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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 nanotubes made of carbon or boron nitride. A second goal of this task was the study of the mass transport through nanotubes with different material and dimensions. In particular the objective was to asses the superlubricity of carbon nanotube to water mass transport. To do so the permeability, i.e. the quantity of mass flows induce by a pressure drop, has to be measured as a function of the radius of the nanotubes and the electronic properties of the nanotube itself. Finally, fluctuations are expected to play an increasingly important role. There is a strong interest in exploring more in details the fluctuation spectrum of ion currents.
2.2 Transport across 2D systems
Beyond its broad exploration in the context of new electronic devices, graphene and 2D devices have a unique potential as new materials to design membranes for fluids with unprecedented efficiency. Membranes are at the heart of many applications and industrial processes, embracing key challenges for our future, such as desalination and water filtration, chemical analysis or energy harvesting. In this part of the project, the goal is to explore the properties of graphene as well as h-BN sheets in the context of fluid transport. Experiments follow basically the same program as the one outlined for carbon and BN nanotubes, with a systematic study of ion and water transport under various forcing (voltage drop, pressure drop, salinity gradients), for various conditions and geometrical characteristics
2.3 Fluid transport and meso-scales properties

Beyond the previous investigations, a long-term and challenging goal is the exploration of the fluidic transport at the mesoscales, where the quantum world of individual atoms – characterized by their specific electronic properties – meets the bulk scale of continuum and classical physics. The generic questions I planned to address are: how fluid transport may connect to electronic properties of the confining materials; and vice-versa may one control fluid transport by tuning the electronic structure of the confining materials?

3. Nanofluidics: from fundamentals to applications

The goal of the third and last WP is the extension of nanofluidics functionalities to industrial applications and it has two main axes or objectives: 1) define new membranes for osmotic energy harvesting; 2) propose new route for desalination process.
3.1 Osmotic energy harvesting
The denomination of osmotic power refers to the free energy that can be extracted from the difference in salinity between salty and fresh water, based on the entropy of mixing of the salt. Two methods are commonly used to extract this energy : PRO (pressure retarded osmosis) and RED (reverse electro-dialysis): both approaches are now well established and, even more, the PRO method is currently explored at an industrial level with a prototype plant developed in Norway by the Statkraft company. However the harvested power is typically of order of a few Watts per meter squared of membranes. Statkraft estimates that commercial viability is ensured above 5 W per meter square of membrane. Goal of this task of the project was to propose new nanofluidic solution in order to bypass the major bottleneck for the harvesting of the osmotic energy. In particular I planned to develop membranes made of boron nitride nanotubes or alternative materials presenting the same physico-chemical characteristics and capable of supporting diffusion-osmotic transport when put between solutions with different salinity.
3.2 New routes for desalination process
Concerning the problem of the desalinisation, the aim of this project is to take benefit of the solutions found by evolved biological system and finally to realize a biomimetic filter able to reproduce the exceptional performace of the human kidney which is 50 times more effective than the state of art membranes for filtration. The ultimate goal of this part of the project is to be able to recreate experimentally a similar kind of biomimetic filter on the basis of the tools that are at our disposal: semi-permeable and/or asymmetric membranes, voltage drops, pH gradients, etc. Going further, one may check the possibility to create a fluidic diode using asymmetric nanochannels: based on an analogy with semi-conducting systems, this geometry can lead to a rectification of the osmotic pressure and ion flux created under an imposed salt gradient
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 have developed a new experimental protocol: by measuring the water jet induced in the external reservoir by the water flow from the nanotube we could obtain a direct measurement of the water velocity in the tube. This measurement has been performed for different nanotubes with varying radius and for different materials. We have observed an enhancement of the permeability respect to classical expectation for carbon nanotube with an enhancing factor depending on the radius (smaller the tube, faster the water flows). In contrast, such behaviour was not observed for boron-nitride nanotubes.

Transport across 2D systems
The study of water and mass transport across bidimensional systems requires the development of a new kind of fluidic devices. Systems of choice are, of course, graphene and boron nitride channels. The realization of this kind of systems is a real challenge and it requires a particular expertise. Thanks to a recent collaboration with Dr. Radha Boya and Prof. Andre Geim at the university of Manchesters, we have been able to study the ionic and fluidic transport through sub-nanometer size channels made of bidimensional systems. A new experimental platform has been realized to integrate such particular devices to the optical detection set-up and new detection schemes based on fluorence microscopy have been implemented.
Fluid transport and meso-scales properties
To study the role of the electronic properties of the confinement materials of fluid behaviours we have performed two different and complementary studies. In the first series of experiments we have studied transport of water and solutions with salts through nanotubes made of different materials: semiconducting carbon and insulating boron nitride. This first series of measurement is in common with the first task of this work package and it follows the same schedule and plan. In parallel we have developed a custom atomic force microscope based of quartz tuning fork sensors. This new experimental set-up allows to study the mechanical and rheological properties of fluids confined at the micro- and nanoscale. We have measured the mechanical impedance of exotic liquids, such as ionic liquids (liquids made of only charges) when they are confined between materials with different electrical conductivity and electronic properties. Accordingly we have related the fluid behaviour to the characteristic of the confining materials. A very surprising result that we have obtained, is that ionic liquids undergo to a phase transition towards a solid state when confined below a critical gap. The value of this threshold is related to the electronic conductivity of the confining material, with more metallic surfaces facilitating solidification. A numerical simulation study, in collaboration with a colleague in Grenoble (France), has allowed to rationalize our findings.
Further by using our newly developed force microscope we have been able to study how confinement modify the behaviour of complex fluids and to address the fundamental origin of shear thickening of colloidal solutions. By approaching a two colloids in a suitable liquid we have been able to show that shear thickening is most likely due to the solid solid friction between the colloids once all the liquid between them is squeezed out because of the extreme confinement.

1.2.3 Work package 3
The objective of this last Work Package is the application of novel nanofluidics functionalities for industrial applications. In agreement with proposed plan, this work package will be substantially implemented in the second period of the project. Nevertheless a certain amount of preliminary studies has been already perfomed to grant the feasibility and success of the WP3.

Osmotic energy harvesting
Key element for the industrial application of osmotic energy harvesting based on nanofluidic transport is the up-scalability of diffusion-osmotic transport to large scale membranes. While the first studies focuses of the the fundamental aspect of diffusion-osmotic transport through nanochannels an important wotk has to be done to assess the feasibility of macroscopic membrane made of suitable materials. We have therefore carried an exhaustive screening of the key characteristics that a material has to satisfy in order be considered a good candidate. If a large surface charge at the solid-liquid interface is the major element that is necessary to develop large diffusion-osmotic currents, it is also important to check if the production at industrial is compatible. We have tested a large number of different candidate and we have found in particular a very promising material : titanium dioxide. TiO2 is a common material that is already used in a large number of industrial applications, such as antireflection coating in standard glasses and antifouling coating for boats. It is easily upscalable and it present a relatively large surface charge. We have performed laboratory studies and we have confirmed the potentiality of TiO2 for energy harvesting applications.
In parallel as a major and for us exciting results of this work package, a start up has been funded based on the results of this wok package. The company Sweetch Energy SAS, aims in particular to the realization and industrialisation of a new class of nanofluidic membranes for the harvesting of the osmotic energy.

New routes for desalination process
To understand the physical origin of rectified transport in nanofluidic channels is the first step to the implementation for desalination and water treatment applications. We have therefore perform two complementary studies to shed a light on this subtle and largely not understood phenomena. Firstly we have performed a detailed experimental study on model systems: we have measured ionic transport under different forcing through asymmetric glass nanocapillaries. Conical channels are realized by laser pulling a glass cylindrical capillaries; the radius of the channel is varying along the length of the channel from 1mm down to hundreds of nanometers. These devices present current rectification when a voltage drop is applied. Experiments by coupling voltage drop to pressure and concentration gradient are perfomed. In parallel a theoretical work based on Poisson Boltzmann formalism are carried on in order to understand the key physical ingredients governing the behaviour of nanofluidics diodes.
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.