Content area
Full text
Introduction
In recent years, with the rapid development of nano-technology, micro/nano-scale devices such as micro-electromechanical system (MEMS) devices and lab-on-a-chip devices have been widely used. The fluid flows in these devices involve a broad range of scales from the atomistic scale to the macroscopic scale.[1]-[3] Simulation of such devices, that is, MEMS and lab-on-a-chip, cannot be carried out relying solely on the continuum models, in particular, the most commonly used Navier-Stokes (NS) equations. The NS equations become generally invalid when the continuum or thermal equilibrium condition breaks down. Molecular techniques, such as molecular dynamics (MD), can describe materials from the nanoscopic perspective and model the physics accurately at these scales. However, due to the computationally expensiveness, they cannot be directly used as a design tool for engineering nano- and micro-scale fluidic systems. In order to investigate these phenomena, researchers usually use the multi-scale modeling methods. The advantages of the multi-scale modeling lie in the ability of revealing properties of multi-scale systems by capturing phenomena that appear a wide range of time and length scales which beyond any single solver and method.[4] Especially, the hybrid atomistic-continuum method (HAC)[5] based on geometric coupling has seen rapid development since it allows on-the-fly information exchange and interaction between multiple simulation regions. The HAC method based on the geometric coupling obtains both simulation accuracy and efficiency, which calculating the majority of the domain by fast continuum solvers (computational fluid dynamics (CFD)) which are several orders of magnitude faster than molecular simulation methods (MD) can speed up the computation tremendously.
The HAC method splits the simulation domain into the continuum region, the atomistic region, and the overlap region, which exchanges the simulation data between the previous two regions to keep the physical properties consistently, that is, density, velocity, temperature, and so on. The top of the overlap region is the junction of the atomistic region and the continuum region, which can be regarded as an open MD simulation boundary. The MD simulation of the open boundary needs to consider the mass, momentum, and energy transfer, which is also an important feature of the simulation of non-equilibrium problem using the coupling method. One of the biggest challenges faced is the problem of the efficient insertion of particles on the...