Two-part simulation method includes molecular-scale details in hydrodynamic boundary conditions

With wide-ranging uses in biology, the field of hydrodynamics relies on a continuum-level description of fluid motion without implementing molecular details such as wall–fluid interactions. One major assumption, the no-slip assumption, states that molecules of a fluid adhere to, rather than slip along, the wall of a container, an assumption that breaks down on the nanoscale.

Shi et al. demonstrated the benefit of including molecular-scale details in hydrodynamic boundary conditions. Combining equilibrium molecular dynamics with non-equilibrium flow simulations, they separated the system into a bulk fluid region that satisfies assumptions of the continuum theory and a near-wall region that addresses nanoscale wall dynamics involved in layers of the fluid that slip.

“The innovation of this work is in redefining hydrodynamic boundaries,” said author Haoyuan Shi said. “Instead of assuming no-slip or slip at the physical wall, we identify a molecularly defined hydrodynamic wall within the fluid, determined directly from equilibrium simulations, that marks the onset of slip. This reveals the counterintuitive fact that all fluids exhibit some slip.”

Previous attempts to account for nanoscale discrepancies in hydrodynamics made ad hoc continuum adjustments and arbitrary interface definitions that failed to provide an unambiguous framework for flows in confined systems.

The group extended the hydrodynamic wall by modifying the slip length to an extrapolated position, allowing them to place slip and nearby stagnant flows under a unified metric linked to surface hydrophobicity. This yielded a hydrodynamic wall that partitions the system at the outermost nanometer, and the bulk fluid exhibited a nearly uniform density profile.

They will next focus on charged interfaces by simulating electrolyte effects on hydrodynamic boundary conditions in confined water systems and their implications on hydrodynamic lubrication forces.

Source: “Incorporating the molecular-scale into a hydrodynamic description of confined aqueous systems,” by Haoyuan Shi, Christopher Jay Mundy, Gregory Schenter, and Jaehun Chun, Journal of Chemical Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0279626 .