publications

2025

1. 

   Momentum tunnelling between nanoscale liquid flows

   Baptiste Coquinot, Anna T. Bui, Damien Toquer, Angelos Michaelides, Nikita Kavokine, Stephen J. Cox, and Lydéric Bocquet

   Nature Nanotechnology, Jan 2025

   Abs DOI Bib

   The world of nanoscales in fluidics is the frontier where the continuum of fluid mechanics meets the atomic, and even quantum, nature of matter. While water dynamics remains largely classical under extreme confinement, several experiments have recently reported coupling between water transport and the electronic degrees of freedom of the confining materials. This avenue prompts us to reconsider nanoscale hydrodynamic flows under the perspective of interacting excitations, akin to condensed matter frameworks. Here we show, using a combination of many-body theory and molecular simulations, that the flow of a liquid can induce the flow of another liquid behind a separating wall, at odds with the prediction of continuum hydrodynamics. We further show that the range of this ‘flow tunnelling’ can be tuned through the solid’s electronic excitations, with a maximum occurring when these are at resonance with the liquid’s charge density fluctuations. Flow tunnelling is expected to play a role in global transport across nanoscale fluidic networks, such as lamellar graphene oxide or MXene membranes. It further suggests exploiting the electronic properties of the confining walls for manipulating liquids via their dielectric spectra, beyond the nature and characteristics of individual molecules.

   @article{Coquinot2025,
     author = {Coquinot, Baptiste and Bui, Anna T. and Toquer, Damien and Michaelides, Angelos and Kavokine, Nikita and Cox, Stephen J. and Bocquet, Lyd{\'e}ric},
     title = {Momentum tunnelling between nanoscale liquid flows},
     journal = {Nature Nanotechnology},
     year = {2025},
     month = jan,
     day = {02},
     issn = {1748-3395},
     doi = {10.1038/s41565-024-01842-8},
     url = {https://doi.org/10.1038/s41565-024-01842-8},
     publisher = {Nature Portfolio},
   }

2. arXiv

   

   Learning classical density functionals for ionic fluids

   Anna T. Bui, and Stephen J. Cox

   Jan 2025

   Abs DOI

   Accurate and efficient theoretical techniques for describing ionic fluids are highly desirable for many applications across the physical, biological and materials sciences. With a rigorous statistical mechanical foundation, classical density functional theory (cDFT) is an appealing approach, but the competition between strong Coulombic interactions and steric repulsion limits the accuracy of current approximate functionals. Here, we extend a recently presented machine learning (ML) approach [Sammüller et al., Proc. Natl. Acad. Sci. USA, 120, e2312484120 (2023)] designed for systems with short-ranged interactions to ionic fluids. By adopting ideas from local molecular field theory, the framework we present amounts to using neural networks to learn the local relationship between the one-body direct correlation functions and inhomogeneous density profiles for a "mimic” short-ranged system, with effects of long-ranged interactions accounted for in a mean-field, yet well-controlled, manner. By comparing to results from molecular simulations, we show that our approach accurately describes the structure and thermodynamics of the prototypical model for electrolyte solutions and ionic liquids: the restricted primitive model. The framework we present acts as an important step toward extending ML approaches for cDFT to systems with accurate interatomic potentials.

2024

1. AIP

   

   A classical density functional theory for solvation across length scales

   Anna T. Bui, and Stephen J. Cox

   J. Chem. Phys., Sep 2024

   Abs DOI Bib

   A central aim of multiscale modeling is to use results from the Schrödinger equation to predict phenomenology on length scales that far exceed those of typical molecular correlations. In this work, we present a new approach rooted in classical density functional theory (cDFT) that allows us to accurately describe the solvation of apolar solutes across length scales. Our approach builds on the Lum–Chandler–Weeks (LCW) theory of hydrophobicity [K. Lum et al., J. Phys. Chem. B 103, 4570 (1999)] by constructing a free energy functional that uses a slowly varying component of the density field as a reference. From a practical viewpoint, the theory we present is numerically simpler and generalizes to solutes with soft-core repulsion more easily than LCW theory. Furthermore, by assessing the local compressibility and its critical scaling behavior, we demonstrate that our LCW-style cDFT approach contains the physics of critical drying, which has been emphasized as an essential aspect of hydrophobicity by recent theories. As our approach is parameterized on the two-body direct correlation function of the uniform fluid and the liquid–vapor surface tension, it straightforwardly captures the temperature dependence of solvation. Moreover, we use our theory to describe solvation at a first-principles level on length scales that vastly exceed what is accessible to molecular simulations.

   @article{Bui2024solvation,
     author = {Bui, Anna T. and Cox, Stephen J.},
     title = {A classical density functional theory for solvation across length scales},
     journal = {J. Chem. Phys.},
     year = {2024},
     month = sep,
     day = {09},
     volume = {161},
     number = {10},
     pages = {104103},
     issn = {0021-9606},
     doi = {10.1063/5.0223750},
     url = {https://doi.org/10.1063/5.0223750},
   }

2. AIP

   

   Revisiting the Green–Kubo relation for friction in nanofluidics

   Anna T. Bui, and Stephen J. Cox

   J. Chem. Phys., Nov 2024

   Abs DOI Bib

   A central aim of statistical mechanics is to establish connections between a system’s microscopic fluctuations and its macroscopic response to a perturbation. For non-equilibrium transport properties, this amounts to establishing Green–Kubo (GK) relationships. In hydrodynamics, relating such GK expressions for liquid–solid friction to macroscopic slip boundary conditions has remained a long-standing problem due to two challenges: (i) The GK running integral of the force autocorrelation function decays to zero rather than reaching a well-defined plateau value, and (ii) debates persist on whether such a transport coefficient measures an intrinsic interfacial friction or an effective friction in the system. Inspired by ideas from the coarse-graining community, we derive a GK relation for liquid–solid friction where the force autocorrelation is sampled with a constraint of momentum conservation in the liquid. Our expression does not suffer from the “plateau problem” and unambiguously measures an effective friction coefficient, in an analogous manner to Stokes’ law. We further establish a link between the derived friction coefficient and the hydrodynamic slip length, enabling a straightforward assessment of continuum hydrodynamics across length scales. We find that continuum hydrodynamics describes the simulation results quantitatively for confinement length scales all the way down to 1 nm. Our approach amounts to a straightforward modification to the present standard method of quantifying interfacial friction from molecular simulations, making possible a sensible comparison between surfaces of vastly different slippage.

   @article{Bui2024gk,
     author = {Bui, Anna T. and Cox, Stephen J.},
     title = {Revisiting the Green--Kubo relation for friction in nanofluidics},
     journal = {J. Chem. Phys.},
     year = {2024},
     month = nov,
     day = {27},
     volume = {161},
     number = {20},
     pages = {201102},
     issn = {0021-9606},
     doi = {10.1063/5.0238363},
     url = {https://doi.org/10.1063/5.0238363},
   }

2023

1. ACS

   

   Classical Quantum Friction at Water–Carbon Interfaces

   Anna T. Bui, Fabian L. Thiemann, Angelos Michaelides, and Stephen J. Cox

   Nano Lett., Jan 2023

   DOI Bib

   @article{Bui2023qf,
     author = {Bui, Anna T. and Thiemann, Fabian L. and Michaelides, Angelos and Cox, Stephen J.},
     title = {Classical Quantum Friction at Water--Carbon Interfaces},
     journal = {Nano Lett.},
     year = {2023},
     month = jan,
     day = {25},
     publisher = {American Chemical Society},
     volume = {23},
     number = {2},
     pages = {580-587},
     issn = {1530-6984},
     doi = {10.1021/acs.nanolett.2c04187},
     url = {https://doi.org/10.1021/acs.nanolett.2c04187},
   }

2022

1. ACS

   

   Trade-Off between Redox Potential and the Strength of Electrochemical CO2 Capture in Quinones

   Anna T. Bui, Niamh A. Hartley, Alex J. W. Thom, and Alexander C. Forse

   J. Phys. Chem. C, Aug 2022

   Abs DOI Bib

   Electrochemical carbon dioxide capture recently emerged as a promising alternative approach to conventional energy-intensive carbon-capture methods. A common electrochemical capture approach is to employ redox-active molecules such as quinones. Upon electrochemical reduction, quinones become activated for the capture of CO2 through a chemical reaction. A key disadvantage of this method is the possibility of side-reactions with oxygen, which is present in almost all gas mixtures of interest for carbon capture. This issue can potentially be mitigated by fine-tuning redox potentials through the introduction of electron-withdrawing groups on the quinone ring. In this article, we investigate the thermodynamics of the electron transfer and chemical steps of CO2 capture in different quinone derivatives with a range of substituents. By combining density functional theory calculations and cyclic voltammetry experiments, we support a previously described trade-off between the redox potential and the strength of CO2 capture. We show that redox potentials can readily be tuned to more positive values to impart stability to oxygen, but significant decreases in CO2 binding free energies are observed as a consequence. Our calculations support this effect for a large series of anthraquinones and benzoquinones. Different trade-off relationships were observed for the two classes of molecules. These trade-offs must be taken into consideration in the design of improved redox-active molecules for electrochemical CO2 capture.

   @article{Bui2022,
     author = {Bui, Anna T. and Hartley, Niamh A. and Thom, Alex J. W. and Forse, Alexander C.},
     title = {Trade-Off between Redox Potential and the Strength of Electrochemical CO2 Capture in Quinones},
     journal = {J. Phys. Chem. C},
     year = {2022},
     month = aug,
     day = {25},
     publisher = {American Chemical Society},
     volume = {126},
     number = {33},
     pages = {14163-14172},
     issn = {1932-7447},
     doi = {10.1021/acs.jpcc.2c03752},
     url = {https://doi.org/10.1021/acs.jpcc.2c03752},
   }