| Microfluidics is one of the most active research fields in MEMS, which have the promise of a wide range of applications in Biomanufactuing, Biomedical engineering, environmental monitoring and military affairs fields. Because microfluidics is a complicated system which involves the microscopic scale, the mesoscopic scale and the macroscopic, there are many uncertainties and debate. Many obstacles with several nanometers around are added into microgeometries in order to increase the fraction extensions and separation efficiency. It is known that the effective diameter of the microchip, the length of the microchip and the length of the polymer are several micons, centimeters and meters, respectively. Because the diversity and coupling in microchips cannot be described with single method, it is necessary to develop the multiscale model and methods.Multiscale phenomena in microfluidics, such as DNA extension, electroosmotic flow DNA folding, and heat conduction are analyzed in this paper, using multiscale finite element method, Brownian dynamics, and Monte Carlo method. The detailed work includes the following parts.1. Study on the DNA extension in microgeometries.DNA stretching is necessary for gene mapping in the implement of Human Genome Plan. DNA extension is small with available experimental technique and microgeometries. In this paper, the combined the finite element method and Brownian dynamics are used to analyze the DNA stretching behavior in single microchips as well as in hybrid microchannels. The effects of initial state of DNA, location of entrance, electric field, transition length of microcontraction, location and radius of cylindrical obstruction on the stretching behavior are analyzed. Tne numerical results show us that the mean fractional extension in the combined microchannel is obvious higher than that in the single hyperbolic contraction due to the pre-stretching effect of the cylindrical obstruction. The location and the radius of the cylindrical obstruction may have effect on stretching behavior. The mean fractional extension of DNA has been got to ninety percent where the location of the centre of the cylindrical obstruction in the referencef frame is ( 3.033×10 ?3m, 1×10 ?4m) with15 micron radius.A multi-domain interpolation algorithm is developed, which can resolve the question of scale coupling. In this algorithm, the refined meshes and the loose meshes are used in the sharp gradient and the slow gradient of variables, respectively.A hybrid microchannel including cylindrical obstruction and hyperbolic contraction is designed to achieve highly uniform and complete stretching of DNA at low De. The related microchip design has been submitted for patent for invention.2. Study on the elelctroosmotic flowElectroosmotic pump is often used in microfluidic chip. Compared with pressure-driven flow, electroosmotic flow does not contain any moving mechanical parts, such as pumps and valves, and can be easily incorporated into microfluidic or nanofluidic chips.The finite element method is used in this paper to study the electroosmotic flow in the rectangular microchannels and trapezoidal microchannels. The effects of zeta potential, section shape and length ration of trapezoidal microchip on the electroosmosis are analyzed. Based on the study on the uniform electroosmotic flow, a multiscale finite element method is presented to solve the electroosmosis in a non-uniform dielectric. The characteristics of electroosmotic in five oscillatory coefficients are analyzed. The numerical results show us that the velocity in non-uniform electroosmosis decrease from the center to the walls of microchips.A multiscale finite element method is presented. In this method, the basis function is elliptic equation which can account for the non-uniform in the cell.It can be found that the velocity profile of electroosmosis in a non-uniform dielectric is not flat.3. Study on the DNA folding.Genomic DNA in eukaryotes is compactly folded into chromatin through several hierarchical packing. The behavior of DNA folding can be used to explain the interaction between DNA and histone as well as the mechanisom of accommodation. The nucleosome which consists of 146 bp of DNA and a histone protein is studied using Brownian dynamics. The analysis of alteration in the shape of nucleosome at varying temperature is presented. Monte Carlo method is used in this paper to study the dynamics of 30 nm fiber. It can be found that the salt concentration has great effect on the diameter, linear mass density and persistence length of fiber. Moreover, the transition from the nucleosome to the fiber is analyzed.The transition of DNA folding is fistly presented in this paper. The compactions of dinucleosomes and trinucleosomes are studied, which can be used to explain the wraping behavior of the multinucleosomes.4. Study on the heat conduction in microchips.It is very important to control the heat conduction for increase the performance and life time of the microchips. Dissipative particle dynamics with energy conservation in mesoscopic scale is firstly used to study the heat flow in a microchannel which consists of two parallel infinite plates. The behavior of heat conduction can be obtained under different density and different temperature gradient.The research results can provide the technical support for the design of microchips and Biomanufactuing. Moreover, the multiscale numerical simulation methods can improve the development of theory of mechanical design.
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