Study On Flow Characteristics Of Microfluidics For MEMS Design

Currently, there is a little knowledge for the flow rules in microfluidic devices basedon MEMS technology. Aiming at this research status for microfluidics, it is necessary tostudy the micro fluidic mechanisms in the domains of microscale, micro structures andmicro functional devices, and find a suitable modeling methodology for the study ofmicrofluidics, then investigate on the optimization of the structures and parameters forthe microfluidic devices. These studies are needed to be proceeded urgently. Theachievements will provide important theoretical basement for the design of newstructures, new devices.Three main modeling methodologies such as continuum medium models, molecularmodels and mesoscale methods is systematically reviewed. The applicable domains forthese methods are investigated respectively, which is very significant to select propermodeling method for a special microfluidic device.The flow resistance characteristics is systematically analyzed and researched formicrofluid in the slip-flow region. Based on the first order slip-flow boundary condition,the analytical solutions of some typical problems such as pressure-difference flows andshear flows in the slip-flow region are obtained after the corresponding detailed analysis.According to the amendment method named equivalent viscosity for pressuredifferential flow in slip-flow region, the modified coefficients of equivalent viscosityand relevant parameters are deduced for several typical crosssection flows, whichsupply a simple method for engineering computation.Based on the first order slip-flow boundary condition, a modified ReynoldsEquation with double momentum compatibility coefficients is put forward for gas filmdamping problems in microstructures based on MEMS. The error analysis of the newmodel shows that the precision requirements of MEMS engineering design can befulfilled by utilizing this model. Based on the modified Reynolds equation brought outin this paper, the damping characteristics of squeezed film are analyzed for thenon-condensable and condensable gas. The analytical expressions of the parameterssuch as pressure distribution, damping force and damping coefficient are deduced then.The analytical solutions of the gas damping problems are well developed for rectangularplates in the conditions of parallel motion and turning motion.Under the slip-flow boundary condition, the slide-film damping characteristics of Couette flow, Stokes flow and Poiseuille flow are researched, and the analyticalsolutions of the three models are presented. Meanwhile, their applications fields arepointed out.Based on the rarefied gas dynamics and the slip-flow modified Reynolds equation,the gas film damping characteristic of the plate capacitive accelerometer is studied, andthe simplified analytical solutions of the damping force and the damping coefficient areobtained. Several methods decreasing the gas film damping are given in this dissertation.The fact is point out that the momentum compatibility coefficient has a significant effecton the gas film damping, especially the squeeze film damping, which is needed a furthertheoretical and experimental study.The damping characteristic of the disk with a role in the center moving in the microfluid is studied. The distribution functions of the pressure and the flow velocity areobtained. The relationship among the compound damping brought by the squeezed-filmand the slide film, structure parameters and the slip length is analyzed, and thecorresponding analytical solution suitable for the engineering practice is obtained. Manystudies on gas film damping for plate structures with multi holes are summarized in thisdissertation. As the gas is non-condensable, the gas film damping characteristic of thethick plate with high via hole-density is studied, and with the above analysis, theanalytical solution of the damping coefficient is presented. The FEM methods forsolving the gas film damping of the multi-hole plate is pieced out, and is extended tocalculate the damping of the plate with rectangular holes.By using the finite difference method, the dynamic damping characteristic of themicro accelerometer is analyzed and studied. The simulation results are in goodagreement with those by experiments. The dynamic coupling model is established forthe electrostatic actuated micro elastic beam. The analytical method is used to analyzethe deformation of the microbeam which is much smaller than the thickness of the gasfilm. Under the effect of the gas film damping, the expressions of the quality factor andthe resonance frequency of the microbeam are obtained. When the maximum deflectionof the microbeam is considerable to the thickness of the gas film, the discretization ofthe dynamic couple equations are proceeded and the differential formats of theequations, the initial conditions and boundary conditions, are obtained using the finitedifference method. Then the dynamic response of the system with constant dampingcoefficient and film damping is simulated.The dynamic characteristic of film damping is studied for the transverse oscillatedresonator. The analytical results are consistent with those by available experiments. For the transverse oscillated microstructures the drag force is one of the most importantfactors to affect the dynamic characteristics of the system, especially to the systemquality factor.A computation model with three adjustable parameters is proposed for thelubrication problems of the ultra-thin gas film. Based on the predicted value of thePoiseuille flow by the linear Boltzmann equation, a set of optimization values for thethree parameters can be conducted by using the least square method. Compared withother methods, in a large scale of 10^-4＜D＜10², the pressure distribution and the loadcapacity calculated by this model keep a better agreement with those predicted valuesby linear Boltzmann equation, which is very helpful for the design of microfluidicdevices.