Continuum and atomistic simulation of electrically-mediated flow through nanometer channels

Fluid transport in nanometer scale channels and pores---referred to as nanofluidic transport---plays an important role in determining the functional characteristics of many biological and engineering devices and systems. In nanofluidic channels or devices, the critical dimension of the channel or device can become comparable to the size of the fluid molecule. In this case, the fluid molecules can be geometrically confined and molecular phenomena such as discreteness of the fluid (e.g. water) and fluid-surface interactions can become important. In addition, when an electrical field is used to transport a fluid or an electrolyte through a nanochannel, the governing fluid transport mechanism can be even more complicated because of ion-fluid and ion-surface interactions. A critical issue---that has not been addressed very rigorously in the literature---is whether a classical theory can be used to describe fluid or ion transport through a nanometer channel.; In this thesis, we study fundamental issues in electrically-mediated fluid flow by performing a detailed comparison between atomistic and continuum simulations. Our results indicate that the molecular nature of the ion and water are important factors influencing the ion concentrations, velocity profiles and other fluid characteristics in nanochannels. As the continuum theories based on the Poisson-Boltzmann and the Navier-Stokes equations account for the various intermolecular interactions in a mean-field fashion, they fail to predict the fluid/ion characteristics accurately. In addition, fluids or electrolytes confined in nanochannels exhibit anomalous behavior, which cannot be explained by the classical theories. Our results also indicate that if the critical channel dimension is larger than ten molecular diameters, then the classical theory can be used to describe fluid/ion characteristics in the central part of the channel, but an atomistic approach is necessary to resolve a few molecular diameters near the channel wall. As a final result, we present embedding multiscale methods which efficiently combine an atomistic model near the channel wall with the classical theory for the central part of the channel. The multiscale models can also be used to explain the anomalous behavior observed in electrically-mediated nanofluidic transport.