Electroconvective Instability Near Ion-selective Surface:Lattice Boltzmann Method Simulations

In recent decades,the development of micromachining technology has provided abundant technological conditions for the research and industrial application of microfluidic devices.Electric driving phenomena has potential advantages for fluid transport in microfluidic systems among the commonly used fluid manipulation techniques.Ion selective surface a kind of microfluidic device widely used in industrial application.Applying a voltage bias electrolyte with an ion-selective surface,a series of complex physical phenomena would appear in the electrolyte solution near the ion exchange surface,which can enhance the ion transport efficiency or realize the desalination of seawater.An interesting convection will appear under high voltage,such physical phenomena has attracted the attention of some scholars.It is of both theoretical significance and practical value to study the instability of electric convection on ion-selective surface.The full coupling electrohydrodynamic governing equations include the electric field module describing ion transport and potential distribution and the flow module describing fluid motion.Existing studying about the electroconvection or electroosmosis of the second kind is based on the governing equations,work research by analyzing,experimenting and simulating the flow stability using the specific physical models.This work establishs a fully coupled lattice Boltzmann method model to solve the concentration,electro potential and flow field distribution from the basic electrohydrodynamic governing equations.Appropriate lattice Boltzmann velocity space discretization schemes are chosen,which can ensure a well calculation accuracy,efficiency and stability.This word simulates the electroosmosis flow and the electroconvection instability near the ion-selective surface in a small scale by using the coupled LBM algorithm.The evolution process from the concentration polarization in hydrostatics state to the occurrence of flow is observed.Typical concentration polarization phenomena are observed,including the electro double layer,the extend space charge layer,the transition layer and the diffusion layer.The simulation results also obtain a pair of convective vortex structure of the electroosmosis the second kind.The variation characteristics of ion current density and maximum velocity of the system are analyzed.It is found that the variation trend of both variables is positively related to the increase of voltage.In the small-scale case that in our simulation,the maximum velocity and current are almost linearly related to the increase of voltage after the overlimiting current appears.The work also study the influence of ion diffusivity and concentration,the result illustrates that a larger flow strength and ion current density corresponding to lager diffusivity and concentration,but increasing the ion diffusivity,the threshold voltage of overlimiting current regime would increasing.The algorithm is optimized by using the GPU parallel and multi block grid,respectively,and the computation ability is improved.The simulation results of GPU parallel program have been verified by using the physical model of the charge injection convection.We get the multi block distribution function exchange format of the convection-diffusion equation and the Poisson equation.Using the multi block multi field coupled algorithm,we simulation the electric convection instability near the ion-selective surface.We find that the current is almost invariant in the limiting regime because the increasing of computation domain,and the result is also consistent with analytical solution.The ion current density increases again with the appearing of flow,which increasing has a nonlinear relationship with voltage.The simulation results present the evolution process of the vortex with time under different applied voltage.At a higher voltage,four vortices appear firstly,and then merged into two large vortices.The results illustrate that the ion transport efficiency of large vortex is higher than the small vortex.Under a lower voltage but trigger the flow,the flow structure directly form two large vortices to influence the current.A long computation region is also adopted in our simulation.The results show that the flow structure transforms from regular vortices to the unsteady flow when increase the voltage,the flow become more chaotic.A negative ion density appears in the electrolyte.We compute average kinetic energy,find the flow strength is concentrated near the ion-selective surface.