Investigating Electrokinetic and Electrochemical Phenomena in Confined Geometries through Multiphysical Modeling

In recent decades, microfluidics and nanofluidics have risen to the forefront of innovation and technological development for a plethora of analytical applications ranging from advanced point-of-care diagnostics and integrated drug delivery systems to multipurpose substance detection. These miniaturized platforms, made possible by emergent microfabrication technologies, often exploit unique features such as increased surface-liquid interactions and small sample volume requirements to efficiently carry out on-chip chemical and/or bioanalytical processes. Moreover, the inherent flexibility of these systems enables a number of processes such as mixing, focusing and separation, visualization and detection, and pumping to be integrated onto a single lab-on-chip platform. However, the physical phenomena that govern these processes tend to be complex and exhibit strong multiphysics coupling, particularly for nanoscale geometries in which finite electric double layers and associated charge-screening effects prevail. Here, numerical simulation offers an avenue for probing the highly coupled nature of electrokinetic and electrochemical effects in confinement, allowing us to elucidate the intricacies of such systems through modeling. By providing an improved fundamental understanding of relevant physical processes, these numerical models enable researchers to optimize existing technologies and develop novel platforms for lab-on-chip applications.In this work, we discuss the modeling of four separate microfluidic and nanofluidic systems suitable for a wide range of analytical processes. First, we discuss flow visualization in a micromixer device driven by electrothermal flow, with an emphasis on how particle image velocimetry measurements can be used to tune simulation results and better represent 3D flow structures in the physical system. Next, we present a nanofluidic analyte focusing and separation technique which leverages field-effect control via wall-embedded electrodes to locally modulate electric double layer properties and induce ion concentration polarization within the channel. Third, we discuss the dynamics of a nanochannel-confined bipolar electrode system and demonstrate how bipolar electrochemistry provides a flexible platform for mixing, preconcentration, and/or analyte detection. Finally, we introduce a variation of the bipolar electrode system which exploits the nonlinear hydrodynamics associated with induced-charge electroosmotic flow to electrokinetically actuate a peristaltic micropumping mechanism through fluid-structure interactions