechnology, gained popularity in microchip applications. Electrokinetically driven flow in microchannels is complex due to the interaction of surface-flow interaction, electrochemistry and multi-physics processes involved. While extensive experimental studies have been carried out in the field since the last two decades (e.g. Dolnik et al., 2000, Hu and Dovichi, 2002), computer simulations have played an important role in the study of the electrokinetically driven flows, including electrophoretic separation in microflows. This is mainly due to the capability in dealing simultaneously with many electrochemical and physical processes involved in the electrophoresis. The previous works by various authors (see Ikuta et al. (1998), Ermakov et al. (1994, 1998, 2000), and Sounart et al. (2000, 2001)) indicate that the phenomena could be described in the framework of the conservation laws of mass and charge and by coupling of the involved electro-chemical kinetics and heat-mass transfer. In the past, many numerical studies of electro-osmosis and electrophoresis have been limited by 1D approximation which could not correctly model the flow and particle migration in the complex geometry of a microfluidic cross-chip. Moreover, few studies have described the full coupling between the processes of ionic transport, electric field variation, and heat transfer, involved in the electrokinetically driven flow and particle migration. The aim of this study is to develop a 3D computational model of electrokinetically driven (or electroosmotic) flow and electrophoretic separation and to validate using available experimental data obtained at the CSIRO microfluidic laboratory. MODEL DESCRIPTION The model consists of a system of differential equations describing the heat, mass, momentum, and ionic species conservations and constitutive dependencies. Equations of mass conservation of ionic species The mass conservation is given by the convectivediffusive equation, via, ) ( ) v ( i i i i i c D c t c ∇ ⋅ ∇ = ⋅ ∇ + ∂ ∂ r . (1) Mobilities of solution and ionic species The bulk buffer solution flow is driven by the electrokinetic force (osmotic flow) while the motion of the charged particles (ions) is governed by capillary electrophoresis. The velocity of the ions is given by,