ABSTRACT
Water confined by hydrophilic materials shows unique transport properties compared to bulk water, thereby offering new opportunities for the development of nanofluidic devices. Recent experimental and numerical studies showed that nanoconfined water undergoes liquid- to solid-phase-like transitions depending on the degree of confinement. In the case of water confined by graphene layers, the van der Waals forces are known to deform the graphene layers, whose bending leads to further nonuniform confinement effects. Despite the extensive studies of nanoconfined water under equilibrium conditions, the interplay between the confinement and rheological water properties, such as viscosity, slip length, and normal stress differences under shear flow conditions, is poorly understood. The current investigation uses a validated all-atom nonequilibrium molecular dynamics model to simultaneously analyze the continuum transport and atomistic structural properties of water in a slit between two moving graphene walls under Couette flow conditions. A range of different slit widths and velocity strain rates are considered. It is shown that under subnanometer confinement, water loses the rotational symmetry of a Newtonian fluid. Under such conditions, water transforms into ice, where the atomistic structure is completely insensitive to the applied shear force and behaves like a frozen slab sliding between the graphene walls. This leads to the shear viscosity increase, although it is not as dramatic as the normal force increase that contributes to the increased friction force reported in previous experimental studies. On the other end of the spectrum, for flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals shear thinning behavior. In this case, water exhibits a constant slip length on the wall, which is typical of liquids in the vicinity of hydrophobic surfaces.
ABSTRACT
A triple-scale model of a molecular liquid, where atomistic, coarse-grained, and hydrodynamic descriptions of the same substance are consistently combined, is developed. Following the two-phase analogy method, the continuum and discrete particle representations of the same substance are coupled together in the framework of conservation laws for mass and momentum that are treated as effective phases of a nominally two-phase flow. The effective phase distribution, which governs the model resolution locally, is a user-defined function. In comparison with the previous models of this kind in the literature which used the classical Molecular Dynamics (MD) for the particulate phase, the current approach uses the Adaptive Resolution Scheme (AdResS) and stochastic integration to smoothen the particle transition from non-bonded atom dynamics to hydrodynamics. Accuracy and robustness of the new AdResS-Fluctuating Hydrodynamics (FH) model for water at equilibrium conditions is compared with the previous implementation of the two-phase analogy model based on the MD-FH method. To demonstrate that the AdResS-FH method can accurately support hydrodynamic fluctuations of mass and momentum, a test problem of high-frequency acoustic wave propagation through a small hybrid computational domain region is considered.
ABSTRACT
A new hybrid molecular dynamics-hydrodynamics method based on the analogy with two-phase flows is implemented that takes into account the feedback of molecular dynamics on hydrodynamics consistently. The consistency is achieved by deriving a discrete system of fluctuating hydrodynamic equations whose solution converges to the locally averaged molecular dynamics field exactly in terms of the locally averaged fields. The new equations can be viewed as a generalisation of the classical continuum Landau-Lifshitz fluctuating hydrodynamics model in statistical mechanics to include a smooth transition from large-scale continuum hydrodynamics that obeys a Gaussian statistics to all-atom molecular dynamics. Similar to the classical Landau-Lifshitz fluctuating hydrodynamics model, the suggested generalised Landau-Lifshitz fluctuating hydrodynamics equations are too complex for analytical solution; hence, a computational scheme for solving these equations is suggested. The scheme is implemented in a popular open-source molecular dynamics code GROMACS (GROningen MAchine for Chemical Simulations), and numerical examples are provided for liquid argon simulations under equilibrium conditions and under macroscopic flow effects.
