Nanofluidic transport using charge-selective interfaces

Abstract

Nanofluidic transport, or fluid motion through nano-sized compartments, has a wide scope of large-scale flow applications, such as in the oil & gas industry, water distribution, and cell biology. In addition, understanding of the mechanism of fluid transport is essential for appropriate designs and fabrications of nano/micro-fluidics for various applications. The flow through nano-sized compartments is fundamentally different from that of its macro-sized counterparts. For instance, the development of surface charge alters the local electric potential distribution and Coulombic forces; the effect of surface tension on the flow is more pronounced. In this doctoral dissertation, we aim to investigate nano-flow and resultant ionic interactions in miniature geometries with non-zero surface charge. Focusing on the engineering application of such nanoflow, we have further analyzed renewable energy generation via salinity gradient, exploiting the ion-selectivity of charged nanochannels. Nanoflow through charged nanoslits under a concentration difference, termed as diffusioosmosis, is dictated by chemiosmotic and electroosmotic effects. We investigated such diffusioosmotic transport in a charged nanochannel and reported a non-dimensional number based on the nanochannel properties that can predict the direction of diffusioosmotic flow through nano-confinements. The non-dimensional number, obtained from the ratio of chemiosmotic and electroosmotic velocities, delineates three flow regimes i.e. along the concentration gradient, opposite to the concentration gradient, and a mixed flow regime. We also extend this analysis to study diffusioosmosis in tapered nanochannels, where we reported a significant dependence of flow velocity field on the angle of the taper, and nanochannel dimensions. The harnessing of salinity gradient energy can be achieved via a process known as reverse electrodialysis (RED), which exploits the charge-selective nature of certain nano-confinements to extract energy when two electrolyte solutions of different concentration are allowed to interact via an ion-selective membrane. We numerically investigated RED in a single nanochannel and quantitatively measured the power output density and energy conversion efficiency of a single nanochannel. In our experimental endeavor, we designed a RED cell where two solutions with a concentration gradient were made to interact through a commercially-available, cation-selective Nafion membrane. Our results reported a power output of the order of mW/m2 from the RED experimental setup. This doctoral dissertation develops a better understanding of diffusioosmotic flow in nano-sized compartments. In the specific case of fluid interaction of two solutions of different concentration via an ion-selective interface, we propose a non-dimensional number, based on the ratio of chemiosmotic and diffusioosmotic velocities to predict the flow direction through the nanochannel. Quantitative analysis of diffusioosmotic flow through asymmetric nanochannel is also presented, paving the way for more systematic utilization of ion current rectification. Lastly, our reports regarding power generation through reverse electrodialysis may be instrumental for designing renewable setups at the river and sea junction, for so-called blue energy technology.