Influence of external environment on fluid/ion transport in confined nanoscale systems

Nanofluidics;Electrokinetics;Ion-channels

Nandigana, Vishal Venkata Raghave

关键词: Nanofluidics;    Electrokinetics;    Ion-channels;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/92723/NANDIGANA-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The advancements in micro-nano-manufacturing technology has lead to a significant leap in understanding the nuances of transport phenomenon in nanopore architecture. The large nanopore surface compared to the volume favors a predominant transport of ions. The environment results in a near ideal ion-selective nanoporous membrane. Theselective nature of ions can be triggered in the fields like single molecule/particle sensing, preconcentration of analytes using electric field focusing and desalination applications. A single nanoporous membrane leveraged with microporous reservoirs manifests the current in the system supporting the engineering and biosciences applications. The external microporous environment influences the transport in a nanopore. My field of contribution contradicts the existing theories and methodologies.Earlier works revealed a non-monotonic growth in the current-voltage characteristics in a nanoporous membrane. The experimental observations predicted a deviation from the classical diffusion-limited current transport theory, which highlights a saturation of the current density at higher applied voltages with an infinite differential resistance. Furthermore, the quasi-equilibrium regime confers a monotonic increase in the current, following the Ohm's law. However, at a critical voltage V_(LI), the current increases infinitesmely postulating a novel limiting resistance regime (LRR) in contradiction to self diffusion mechanism. A push in voltage manifests a second critical voltage V_(LII), referred to as the overlimiting current regime with a higher slope, and constant conductivity.Many plausible mechanisms are discussed to explain the overlimiting current characteristics. Using a 1-D ideal ion-selective membrane model a possible mechanism for the limiting resistance region deviating from the self diffusion mechanism was set forth. Further, the theory predicted a region of induced charge arising in the unstable depletion micro-nano junction owing to the convective fluid instabilities.Experimental works were navigated this field of research and postulated a probable convective fluid and charge instabilities for the overlimiting current characteristics in micro-nanostructures. However, an exact physical understanding of the large, yet finite differential resistance, in the limiting resistance regime, in conjecture with the transition to the overlimiting regime and the true overlimiting current mechanism is still unexplored.In this thesis, I have explained the fundamental physics unraveling the mystery of overlimiting current in micro-nano interconnect architecture. A comprehensive physical model to discuss all the three current regions are numerically predicted by considering the microporous membrane interconnected with nanochannel. I tackled this problem by using a detailed 2-D nonlinear, nonideal ion-selective model. Further, I discuss the necessary governing equations required to model the system. The model illustrates a constant conductivity in the overlimiting current, consistent with all the experimental observations. I highlight the mechanism, by analyzing the behavior of the ionic concentration, near the depletion junction of the cathodic microporous membrane integrated with nanochannel. The redistribution of charges at the depletion junction manifests in the overlimiting current, in contradiction to convective charge/fluid instabilities at this junction. My understanding propelled me to investigate the charge dynamics in the nanoporous membranes influenced by external microporous architecture. To this end, I first reduce the order of the governing equations, developing a novel dynamical area-averaged multi-ion transport model (DAAM). The concentration and electric potential are integrated in the radial direction. Furthermore, the convective fluid flow contribution plays an insignificant role in the mechanism, and hence, is neglected in my DAAM model. To understand the charge dynamics, I proposed a novel computational impedance spectroscopy model (CIS). Henceforth, we apply a harmonic electric potential in the system and investigate the harmonic output current. I infer the phase effects between the concentration and the electric potential wave, and determine the system dynamics under equilibrium, quasi-equilibrium (Ohmic regime), limiting resistance regime and in the overlimiting current regime.My findings led to a fruitful collaboration with the experimentalists to invent a novel nano-diode. The invention is in conjecture with a bio-inspired gating channel, with an added advantage of their application in radioactive environment. The semiconductor diodes fail to serve this purpose. The advantage stems from the fact that the current rectification (ON/OFF states) can be put forth by the change in the the cross-sectional area of the external microporous environment leveraged with a nanopore in addition to the migration of the enriched and depleted concentration zones.Temporal power spectra map of the ON state current dynamics postulated an electroconvective origin from our experimental observations.However, I present an advanced theoretical correlation between the ON state current dynamics (for nano-diodes) with the chaotic clustering of ions at the anodic micro-nano enriched/Avalanche regime. I mathematically quantify chaos in terms of maximum Lyapunov exponent. The maximum Lyapunov exponent increases monotonically with an increase in the applied bias voltage and the macropore reservoir diameter. Furthermore, I postulate a low frequency "1/f" type dynamics for the voltage dominated chaos and "1/(f^2)" type dynamics for the macropore reservoir dominated chaos.Finally, I establish a new route for depletion induced chaos postulated by earlier works using a deterministic hypothesis. The depletion chaos is manifested at near ideal ion-selective condition and under nonequilibrium potential. The maximum Lyapunov exponent monotonically increases with the ideality and is independent of the applied voltage. The current oscillations are inherited at the theoretical ideal limit highlighting the propagation of the ion depletion region from the external microporous membrane towards the nanostructure. The nonequilbrium instability in potential navigates this result. The potential instability leads to bidirectional ion hopping, fabricating in chaotic motion of the ions. I postulate a novel correlation between the temporal map and the depletion chaos and propose a new mechanism for 1/f type dynamics in nanoporous membranes.