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

Berichtszeitraum: 2019-10-01 bis 2021-09-30

Nanofluidics is the field of physics that studies the fluid behaviour at the nanometer level, scale where the standard description based on classic macroscopic laws is no longer valid. The importance and relevance of nanotechnology is nowadays well established, after decades of intense studies, and the evidence of exotic behaviors coming from the influence of surface or quantum effect at the smallest scale has been repeatedly proven. Coming back to Nanofluidics and more in general to soft condensed matter, one may ask if we could expect some peculiarities at the smallest scale: exotic phenomena could rise because of the interaction between ions and fluid molecules; as a matter of fact, Navier-stokes equations for fluid transport have been shown to be inadequate when the lengthscale reaches the nanometer and new models are expected to emerge from the confinement of liquids comparable with the liquid molecular size. Further, and in analogy to other field of nanophysics such as nanoelectronics, nanophotonics and nanotribology, the role of surfaces and interfaces and fluctuations should become more and more important and new functionalities could be developed taking benefit of the particular properties of the matter at smallest case.
A first objective of the project NanoSOFT was to develop a general fundamental understanding of fluid transport at the nanoscale, in the search of new behaviors and properties of fluids dynamics at these scales. A long-term and challenging objective is to explore the interplay between fluidic transport and the solid-state electronic properties of the confining materials, in a quest to make the link between soft and hard condensed matter.
A second objective was to provide a link between fundamental research on nanofluid transport and energy harvesting and desalination questions, thereby ensuring a succesful transfer between the fundamental findings and new applications.
The project was organized around 4 axis: (1) new experimental tools for nanofluidics; (2) new theoretical tools for nanofluidics; (3) fluid transport at the nanoscales, from continuum to mesoscale behaviour: investigation of advanced fluidic functionalities; (4) nanofluidics, from fundamentals to applications.
Thanks to support of the ERC StG, we have been able to propose and realize novel fluidic devices for confinement at nanoscale and molecular scale by using artificial nanomaterials. We have been able to investigate exotic traport of charges and mass in carbon nanotubes and graphene channels. Our studies demontrate the exceptional properties of carbon material in term of water permeability but also to point out unexpected behaviours for ions transport. In particular we have shown that ion trasport is strongly dependent on the properties of the confining material and we have reported, for the first time, an activated trasport that is externally tuned by controlled parameter. Doing this we have demonstrated that single digit carbon nanotubes shares some feature typical of biological ion channel and paving the way for bioinspired artificial devices. Further bidimensional graphene based fluidic channels have been exploited to create the first exemple of memory response with ionic trasport dependent on the history of the device.
Finally it has shown that novel nanofluidic functionalities can be harvested to propose novel solutions for the conversion of osmotic energy but also for the creation of performing filters for water purification and desalination
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 a phd sutdend and post-doctoral researcher hired within the framework of the ERC project, we have developed a novel nanofluidic device based on 2D materials such as graphene and hexagonal boron-nitride. Using standard lithography and Scanning Electron Microscope local milling, nanochannels are created on a thin crystal of the 2D material. The crystal is further placed between to thick crystals of the suitable material. This technique allowed us to study flow and ionic transport in channels of extremely small dimensions ranging from few nanometers down to 0.6 nm dimension comparable with the size of a water molecule. This new geometry is of particular interest in the context of measurement nonlinear coupling of mass and charge transport.
Developing investigations techniques
Concerning the second part of the WP, we have developed a novel flow measurement plateform based on optical microscopy. Mass transport through nanochannels can be measured by detecting the passage of sub-nanometer size fluorescent particle: when the particle cross the channel, it encounters a focused laser beam placed at the exit of the channel. This induced the emission of the fluorescence from the particle that can be detected by photodetector with a filter centered around the emission peak of the particle. The evolution of the intensity vs time while applying different forcings (voltage or pressure) gives a direct information on the mass transport. This novel plateform has been show a sensitivity of approx 100 attoL/s, 3 orders of magnitude lower than what presented before in the literature.
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 single digit C nanotubes
Using the transmembrane nanotube devices, we have been able to measure ionic transport through individual single digit carbon nanotubes. Here, we focus on ionic transport through 2-nm-radius individual multiwalled carbon nanotubes under the combination of mechanical and electrical forcings. Our findings evidence mechanically activated ionic transport in the form of an ionic conductance that depends quadratically on the applied pressure. Our theoretical study relates this behaviour to the complex interplay between electrical and mechanical drivings, and shows that the superlubricity of the carbon nanotubes is a prerequisite to attaining mechanically activated transport. The pressure sensitivity shares similarities with the response of biological mechanosensitive ion channel but observed here in an artificial system. This paves the way to build new active nanofluidic functionalities inspired by complex biological machinery.
These results were subjects of publication in leading international refereed journals:
1) Marcotte et al, Nature Materials 19 , 1057 (2020)
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. Such channels push fluid confinement to the molecular scale, wherein the limits of continuum transport equations are challenged. Water films on this scale can rearrange into one or two layers with strongly suppressed dielectric permittivity or form a room-temperature ice phase. Ionic motion in such confined channels is affected by direct interactions between the channel walls and the hydration shells of the ions, and water transport becomes strongly dependent on the channel wall material. We explore how water and ionic transport are coupled in such confinement. Here we report measurements of ionic fluid transport through molecular-sized slit-like channels. The transport, driven by pressure and by an applied electric field, reveals a transistor-like electrohydrodynamic effect. An applied bias of a fraction of a volt increases the measured pressure-driven ionic transport (characterized by streaming mobilities) by up to 20 times. This gating effect is observed in both graphite and hexagonal boron nitride channels but exhibits marked material-dependent differences. We use a modified continuum framework accounting for the material-dependent frictional interaction of water molecules, ions and the confining surfaces to explain the differences observed between channels made of graphene and hexagonal boron nitride. This highly nonlinear gating of fluid transport under molecular-scale confinement may offer new routes to control molecular and ion transport, and to explore electromechanical couplings that may have a role in recently discovered mechanosensitive ionic channels
These results were subjects of publication in leading international refereed journals:
1) Mouterde et al, Nature 567 , 87 (2019)
Fluid transport and meso-scales properties
Recent advances in nanofluidics have allowed the exploration of ion transport down to molecular-scale confinement, yet artificial porins are still far from reaching the advanced functionalities of biological ion machinery. Achieving single ion transport that is tunable by an external gate—the ionic analogue of electronic Coulomb blockade—would open new avenues in this quest. However, an understanding of ionic Coulomb blockade beyond the electronic analogy is still lacking. We developed a novel theoretical framework and we found that ionic Coulomb blockade occurs when, upon sufficient confinement, oppositely charged ions form ‘Bjerrum pairs’, and the conduction proceeds through a mechanism reminiscent of Onsager’s Wien effect. Our findings open the way to novel nanofluidic functionalities, such as an ion pump based on ionic Coulomb blockade, inspired by its electronic counterpart.
Further we adress the behaviour of atomic scale glod junction and we studied by atomic force microscopy their mechanical response when subjected to an external shear. By subjecting the junction to increasing subnanometric deformations we observe a transition from a purely elastic regime to a plastic one, and eventually to a viscous-like fluidized regime, similar to the rheology of soft yielding materials, although orders of magnitude different in length scale. The fluidized state furthermore exhibits capillary attraction, as expected for liquid capillary bridges. This shear fluidization cannot be captured by classical models of friction between atomic planes and points to an unexpected dissipative behaviour of defect-free metallic junctions at ultimate scales. Atomic rheology is therefore a powerful tool that can be used to probe the structural reorganization of atomic contacts.

These results were subjects of publication in leading international refereed journals:
Comtet et al. Nature 569, 393 (2019)

1.2.3 Work package 3
The objective of this last Work Package is the application of novel nanofluidics functionalities for industrial applications. While in the previous periods we focused on energy harvesting, during this last reporting period we developed novel techniques for water filtration and purification.
New routes for desalination process
Access to fresh water is among the key challenges facing our modern society. Standard purification and filtration techniques are based on semipermeable membranes through which the water to be treated is push with very large mechanical pressures leading to a very energetically inefficient and costly process. Based on a novel fundamental nanofluidic approach, we show here a new ultrafiltration scheme based on voltage induced reverse electro osmosis. New composite membranes based on Graphene Oxide on top of a charged support allow to move up to 238 L h−1 m−2 with full rejection of sub-nanometer size particle and hormones by applying a constant current of 0.5 mA. This system pave the way to new, efficient and easy to implement plateforme to filter and purify water
This result has lead to the filing of an international patent:
1) J. Carvaljo Perez, A. M’Barki, L. Bocquet, A. Siria, Méthode de filtration par osmose inverse induite par champ électrique, Europe N° 20305110.7 (2020)
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 PI research activity with new goals with potential high impact. In particular three different lines of research can 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 phase changing 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) Novel nanofluidic techniques for water treatment.
The increase of population and the improvement of life quality have made the access to clean water a central problem in many parts of the world. Drinkable water undergoes to contamination with a large variety of pollutants[2, 3, 4] coming from different sources, like colorants, pesticides, drugs and hormones. Hormones spreaded to stream water by the primary sectors, like farming, are one of the most common contaminants nowadays in drinkable water and they are known to present an important environmental and human health hazard. The low concentration and small size of hormone molecules, do not only impede detection by routine analytical methods of water pollutants determinations, also impede the removal of these kind by general purification methods. Nanofluidics transport indeed allows going beyond state-of-the-art passive membranes based on standard sieving processes and may pave the way to new and more efficient functionalities. we propose an ultrafiltration process based on a thin semipermeable membrane, composed by Graphene Oxide and polycarbonate, where the flow is obtained by applying a low energy electric force. To demonstrate the feasibility of the approach we first design an experimental set-up consisting in to reservoirs separated by a semipermeable membrane where a voltage drop can be applied between the faces of the membrane. Then, by using a membrane with controlled porosity of ≈ 1-2 nm, we test our system in terms of flux and rejection rate, initially with a probe molecule to optimize the operative conditions and finally with a pollutant that it is often contaminating drinkable water, testosterone.