Vishal V. R. Nandigana

We demonstrate a novel nanofluidic diode that produces rectification factors in excess of 1000. The nanofluidic diode consists of ion permselective nanopores that connect two reservoirs of different diameters- a micropore reservoir and a macropore reservoir. On the application of +100 V to the micropore, a low OFF state current is observed. The OFF state is caused by formation of the ion depleted zone in the micropore because the anions are prevented from entering the nanopores from the micropore and the cations are depleted in this region to maintain charge neutrality. On the application of −100 V, we observe a high ON state current. The ON state is caused by formation of the ion enriched zone in the microchannel because the anions cannot pass through the nanopores and accumulate in the microchannel. To maintain charge neutrality the cations also become enriched in the microchannel. The ratio of ON state current to the OFF state current gives the rectification of current. Here, plasma oxidation is used to achieve a nanopore with a large wall surface charge density of σn = −55 mC/m2 which yields a rectification of current on the order of 3500 that is nearly two orders of magnitude higher than those reported thus far. In contrast to the other nanofluidic diodes, this nanofluidic diode does not introduce asymmetry to the nanopore, but asymmetry is produced by having the nanopores join a micropore and a macropore. Introduction of asymmetry into the fluidic reservoirs which the nanopores connect is quite simple. Hence, the nanofluidic diode is easy to scale up to industrial level.

Osmotic power generation, the extraction of power from mixing salt solutions of different concentrations, can provide an efficient power source for both nanoscale and industrial-level applications. Power is generated using ion-selective channels or pores of nanometric dimensions in synthetic membrane materials. 2D materials such as graphene and MoS2 provide energy extraction efficiencies that are several orders of magnitude higher than those of more established bulky membranes. In this Review, we survey the current state of the art in power generation with both 2D materials and solid-state devices. We discuss the current understanding of the processes underlying power generation in boron nitride nanotubes and 2D materials, as well as the available fabrication methods and their impact on power generation. Finally, we overview future directions of research, which include increasing efficiency, upscaling single pores to porous membranes and solving other issues related to the potential practical application of 2D materials for osmotic power generation.

Nanopores have been used for myriad applications ranging from water desalination, gas separation, fluidic circuits, DNA sequencing, and preconcentration of ions. In all of these applications noise is an important factor during signal measurement. Noisy signals disrupt the exact measuring signal in almost all of these applications. In this paper, we rationalize whether current oscillations should be classified only as noise or the physical disturbance in ionic charges has some other meaning. We infer that the physical disturbance in ionic charges and the current oscillations are not noise but can be chaos. Chaos is present in the system due to depletion of the ions, created by nonequilibrium anharmonic distribution in the electrostatic potential. In other words, multiple electric potential wells are observed in the nanoporous system. The multiple electric potential wells leads to bi-directional hopping of ions as the ions transport through the pore. The bi-directional hopping results in current oscillations. This paper suggests that chaos exists from a deterministic perspective and that there is no stochastic element leading to current oscillations. We prove this case by considering a simple oscillator model involving the electrostatic and dissipative forces in order to model ionic current. We observed current oscillations even in the absence of a stochastic noise force. Hence, we state that current oscillations in nanopores can be due to chaos as well and not necessarily due to noise. Furthermore, the color associated with the chaotic spectrum is not brown but pink, with 1/f type dynamics similar to the 1/f type pink noise presented by theorists and experimentalists. However, the 1/f type pink chaos exists due to deterministic current oscillations and not due to a stochastic fluctuating noise force.