Fully-Coupled Modelling of Electrokinetic Flow and Migration of Electrolytes in Microfluidic Devices

We investigate the electrokinetic flow and mass transport in a microchannel junction, serving as an injector of a microelectrophoresis device. In order to consider all essential features of this complex system, the electrical situation, the fluid dynamics, and the (physical) chemistry is taken into account. The electrical situation is modeled by a combination of an electrostatic and an electrodynamic approach. The fluid dynamics can be described by the Navier-Stokes equations, extended by an additional force term. The chemistry of the system is represented by source terms in the mass transport equations. Moreover, the interaction between the buffer concentration and the physicochemical properties of the channel wall is taken into consideration by an empirical approach. All equations are encoded in a dimensionless form so that dimensionless groups control the problem. Approximative analytical solutions for the phenomena within the electrical double layer can be found and, thus, we can reduce the numerical costs due to an asymptotic matching procedure. The models are implemented in a Finite Element Method (FEM) code and time-dependent, two-dimensional simulations are performed. The results of the simulations show the strong coupling between the involved physicochemical processes.