Theoretical and experimental study on electrokinetic energy conversion efficiency in micro- and nanochannels

Abstract

Microfluidic devices based on the electrokinetic energy conversion phenomena can be used as a pump delivering liquids in small sized channels or as a power generator supplying very small amounts of electrical power. In this work, we propose theoretical and numerical modeling methods to accurately predict the performance and efficiency of these microfluidic electrokinetic energy conversion devices. For this, we investigate the fundamental transport mechanism in silica micro- and nanochannels analytically and numerically. We propose the self-consistent Poisson-Boltzmann (PB) model derived from the complete set of constitutive equations. This model describes the general electrokinetic transport process in the microchannel. Solute flow rate, current, and volume flow rate are cross-coupled with input forces such as the hydrostatic pressure, the electromotive force, and the osmotic pressure through the Onsager reciprocal theorem. We propose a simple and reliable physicochemical boundary condition at the interface between the channel surface and the solution that takes into account the effect of the Stern layer conduction and the dependence of the surface charge density on solution pH and ionic concentration. It is found that the Stern layer conduction always lowers the energy conversion efficiency and it is becoming more distinct in the nanochannel where the electric double layer is prone to overlap. We validate the self-consistent PB model by comparison with the experimental results for a fused silica capillary with an inner diameter of 20μm. It is found that the theoretical model predicts the systematic dependencies of the streaming current, the streaming potential, and the electrokinetic energy conversion efficiency on pH and ionic concentration well. We propose the self-consistent Nernst-Planck (NP) model to characterize the electrokinetic transport in the nanochannel. The proposed model successfully describes nonlinear behavior such as the concentration po...