MOLECULAR DYNAMICS AND ITS HYBRID SIMULATION FOR MICROFLUIDICS

摘要

Molecular dynamics (MD) is an effective tool to treat the surface problems, which is important in thermal MEMS and microfluidics. However, its limited computational scale can not give a complete picture of flow physics in microsystems. Our work consists of two parts. In the first part, we develop a new criterion number from basic quasi-LJ potential equations between solid and liquids, governing boundary conditions. The critical criterion number of 0.757 leads to the non-slip flow, accompanying the substantial epitaxial ordering liquid structures close to solid walls. Deviation from the critical value results in uniform liquid distributions close to the solid walls and slip flow occurs. In the second part, we develop a hybrid model adjoining the MD simulation and the continuum fluid mechanics. The whole computation domain is divided into MD, P and an overlap region. The later one consists of two-way coupling that conveys mass, momentum and energy fluxes. We reproduce the three types of boundary conditions over the multiscale channel size from nano to millimeter. It is found that the slip lengths are mainly dependent on the interfacial parameters at the solid/liquid surface while channel sizes have less effect. The three boundary conditions (slip, non-slip and locking) existing in nanoscale still occur in macroscale. This is the absoluteness of the boundary conditions. However, the slip velocities relative to that of solid wall approach those with non-slip boundary conditions when the channel size is larger than thousands of molecular diameters. This is the relativity of the boundary conditions.