Nanofluidic Energy Conversion with (re)Active suRfaces – NECtAR

During this project, we need to extend the current theoretical framework and develop new numerical and experimental tools. On the numerical part, we need to develop accurate and reliable approaches to extract the friction properties of complex liquid-solid interfaces from ab initio molecular dynamics (AIMD) simulations. Moreover, we also need to develop suitable tools to measure the EK response with AIMD, and in particular we will explore the possibilities of non-equilibrium AIMD. Finally, we will focus on applications to waste heat harvesting. To that aim, we will determine the response of liquid/solid interfaces to thermal gradients. Here again, we will need to develop the suitable numerical framework. On the experimental part, different challenges must be tackled. A first lockout will be the generation of almost defect free surfaces to fit the numerical predictions, and to achieve a complete characterization of these surfaces. The second step to get a better fundamental understanding on how surface reactivity and EK transport are coupled, is to probe both effects at the same time. We then propose to couple in the same experiment a technique allowing surface reactivity optical characterization (Second Harmonic Generation – SHG) with macroscopic EK measurements. Even though both techniques are well developed separately, such a simultaneous measurement remains a technical challenge. To disentangle bulk and surface contributions, a special effort will be also devoted to track EK transport in very confined systems (a few nanometer thick). Lastly, to go beyond this state of the art understanding, we propose an alternative way to control surface reactivity and subsequent transport by using optically active materials, which represents also a significant challenge.

Waste heat harvesting is one of the greatest challenges of our society. Nanoporous membranes could play a role thanks to thermo-osmotic flows, generated at surfaces by temperature gradients. We have used atomistic simulations to explore the underlying molecular mechanisms, and understand the effect of the wetting properties of the liquid on the solid surfaces. We have shown the critical role of interfacial hydrodynamics, which can reverse the direction of the flow, and strongly amplify it. In particular, we have predicted giant thermo-osmotic flows at the water-graphene interface. These theoretical results open many perspectives for the efficient generation of flows using waste heat, with could be applied for instance to sea water desalination. In particular, we have combined theoretical modeling and atomistic simulations to explore the possibility to harvest waste heat with nanoporous membranes, focusing on model membranes built from carbon nanotubes. We have shown that when such membranes are submitted to temperature differences, extremely fast and robust flows can be produced, which could be used for pumping or desalination applications. In contrast with the traditional understanding of these systems, we have shown that the performance of the membrane depends crucially on hydrodynamics in the pore and at the entrances. Our model could be used to guide the search for innovative membranes for waste heat harvesting.

Nanofluidics is a blooming research field showing great promises for applications related to clean, safe and efficient energy, where technological development is limited by a lack of fundamental understanding of nanofluidic transport. By investigating the role of interfacial reactivity, we aim at overcoming the limits of current descriptions. We expect that a better understanding of the mechanisms underlying nanofluidic energy conversion will in fine help to develop better devices. Specifically, in this project we explore how nanofluidic systems can help to harvest a particularly elusive energy: waste heat. Nanofluidic devices could provide an interesting alternative to thermoelectric materials, or they could use low grade waste heat to desalinate water. This would be a breakthrough in many industrial applications involving too small temperature differences for electricity production with standard methods, or could even be included in smart clothes to harvest body heat.

5. R. Hartkamp, A.-L. Biance, L. Fu, J.-F. Dufrêche, O. Bonhomme, L. Joly: “Measuring surface charge: why experimental characterization and molecular modeling should be coupled”, invited review article submitted to Current Opinion in Colloid and Interface Science
4. L. Fu, S. Merabia, L. Joly: «Understanding Fast and Robust Thermo-Osmotic Flows through Carbon Nanotube Membranes: Thermodynamics Meets Hydrodynamics«, J. Phys. Chem. Lett. 9, 2086 (2018)
3. L. Fu, S. Merabia, L. Joly: «What Controls Thermo-osmosis? Molecular Simulations Show the Critical Role of Interfacial Hydrodynamics«, Phys. Rev. Lett. 119, 214501 (2017)
2. O. Bonhomme, B. Blanc, L. Joly, C. Ybert, A.-L. Biance: “Electrokinetic transport in liquid foams”, Adv. Colloid Interface Sci. 247, 477 (2017)
1. A. Barbosa de Lima, L. Joly: “Electro-osmosis at surfactant-laden liquid-gas interfaces: beyond standard models”, Soft Matter 13, 3341 (2017)

Water desalination and sustainable energy harvesting are among the greatest challenges of our society, and nanofluidics offers promising solutions to address them. Nanofluidic energy conversion systems rely on electrokinetic (EK) effects, which couple different types of transport (hydrodynamic, electric, ionic, thermal…) at interfaces. EK effects are sensitive to the molecular detail of interfaces, and should thus depend on their physical chemistry. Yet the possibility to couple surface reactivity and EK transport to enhance the performance of nanofluidic devices has never been studied to the best of our knowledge.
Here we will explore these couplings in order to design innovative nanofluidic systems for sustainable energies, focusing on waste heat harvesting. To that aim, we will gather experimentalists and theoreticians who will work on the same systems, and who will probe simultaneously the physical chemistry of the interfaces as well as their transport properties. This strategy opens the way to find optimal situations where reactive interfaces have a large impact on nanofluidic energy conversion. (tasks 1 and 2). Finally, we will use this approach combining modeling and experiments to design a proof-of-principle nanofluidic device using waste heat to produce electricity or drinking water (task 3).
In task 1, we will investigate the coupling between reactivity and fluidic transport at water/ceramics interfaces, ubiquitous in solid-state nanofluidic systems. In particular, we will explore the performance of oxide surfaces such as MgO and ZnO, where water can dissociate to form “super-ionic” water. We will use ab initio molecular dynamics simulations to test the role of interfacial chemistry and quantum effects on transport. In parallel, we will combine in a unique experimental setup macroscopic measures of EK transport with non-linear optics characterization of the nanometric structure and dynamics of interfaces. We also propose to adjust the electric charge of some surfaces by using light as an external switch, and to use these surfaces to control and enhance interfacial transport.
In task 2, we will consider hydrophobic surfaces, which can amplify EK effects through hydrodynamic slip. The electric charge of these surfaces is generally mobile and can react to the flow. The aim of this task is to provide a fundamental understanding of these effects, so as to propose efficient systems for large energy conversion. On the numerical side, we will investigate the role of charge mobility at various hydrophobic surfaces, using classical molecular dynamics simulations. On the experimental side, we will adapt the methods developed in task 1 to the specific details of hydrophobic surfaces.
The last task will be devoted to the study of waste heat harvesting, using EK effects induced by thermal gradients. Indeed, thermal gradients are expected to induce a thermoosmotic flow of water and a thermophoretic motion of ions, generating electricity. Since these phenomena have deserved little attention previously, we will first characterize their amplitude near ceramic and hydrophobic surfaces considered in the preceding tasks, combining simulations and experiments. We will then explore different membrane-based processes to harvest waste heat, in order to propose a proof-of-concept at the end of the project, whose performance will be quantified.