|Ph.D Student||Zalmanovich Neta|
|Subject||Electrochemical Characterization of the Transient Response|
of a Microchannel-Nanoslot Fluidic Devices
|Department||Department of Mechanical Engineering||Supervisor||Professor Gilad Yossifon|
|Full Thesis text|
Experimental and numerical results reveal non-monotonic chronopotentiometric (galvanostatic operation) and chronoamperometric (potentiostatic operation) response of a microchannel-nanoslot system. In shallow microchannel (4µm depth) we reported evidence of variation in ion selectivity of a fabricated microchannel-nanochannel device resulting in the appearance of a distinct local maximum in the overlimiting chronopotentiometric response. In this system consisting of shallow microchannels joined by a nanochannel, viscous shear at the microchannel walls suppresses the electro-osmotic instability and prevents any associated contribution to the nonmonotonic response. Thus, this response is primarily electrodiffusive. Numerical simulations indicate that concentration polarization develops not only within the microchannel but also within the nanoslot itself, with a local voltage maximum in the chronopotentiometric response correlated with interfacial depletion. In contrast, in deep microchannel (95µm) we observed a distinct signature of electroconvective instability in the transient response for a microchannel-nanoslot system. We studied the development of the electroconvective instability from equilibrium during the transient chronopotentiometric/chronoamperometric responses at overlimiting current conditions and observe a distinct signature of nonmonotonic transient response resulting from the emergence of electroconvective instability. This stands in contrast to previously reported experimental non-monotonic transient responses in heterogeneous membrane systems associated with linear electroosmotic flow. Trying to understand how these different mechanisms compete with each other through a numerical investigation using a Comsol Multiphysics 5.3 is done. First we compare the solution of 1layer to the 2D finite element used in former work, then we expand to 3layer system. We further study the effect of the geometrical sinusoidal undulation of the micro-nanochannel interface using a constant amplitude and varying wavelengths on its electrical response. When observing the development of the depletion layers, the emergence of electroconvection vortices reminiscent of electro-convection instability mechanism occur at large wavelength while Dukhin’s elctroconvection seems to dominate at small wavelengths. A non-monotonic electrical response with relation to the undulated interface wavelength makes it hard to reach a clear conclusion regarding its overall effect on the system. To summarize, understanding the transient response of non-ideal permselective systems is essential for obtaining fundamental insight and for optimizing efficient operation of practical fabricated nanofluidic and membrane devices that are important for electrodialysis, sorting, preconcentration and biosensing.