A continuum and atomistic simulation study of ion transport in multilayered graphene membranes

Graphene membrane as a staggered multilayer structure was demonstrated to be a promising filter membrane for the gas and liquid separation. The superior property of graphene membrane is owing to the exotic behaviour of fluid confined in the graphene nanochannel (< 10 nm), namely the booming nanofluidics field. Unlike the 1-D nanochannel in lab-on-a-chip devices, the graphene membrane has a unique cascading nano-slit system. Understanding of ion transport in graphene membranes is very limited. Thus this thesis is devoted to study the microstructures and novel physical phenomena and mechanism that govern the ion transport inside graphene membranes via continuum and atomistic simulations. A quantitative and statistical representative microstructure model is obtained for graphene membranes by correlating diffusion permeation experimental results with continuum simulation results. This task is achieved by taking advantages of the tuneable graphene membrane platform recently developed in our group. The crucial role of the pin-hole defects in graphene sheets is revealed. Based on this structure model, comprehensive continuum simulations were performed to study the electrokinetic properties of ion transport inside graphene membranes. Comparison with direct experimental measurements leads to an interesting scaling law that correlates the relative conductance with channel size. We find some novel electric double layer (EDL) structures, such as EDL caused by external electric field (coined as binary boundary layer (BBL) in this thesis) and EDLs at the pore aperture regions. Influences of these EDL structures and the channel surface charges on the driving force distribution and ion concentration inside the cascading nanoslit system are carefully studied. With the obtained information, ion transport in graphene membranes was analysed. To understand the unusual ion transport behaviour observed by the experiments at molecular scale, atomistic simulations for ion electro-kinetic flow through the membranes with and without surface charge were performed. The EDL structures, ion concentrations, and ion transport properties were carefully studied and compared with continuum simulations. For graphene membranes with zero surface charge, our MD simulations showed a strong BBL caused by external electric field. As a result, a novel polarized electro-osmosis flow (EOF) phenomenon is observed. Unlike the conventional EOF, the polarized EOF has two flows in opposite directions next to the two opposite walls of a slit, respectively. The polarized EOF accelerates the ion transport inside the membrane and could be utilized as a novel nano-mixer. With surface charges, the ion transport is a combination of ion electrophoresis, conventional EOF, and the polarised EOF. In small slits, the conventional EOF is dominant for enhancing electrokinetic conductivity. Our MD simulations provide novel physical insights that are missed in continuum simulations, which should be considered in future continuum simulations and models. Overall, in this thesis, the combination of continuum and atomistic simulations, complemented by experimental results provide us much needed knowledge of ion transport in graphene membranes. This study should provide a basis for further fundamental study and practical applications of graphene membranes