טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRosentsvit Leonid
SubjectThe Effect of Geometrical Symmatry Breaking and
Advection on Concentration- Polarzation
Phenomenon in Micro/Nano- Fluidic
Devices
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Gilad Yossifon
Full Thesis textFull thesis text - English Version


Abstract

Understanding ion transport processes through ion-permselective nanoporous membranes or nanochannels with overlapping Debye layers is of great importance in realizing optimal designs of desalination, bimolecular sensors, lab-on-a-chip and fuel cell devices. In such systems, under the application of an external electric field, the ion-permselectivity symmetry breaking phenomenon results in ionic concentration-polarization (ICP), i.e., the formation of ionic concentration gradients. Various effects may influence ICP in our systems and we investigated two such interactions. Ion current rectification (ICR) inversion was observed in a symmetry-broken funnel-shaped nanochannel geometry above a threshold voltage, roughly corresponding to the under-limiting to over-limiting current transition. Previous experimental studies have examined ICR at either low voltage (under-limiting current region) for conical nanopores/funnel-shaped nanochannels or at high voltage (over-limiting region) for straight nanochannels with asymmetric entrances or asymmetric interfacing microchannels. The observed ICR inversion occurs because the system resistance is shifted, beyond a threshold voltage, from being controlled by intra-channel ICP to being controlled by external ICP. Additionally, strong hysteresis effects, due to residual ICP, manifest themselves through the dependence of the transient ICR on voltage scan rate. In addition, we studied the effect of advection on the transient ICP phenomenon in microchannel-membrane systems. Specifically, the temporal evolution of the depletion layer in a system that supports net flow rates with varying Péclet values was examined. Experiments complemented with simplified analytical one-dimensional semi-infinite modeling and numerical simulations, demonstrated either suppression or enhancement of the depletion layer propagation against or with the direction of the net flow, respectively. Of particular interest was the third-species fluorescent dye ion concentration polarization dynamics which was further explained using two-dimensional numerical simulations that accounted for the device complex geometry.