Vidhan Kashyap

Boiling is a striking example of a multiscale process, where the dynamics of bubbles is governed by the interplay between the molecular interactions responsible for nucleation, and the macroscale hydrodynamic and thermal boundary layers. A complete description of this phenomenon requires coupling molecular- and continuum-scale fluid mechanics into a single modelling framework. This article presents a hybrid atomistic–continuum computational model for coupled simulations of nucleate boiling. A domain decomposition coupling method is utilised, where the near-wall region is solved by a Molecular Dynamics description, which handles nucleation and the moving contact lines, while the bulk flow region is solved by a continuum-scale description based on the Navier–Stokes equations. The latter employs a Volume Of Fluid method to track the evolution of the liquid–vapour interface and the interphase mass transfer is computed via the Hertz–Knudsen–Schrage relationship. Boiling of a Lennard-Jones fluid over a heated wall is simulated and the hybrid solution is validated against a fully molecular solution. The results obtained with the coupled framework in terms of time-dependent bubble volume, phase-change rates, bubble dynamics and evolution of the temperature field agree quantitatively with those achieved by a MD-only simulation. The coupled framework reproduces the bubble growth rate over time from nucleation until a bubble diameter of about 70 nm, demonstrating the accuracy and robustness of the coupling architecture. This also demonstrates that the fluid dynamics description based on the Navier–Stokes equations is capable of correctly capturing the main heat and mass transfer mechanisms responsible for bubble growth at the nanoscale. The proposed modelling framework paves the way towards multiscale simulations of boiling, where the necessary molecular-level physics is retained in a computational fluid dynamics solver.

This work investigates electrokinetic flow and the zeta potential in slit channels with nanoscale surface features, using molecular dynamics simulations and continuum-level analyses. It introduces a simple analytical model to account for the effects of nanoscale roughness. The model uses matched asymptotic solutions for charge density and fluid flow, ensuring conditions like electroneutrality and the Onsager reciprocal relation (linking electroosmotic flow and streaming current) are met. The analytical predictions align with molecular dynamics results, which include effects like ion solvation and surface hydration layers. The study highlights that measuring both electroosmotic and pressure-driven flow rates or streaming currents is key to accurately determining the zeta potential in channels with nanoscale features.