Enhancement of solute mixing due to thermo-osmotic flow of ternary electrolyte solutions through a nanochannel modulated by periodic charged blocks

The study of uncharged species transport within a micro/nano-fluidic device is crucial for achieving efficient chemical/biochemical sample mixing in various industrial and biomedical applications. However, rapid solute mixing within a micro/nano-fluidic device is extremely challenging due to the negligible inertial effects within the channel, and it requires an extrinsic stimulus to create disturbances within the flow domain. In this work, the passive solute mixing due to the thermo-osmotic flow of three immiscible electrolyte solutions within a nanochannel whose walls are modulated by periodically arranged charged blocks, aligned vertically with the axial electric field, is investigated. Our primary objective is to investigate how surface heterogeneity and longitudinal temperature gradient modify the ion distribution and the flow characteristics of three fluid layers, resulting in a novel micro-mixing process within the channel. Therefore, the complete non-linear flow governing equations based on the Poisson-Nernst-Planck model are solved numerically using a staggered grid-based finite volume method, considering higher interfacial tension coefficients along two liquid-liquid interfaces. An analytical expression for the flow characteristics is derived under isothermal conditions, and it is observed that the analytical results correspond well with the numerical results for low wall zeta potential. Our results reveal that the surface heterogeneity and the axial temperature gradient lead to complex flow patterns of fluids and ions, creating various vortex zones within the diffuse layer, which enhance the simultaneous mixing of three distinct solutes within each immiscible fluid layer.