טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYoav Green
SubjectElectrokinetic Ion Transport in Heterogeneous Ion
Permselective Systems
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Yossifon Gilad
Full Thesis textFull thesis text - English Version


Abstract


In classical fluid mechanics the usual driving forces are either pressure or gravity. As the size of the system is decreased additional forces become more important. For nanoscale systems, electrical forces become the dominant force. The introduction of electrical effects not only increases the richness of the resultant physics but it also increases the complexity of the governing mathematical equations and the underlying physics themselves. This complexity can be observed through numerous non-linear processes that are non-existent in classical flows. One such example is the formation of a hydrodynamical instability in Low Reynolds flows.

For more than a half century seawater has been desalinated using electrodialysis systems that are based on nanoporous membranes that exhibit the capability to reject ions based on the electrical charge. By applying an electric potential and accounting for the rejection property, also termed permselectivity, one could remove charged ions from seawater to produce desalted water. The desalination process was characterized by a current-voltage response curve comprised of three different regions: I) Ohmic, II) limiting-current, III) over-limiting current. For numerous decades, the physics of the first two regions were very well understood. The presence of the third region baffled the community for a long time and the reason for its existence remained unexplained. This third region is perhaps the most important in desalination as it is responsible for the removal of more salts and as such was highly investigated. Roughly 15 years ago, it was hypothesized that at the interface of the nanoporous membrane, non-linear effects resulted in the onset of an electrically driven hydrodynamic instability. This was confirmed experimentally seven years ago. One work used a standard nanoporous membrane used in desalination system while the other used a simpler system comprised of single nanochannel. Unlike membranes, the geometry of a nanochannel can be fabricated to exact specifications. The nanochannel and the membrane share the property of permselectivity, which allows the nanochannel to serve as simplified model of the membrane - and allows investigating the fundamental properties of a single nanochannel/nanopore.  

We set out to understand the impact of the nanochannel geometry on the overall response of the system. As a first step, focus was placed upon the Ohmic and limiting-current regions. In these two regions, we theoretically modeled our system to account for the 3D geometries of our experimentally used nanochannel systems. A current-voltage response was derived which accounts for the effects of the changing geometry of the nanochannel as well as the adjoining microchambers. The current-voltage response was verified by finite-element simulations and by experiments. These experiments, also, allowed us to investigate the third region (the over-limiting-current) in depth. While previous works had shown that the electroconvective instability was responsible for the over-limiting-current in nanoporous-membrane systems, recent works had indicated that in nanochannel systems an additional electroconvective mechanism might be responsible for the over-limiting-current. Our experiments investigate the relation between these different mechanisms. We show that it is the geometry of the system that determines the extent at which each mechanism is expressed.