Modeling Complex Electrokinetic Nanofluidic Systems

The electrical double layer (EDL) nano-structure at the interface between electrolytes and charged surfaces dominates the performance of a myriad of electrokinetic and electrochemical processes. A complete understanding of the EDL nano-structure allows for a predictive tool for various systems such as supercapacitors, desalination, and nano-particle manipulation. My work involves developing theoretical models to elucidate the nano-structure of the EDL and the consequent effects on fluid flow and species transport in such systems. These include models explaining dispersion of ions in channels with thick EDLs, surface-charge-based ion conductivity changes, nanofluidic-based DNA hybridization, nanofluidic isotachophoresis, charge inversion due to large ions, and nanofluidic systems with heterogeneous surface charges. Collectively, these studies have enriched our understanding of complex electrokinetic nanochannel transport.First, I describe a model for the EDL in nanofluidic channels, showing experimentally validated theoretical regimes where dispersion and/or significant EDL size might affect experimental results, as well as methods to account for these effects. Understanding these effects is essential to accurately interpret experiments as well as design of future experiments and subsequent applications. This model can further explain other micro- and nanoscale electrokinetic transport physics. For example, 1) this theory can explain nanochannel conductivity changes due to changes in surface charge, 2) accounting for reaction terms, it can accurately model non-equilibrium DNA hybridization as well as the effect of nano-confinement on such hybridization in electrokinetic capillary electrophoresis-based systems, 3) it can predict an isotachophoretic-like standing front in nanochannels with surface-charge-inverting complex ionic species that induce fluid flow reversal, and 4) it can describe behavior with heterogeneous surface charge. To explain the behavior of nanofluidic systems with heterogeneous surface charge and complex ionic species, I refined the model by accounting for hard-sphere ion size and more complex near-field screening effects using classical Density Functional Theory. I conducted a theoretical study to explore heterogeneous surface charge in nanochannels with embedded, addressable electrodes that allow us to fully probe EDL structure. I developed a more complete EDL model and performed a systematic theoretical study of EDL nano-structure by varying ion diameter, valence, and concentration, as well as surface charge in order to elucidate EDL nano-structure, fluid flow, and species transport in nanochannels. Thus far we have preliminary model validation using custom-fabricated nanochannels with complex ions, and further experiments will both interpret nanochannel physics through theory as well as improve the model via experimental feedback, overall enabling a more complete predictive theory for future experimental and application design.