Application Of Random Walk Theory In The Simulation Study Of Micro Separation Systems

Manz et al. proposed the concept of micro total analysis system (μTAS) in1990s, since then the idea of using artificial micro regular structure as molecular sieve has been developed. The μTAS has the advantages of miniaturization, integration, quickly analysis, low sample consumption, real time and continuous detection. Further, it can be used in inaccessible microenvironment. These bring a significant innovation for biological and chemical analyses. With the development of micro/nano technology, molecular sieving in micro/nano scale has become an alternative separation technique compared to traditional gel electrophoresis and gel exclusion chroma to graphy. Molecular motion, especially in micro/nano confined space, is a key factor that affects the separation performance. Therefore, an in-depth understanding of molecular motion in micro/nano confined space is very important for the development and application of these micro separation devices.As an important means for theoretical research, computer simulation has significant superiority in studying the molecular motion The random walk theory extracts the critical information that affects the molecular diffusion and makes reasonable simplification of the actual processes, so that the simulation efficiency is improved effectively. Thus, the random walk theory can well describe the molecular motion in micro/nano scale. Based on the random walk simulation program developed by our group, we studied the particle diffusion processes in micro/nano confined space with appropriate models. The factors affecting the diffusion behavior were investigated, and the principle of particle diffusion in the micro/nano confined space was concluded. Moreover, with establishing different models of micro separation devices, we explored the performance of these micro devices. The simulation method we proposed would have certain reference significance for developing micro separation devices and optimizing the operating parameters.1. With simulating the separation processes in several simple barrier systems, we systematically investigated the influence of barrier, compartment size and lateral drift velocity on the particle diffusion behaviors. The simulation results were consistent with theoretical prediction or experimental results, which demonstrated the validity of random walk simulation program in studying the particle movement in micro/nano confined space. Further studies found that the anomalous diffusion mode is not simply due to the restriction of barriers but rather to the formation of compartment by barriers. Besides, the presence of barriers or lateral drifting velocity alone does not create bias on the location of the particle, whereas the combination of both does.2. A micro separation system based on asymmetric obstacles was constructed, and the particle diffusion processes in this system were simulated. The simulation results indicated that molecules with different diffusion coefficients would be deflected away from the field direction by the asymmetric obstacles in different degrees so that a mixture of molecules was sorted in the two-dimensional space in the direction transverse to the field. The separation performance would be improved by selecting appropriate drift velocity. Further, by analyzing the band broadening effect, we predicted the performance of the separation sieve with different length quantitatively.3. We also constructed a model of miniature single-channel separation system, which was a quasi one-dimensional separation system made of inversely asymmetric Brownian ratchets, and simulated the molecular diffusion in this system based on random walk theory. The separation mechanism and the influence of different conditions on the diffusion behaviors were discussed. It was concluded that the interaction between the particle and the Brownian ratchets could be adjusted by the switching time of the external force properly, which enabled different particles separated into two different directions in the single-channel system. The separation of multi-components in this single-channel separation system could be achieved by selecting appropriate conditions. The ratio of the diffusion speed to drift velocity for particles that can be applied to this separation system was about ranging from0.02to7. Furthermore, it was demonstrated that with adjustment of the structural parameters of the separation device, time cost of the separation process could be saved effectively while achieving optimal separation.