|M.Sc Student||Liel Uri|
|Subject||Overlimiting Current in Microchannel-Nanochannel|
|Department||Department of Mechanical Engineering||Supervisor||Professor Gilad Yossifon|
|Full Thesis text|
Similarly to membranes, upon application of an electric field, the ion permselectivity of a nanochannel leads to concentration enrichment and depletion on its opposite sides, termed concentration polarization. Upon further increase of voltage, when the ionic concentration on the anodic side of the nanochannel/membrane approaches zero, an extended space charge (ESC) layer appears to support overlimiting current, i.e. current beyond the classical diffusion limit.
This work presents results demonstrating the ESC-mediated transition between classical, diffusion-limited current and overlimiting current in micro-nanochannel devices. The ESC is correlated with a distinctive maximum in the dc resistance. Experimental results for a shallow surface-conduction dominated system are compared with theoretical models, allowing estimates of the effective surface charge at high voltage to be obtained. Further, this work extends the study to microchannels of moderate to large depths where the role of various electroconvection mechanisms such as electroosmosis of the second kind and electro-osmotic instability become dominant. It is shown that the dc resistance maximum flattens and shifts to higher voltage with increasing field-focusing (i.e. increasing microchannel depth) and/or increasing concentration and disappears completely at sufficiently large field-focusing/concentration. These trends are substantiated by 2D numerical simulations.