Length-dependent instability of shear electroconvective flow: From electroconvective instability to Rayleigh-Bénard instability

Abstract

Ion and water transport by electroconvection continually finds new applications, arousing considerable research interest. This paper is devoted to the important issue of the effects caused by shear flow, as this flow always occurs in various electrochemical applications, such as electrodeposition, electroplating, and electrodialysis. In this paper, the dimensionless Poiseuille-Navier-Stokes and Poisson-Nernst-Planck model is proposed, which contains the buoyancy force induced by ion concentration polarization. The numerical results show that in the existing literature, the Rayleigh-Bénard convection is neglected and the Debye layer effect is overestimated, leading to a large difference between the simulation results and the experimental data. In addition, the chaotic phenomenon of shear flow is discussed in detail based on the proposed model. The main contributions are as follows: (i) There are two distinct instability phenomena, namely, electroconvective instability, caused by the electric force, and Rayleigh-Bénard instability, caused by the buoyancy force. (ii) For electroconvective instability, the fully overlapping vortex structures in the microchannel are obtained numerically for the first time. In addition, the shear sheltering effect is verified numerically. (iii) The effects of the characteristic length and electrohydrodynamic coupling constant on the Rayleigh-Bénard instability are studied. (iv) The transition condition from electroconvective instability to Rayleigh-Bénard instability is investigated. The analysis shows that choosing a characteristic length consistent with the actual structure is a necessary condition for achieving high-precision analysis of fluid behaviors such as the flow pattern. This conclusion provides important guidance for the design and optimization of the concentration microfluidic chip.