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Fluid transport at the nano- and meso- scales : from fundamentals to applications in energy harvesting and desalination process

Periodic Reporting for period 3 - NanoSOFT (Fluid transport at the nano- and meso- scales : from fundamentals to applications in energy harvesting and desalination process)

Reporting period: 2018-04-01 to 2019-09-30

The project NanoSOFT, as stated in the Description of Action (DoA), is divided in three main tasks or Work Packages: 1) New experimental tools for nanofluidics; 2) Fluid transport at nanoscales, from continuum to mesoscale behaviour; 3) Nanofluidics: from fundamentals to applications. In the following the objectives of each WP will be presented and described.
1. New experimental tools for nanofluidics

This first WP deals with the realization of new experimental tools for nanofluidics and it has two distinct objctives : 1) realization of new nanofluidic devices, allowing for the detailed study of fluidic transport under diverse forcings, such as electric fields, pressure drops, chemical gradients, or combinations of these ; 2) developement of new experimental techniques to investigate the transport properties : methods to measure electric currents with high sensitivity, as well as new methods to measure water or solute fluxes.

1.1 Developing nanofluidic devices: towards new geometries and materials
In the project description this objective was further divided in two complementary tasks.
New geometries : using nanomanipulation, I planned to fabricate new kind of nanofluidic systems. In the spirit of the patch-clamp technique developed by Neher and Sakmann, I proposed to connect a single nanotube at the end of a micropipette, thereby creating nanopipettes . This set-up has many key advantages since it can be easily associated with optical techniques to visualize directly translocation of fluorescent nanoparticles (or macromolecules) inside the nanotubes. As a byproduct, this allows for direct flow measurements using fluorescent dyes as passive tracers.
New Materials : h-BN, h-Cg and composite h-BN-C layers : in order to perform a dedicated study to explore the properties of different materials at the interface of fluids and the role of the electronic properties, which are largely unexplored up to now, to the behaviour of confined fluids I planned to realize nanoufluidic systems based on individual nanotubes made of Boron Nitride and Carbon, two materials with the same crystallography but radical different electronic conductivity. In parallel I plan to realize also systems made of planar bidimensional systems such as graphene, hexagonal boron nitride and a mix of the two.
1.2 Developing investigations techniques
The main objectives of this task is the realization of new experimental techniques amenable for a systematic study of ionic and fluidic transport. In particular I planned to upgrade the former experimental set-up for the measurement of ionic transport through nanochannel in order to increase the sensitivity (therefore having access to smaller systems) and the frequency range (in order to study current noise). In parallel I also planned to realize a new experimental set-up based on optical techniques, such as optical microscope, fluorence and confocal microscope in order to investigate the mass transport through individual nanotubes.

2. Fluid transport at nanoscales, from continuum to mesoscale behaviour
The goal of the second WP is the study of the properties of the fluids confined at the nanometer level from a fundamental point of view. The research plan is organized around three main objectives: 1) the thorough characterization of transport through single nanotubes,; 2) the exploration of “2D systems” made of h-BN or graphene; 3) exploration of the interplay between fluid properties and the solide-state electronic properties of the confining materials.
2.1 Transport inside individual BN and C nanotubes
The goal of this first task of the WP2 is the full characterization of transport inside individual nanotubes made of different materials. By using the devices and techniques developed in the WP1, I planned to study how ionic transport is affected by the different confining materials: the flow of ions induced by different forcings, being voltage, pressure or concentration gradient, has to be studied for different
1.2.1 Work Package 1
In agreement with the objectives detailed in the project proposal and DoA, two parallel routes have been followed in this this first period: firstly, the realization of new experimental devices (following the underlying idea of new materials and new geometry) and secondly the development of novel experimental set-ups.
Developing nanofluidic devices: towards new geometries and materials
Concerning the first part, thanks to the post-doctoral researcher hired within the framework of the ERC project, we have generalized our nanoassembling route, previously used to create an individual transmembrane boron-nitride nanotube, to other kind of systems. In particular we have been able to apply the same technique, consisting on nanomanipulation in side a scanning electron microscope, to individual nanotubes made of Carbon. If the task may seem straightforward given our previous experience, we want to stress that the characteristics of carbon nanotubes – in terms or rigidity and charging – made this objective quite challenging. Nonetheless, modifying the manipulation plateform to an upgrade and more stable version, made possible to create transmembrane devices where one individual nanotube made of carbon is connecting two reservoirs. We have been able to apply the technique to nanotubes with different radii (from 50nm down to 3.5nm) and growth technique (chemical vapour deposition and discharge arc). Further we have applied the same technique to realize alternative geometries: more into the details an individual nanotube has been connected at the extremity of a laser pulled glassed capillary. This new geometry is of particular interest in the context of measurement of mass transport.

Developing investigations techniques
Concerning the second part of the WP, we have upgraded the former electrokinetic measurement platform to increase the sensitivity and the frequency bandwidth. A low noise current amplifier coupled to a custom faraday cage and dedicated electrical circuit allowed to increase the ionic current sensitivity down to 50 fA level (compared to the 1pA of the previous version, i.e. a factor 20) with a 100 kHz frequency range. In parallel a new optical detection scheme has been developed on the basis of a commercial inverted optical microscope coupled to a home made compatible fluidic cell.

1.2.2 Work package 2
Thanks to the development of new experimental devices and techniques as described in WP1, we have been able to address several interesting objectives of the Work package 2.
Transport inside individual BN and C nanotubes
Using the transmembrane nanotube devices, we have been able to measure ionic transport through individual nanotubes made of different materials such as Carbon and Boron Nitride and compare the results between different systems. We have measured the electrokinetic properties for different working conditions by changing the salt concentration in the reservoir and the pH level. We have characterized the full transport matrix by applying different forcing being voltage, pressure and concentration gradient. The experiments have been carried for different nanotubes with different radii and they point out a very different surface chemistry for boron-nitride and carbon materials. To understand and rationalize these findings, numerical simulations have been performed in collaboration with colleagues of the chemistry department of the Ecole Normale Superieure. In parallel, we have also performed measurement of ionic fluctuations inside nanotubes. To do so we have analyse the frequency response of ionic current when the nanotubes are subjected to various external conditions. Once again our results point out a very different behaviour for carbon and boron-nitride materials. Finally, by using the new nanocapillary devices and the optical detection set-up we have addressed the water mass transport in individual nanotube. In practice to have access to the permeability of such a tiny pipes, we
This first period of the ERC project NanoSOFT has been particularly positive with a large number of objectives that have been accomplished. This is of course very exciting and it opens new perspectives for the following of project with new goals with potential high impact. In particular three different lines of research have now to be followed:
1) Non linear fluidic transport in ultra-confined systems: Our first measurements in sub nanometer size channel point out a very exotic transport behaviour. Ions flow through these small channels in a way that cannot be understood within the framework of classical hydrodynamic couple with electrokinetics: in particular the coupling by ion transport and flow transport pave the way to non-linear transport that lead to new paradigms for nanofluidics. We want now to address these questions by systematically measure transport through angstrom size slits made of bidimensional materials such as graphene or hexagonal boron nitide, as well as single walled nanotubes. The goal, and definitely the challenge, is to observe many body features such as coulomb blockade ionic transport, where ions are passing through the channel one by one, that are theoretically foreseen but not experimentally proven.
2) Confinement induced phasechanging ionic liquids: Ionic liquids are particular and unusual liquids that are composed by only charges. They are of interest from a fundamental point of view because they allow to challenge the standard description of electrolytes but further they are subjects of intense research because of their potential applications. They have been proposed as ideal systems for the development of new and performing energy stocking devices as supercapacitors. In supercapacitors ionic liquids are confined in nanoporous electrodes made of carbon and other suitable compounds. Understanding their behaviour in such extreme conditions is therefore of crucial importance in view of development of new devices. Our results on phase change under confinement point out new and unforeseen behaviours for such liquids that may drastically impact their performances for supercapacitors. We aim now to fully investigate the behaviour of ionic liquids in confined geometries and understand the role of disorder to their transport dynamical properties in order to asses the properties as electrolytes for supercapacitors.
3) Membrane up scaling for energy harvesting: Harvesting the energy coming from the mixing of solution with different salinity has been proposed as an alternative and clean source of energy. The key to make this source of energy at an industrial level is to reach a conversion efficieny of the order of 5 Watts per square meter of membrane. Several routes have been proposed over the last decades but so far non has been able to pass this economical threshold. Our results on Boron-Nitride nanotube first, and then on nanoporous Titanium Dioxide pushed the efficiency up to several thousands of watts per square meter. We still need to prove that such new interesting materials can be upscaled to large membranes. To this goal, a start-up company, Sweetch Energy SAS, has been created aiming to the reaslization and eventually commercialization of new membranes. This will be consequently an important aspect of the work that I plan to carry in the second part of the project. Working in close contact with the company we will study new solutions and propose new technologies for the creation of nanfluidic membranes. While the company will focus more on the industrial implementation of membranes made of TiO2 or alternative materials, in the group we will look for new nanofluidic functionalities, such as fluidic and ionic diodes a priori able to boost even more the conversion efficiency. In this context it is worth noting that an application for the next ERC-PoC is suitable.