Simulation Of Particle Diffusion In The Micro/Nano Confined Space Based On Random Walk

Research of molecular diffusion with microstructures in submicron dimension is valuable from both scientific and technological viewpoints in recent years. With the development of micro/nano devices, researchers focus more on processing technology. However, the internal unconventional physics mechanism of micro/nano devices is not deeply investigated. This leads to the theoretical research lags behind the processing technology. It has become the bottleneck that restricts design, manufacturing, optimization, and application of micro/nano devices. Therefore, strengthen basic research for micro/nano scale unconventional physics problems has become a consensus. Molecules or ions diffusion is one of the key problems of unconventional physics problems in micro/nano scale. It is also the foundation for studying other related technologies. The particle diffusion behavior in micro/nano scale is obviously different from that in conventional scale. The assumption of continuous media is no longer work when the characteristic size of micro/nano channel is comparable to the mean free path of the diffusion particle. In addition, in the micro/nano scales, the importance of various interactions which affect the particle diffusion will change. To overcome this limitation, in this paper, we propose a fresh approach to study the particle diffusion behavior in the micro/nano confined space based on random walk theory. We developed a software named random walk system in chemistry (RWS-C) to simulate the particle diffusion behavior with the following micro/nano confined space.1. The diffusion behavior of the dye-labeled molecules in self-spreading lipid bilayer on a substrate with nanoscale barriers was simulated. Suppressed diffusivity is observed at this system with the barrier gap size near to tens of nanometers. The diffusion mode of the particle change from normal to subdiffusion. Long-range diffusion coefficients of the dye-labeled molecules were significantly reduced by the presence of the barriers. In additional, the simulation results were compared with experiments of Murakoshi’s group. A new interaction model was proposed to describe the interaction between the dye-labeled molecules and the barriers. The simulation results show that this model can be used in this system very well.2. Three kinds of Brownian ratchet obstacles with narrow gap and the channel structure was modeled and simulated. The influence of the self-spreading velocity, the shape of the ratchet obstacle and the step number of the simulation on the separation angle distribution were discussed respectively. Comparison between the results of simulation and experiments shows that this random walk method can provide theoretical reference for micro/nano ultra small separation device design and optimization.3. The particle diffusion behaviors in the micro/nano membrane channel which have Voronoi fractal structure were simulated. The influence of the membrane porosity, the average diameter of the membrane pores, irregularity and the interaction between the particle and the membrane on the particle diffusion behavior was discussed respectively. The results show that the long-range diffusion coefficients of the particle were significantly reduced by the decreasing of membrane porosity and average membrane pore diameter. However, the particle diffusion is independent of the irregularity of the membrane structure.4. We proposed a new diffusion model describing the diffusion behaviors of hydrated ions in the process of nanofiltration based on the random walk theory when the nanofiltration membrane is uncharged or low charged. In this model, the hydration of ions and their deformation capacity were considered. The structure of the membrane was idealised into a lozenge shape and the diameter of membrane pore was defined as gapsize. Six familiar ions Li+, Na+, Mg2+, Al3+, K+and Ca2+were investigated. The diffusivities of hydrated ions were calculated and discussed. The results show that the hydration of ions cannot be ignored in nanofiltration process when the membrane pore size is comparable to the dimensions of the hydrated ions.