Optimum Interfacial Friction and Electrohydrodynamic Drag Achieves Nanoscale Fluid Control

The behaviour of fluids at solid surfaces is fundamental to a wide range of technologies, from energy storage to catalysis, yet a complete understanding of their nanoscale interactions has remained elusive. Cecilia Herrero from the Institut Laue-Langevin, alongside Lyderic Bocquet from Laboratoire de Physique de l’Ecole Normale Supérieure and Benoit Coasne also of the Institut Laue-Langevin, and their colleagues, have now shed new light on these complex processes. Their research focuses on the interplay between charge relaxation within metals and the behaviour of adjacent fluids, utilising a novel molecular simulation approach termed ‘Virtual Thomas-Fermi fluids’. This work is significant because it reveals a surprising relationship between interfacial friction and the metallic properties of a surface, identifying an optimum point where friction is maximised due to strong overlap between solid and fluid dynamic structure factors. Furthermore, the team directly observed electrohydrodynamic drag, demonstrating momentum transfer between solid and liquid driven by dynamic electrostatic interactions.

Phenomena occurring near solid surfaces are central to applications in energy storage and conversion, electrochemistry and electrowetting, and adsorption and catalysis, yet their nanoscale behaviour remains incompletely understood. Recent experimental and theoretical studies on metallic surfaces have revealed exotic peculiarities including complex electrostatic screening, unexpected wetting transitions, and interfacial quantum friction. This work utilises a combined approach of density functional theory (DFT) and molecular dynamics (MD) simulations to investigate these interfacial phenomena, aiming to elucidate the underlying mechanisms governing charge transfer and molecular interactions at metal-liquid interfaces, with a specific focus on platinum (Pt) surfaces in contact with ionic liquids containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]).

Hydroelectronic Drag and Nanoscale Energy Harvesting

This research details a study into the fundamental physics at the interface between liquids and solid materials, specifically focusing on the potential to harness energy from nanoscale fluid flows. Researchers discovered a phenomenon called “hydroelectronic drag,” where fluid flow across a metallic surface generates an electrical current within the metal itself. This current originates from a quantum mechanical effect related to the fluid’s momentum being transferred to electrons in the metal, rather than classical friction. The strength of this effect, and the generated current, is strongly dependent on the metallicity of the surface, with more metallic surfaces exhibiting stronger responses.

The research suggests the potential to capture energy from “waste” flows , small-scale fluid movements typically lost, such as those in nanopores or microfluidic devices. Interestingly, the generated electrical current can reduce overall friction between the fluid and the surface, creating a negative contribution to friction. Molecular dynamics simulations, combined with a Thomas-Fermi model to represent the electronic structure of the metal surfaces, were used to model the interaction between water and metallic surfaces and develop a theoretical framework to explain the observed hydroelectronic drag. This research opens up possibilities for developing new nanoscale energy sources, designing surfaces that minimize friction, and gaining a deeper understanding of the complex interactions between fluids and solid materials. The paper demonstrates a novel way to convert kinetic energy into electrical energy at the nanoscale, potentially offering a sustainable and efficient energy source for future technologies.

Fluid Friction Peaks with Metallic Charge Dynamics

Scientists achieved a breakthrough in understanding fluid behavior at the nanoscale, focusing on the complex interactions between fluids and solid surfaces. They employed a novel molecular simulation approach, utilising Virtual Thomas-Fermi fluids, to investigate interfacial transport and the coupling between charge relaxation in metals and molecular behaviour in adjacent fluids. This atom-scale strategy provides a realistic description of solid excitation spectra, including charge relaxation modes and conductivity, surpassing conventional techniques. Experiments revealed a non-monotonous dependence of fluid/solid friction on metallicity, peaking when the charge dynamic structure factors of the solid and fluid strongly overlapped.

Measurements of charge/charge correlation functions demonstrated that the relaxation timescales for both the solid and water film increased significantly upon contact, providing evidence for dynamic coupling at the interface. This coupling slows down the dynamics of both phases, indicating an interfacial friction arising from electrostatic interactions. Further analysis determined the imaginary part of the surface response function to illustrate the full wavevector and energy transfer spectra. Results highlighted the interplay between the wavevector-dependent dynamics of the interfacial fluid and the electrostatic response of the solid surface, with a marked slowdown in the solid’s dynamic spectrum when in contact with water. This dynamic coupling isn’t attributable to phonon-mediated momentum transfer, but rather to a quantum friction arising from the coupling between charge relaxations within the metal and molecular modes in the fluid. The breakthrough delivers a new understanding of interfacial friction, quantified by the friction parameter λ, which governs molecular flow and energy dissipation at the interface, with profound implications for applications in areas like energy storage and electrochemistry.

Interfacial Friction and Metallicity’s Role in Transport

This research presents a novel molecular simulation approach, utilising Virtual Thomas-Fermi fluids, to investigate interfacial transport between fluids and metallic surfaces. The study successfully models the complex interplay of charge relaxation within the metal and the behaviour of adjacent fluid molecules, something conventional techniques often simplify. Researchers demonstrated that interfacial friction exhibits a non-monotonous relationship with metallicity, peaking when the dynamic structure factors of the solid and fluid significantly overlap, revealing a key microscopic mechanism governing nanofluidic transport. The work provides a direct observation of electrohydrodynamic drag, the induction of fluid flow by electrical current within the metallic material. This drag arises from momentum transfer between the solid and liquid, mediated by dynamic electrostatic interactions and interfacial friction, and is dependent on a balance between intrinsic and cross-coupling friction terms. Understanding these fundamental mechanisms at metal/fluid interfaces will be crucial for designing high-performance devices for applications like energy storage and catalysis.