Non-equilibrium response

2024

1. 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

   Abs DOI Bib

   Friction at water–carbon interfaces remains a major puzzle with theories and simulations unable to explain experimental trends in nanoscale waterflow. A recent theoretical framework—quantum friction (QF)—proposes to resolve these experimental observations by considering nonadiabatic coupling between dielectric fluctuations in water and graphitic surfaces. Here, using a classical model that enables fine-tuning of the solid’s dielectric spectrum, we provide evidence from simulations in general support of QF. In particular, as features in the solid’s dielectric spectrum begin to overlap with water’s librational and Debye modes, we find an increase in friction in line with that proposed by QF. At the microscopic level, we find that this contribution to friction manifests more distinctly in the dynamics of the solid’s charge density than that of water. Our findings suggest that experimental signatures of QF may be more pronounced in the solid’s response rather than liquid water’s.

   @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},
   }