Abstract
In molecular-level fluidic transport, where the discrete characteristics of a molecular system are not negligible (in contrast to a continuum description), the response of the molecular water system might still be similar to the continuum description if the time and ensemble averages satisfy the ergodic hypothesis and the scale of the average is enough to recover the classical thermodynamic properties. However, even in such cases, the continuum description breaks down on the material interfaces. In short, molecular-level liquid flows exhibit substantially different physics from classical fluid transport theories because of (i) the interface/surface force field, (ii) thermal/velocity slip, (iii) the discreteness of fluid molecules at the interface and (iv) local viscosity. Therefore, in this study, we present the result of our investigations using molecular dynamics (MD) simulations with continuum-based energy equations and check the validity and limitations of the continuum hypothesis. Our study shows that when the continuum description is subjected to the proper treatment of the interface effects via modified boundary conditions, the so-called continuum-based modified-analytical solutions, they can adequately predict nanoscale fluid transport phenomena. The findings in this work have broad effects in overcoming current limitations in modeling/predicting the fluid behaviors of molecular fluidic devices.
Highlights
In molecular-level fluidic transport, where the discrete characteristics of a molecular system are not negligible, the response of the molecular water system might still be similar to the continuum description if the time and ensemble averages satisfy the ergodic hypothesis and the scale of the average is enough to recover the classical thermodynamic properties
The discrete characteristics of molecular systems are not negligible, the response of the molecular water system might be similar to the continuum model if the ergodic hypothesis develops and the scale of average is large enough to recover the classical thermodynamic properties
In this study, we present the result of our investigations using molecular dynamics (MD) simulation method with continuum-based energy equations and check
Summary
In molecular-level fluidic transport, where the discrete characteristics of a molecular system are not negligible (in contrast to a continuum description), the response of the molecular water system might still be similar to the continuum description if the time and ensemble averages satisfy the ergodic hypothesis and the scale of the average is enough to recover the classical thermodynamic properties Even in such cases, the continuum description breaks down on the material interfaces. The substantially different physics between fluid flows at molecular-levels and in continuum models can be interpreted using four factors: (i) interface/surface force field, (ii) thermal/velocity slip, (iii) discreteness of fluid molecules at the interface and (iv) local viscosity. The total solid-liquid interfacial energies (Etotal) with the contributions of water-Cu (EWa-Cu) and water-graphene (EWa-Gra) for corresponding cases are calculated using equation (1). (c) Couette flows of water in graphenecoated Cu nanochannel 7-nm-high with a shearing velocity of 50 m/s
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