Boundary condition, an old but not well solved problem, linked from nanometer to macroscale

Abstract

A key issue of the flow and heat transfer in channels is to determine the boundary condition, which is affected by various parameters, such as the solid-fluid potential interactions, solid wall roughness, surface wettability etc. We summarize the progress that has been made in recent years. In the first part of this paper, we reviewed the three-atom-model, leading to an important criterion number governing the boundary conditions. The molecular dynamics (MD) simulations verified the effectiveness of the criterion number. The second part of this paper reports the multiscale simulation of the flow field in channels, adjoining the molecular dynamics (MD) simulation and the continuum fluid mechanics. Three types of boundary conditions (slip, non-slip and locking) were identified over the multiscale channel sizes. The slip lengths are found to be mainly dependent on the interfacial parameters with the fixed apparent shear rate. The channel size has little effects on the slip lengths if the size is above a critical value within a couple of tens of molecular diameters. The slip, non-slip and locking interfacial parameters yield positive, zero and negative slip lengths, respectively. The three types of boundary conditions existing in “microscale” still occur in “macroscale”. However, the weak dependence of the slip lengths on the channel size yields decreased slip velocities with increases in channel sizes for all three types of interfacial parameters.