Numerical Simulation Of Pressure-driven Flow In Microchannels

Microfluidic chips, which is based on micromachining technology and microelectronic technology, mainly researches life science and analytical chemistry, studies the sampling, reaction, detection and separation process of the reagent in the microchannels network, the major working principle of this process is to control the reagent flow in the microchannels. This paper focused on the major issues of the scale effect, skin effect and the slip existing in the microfluidic clips, employed the combining methods of theoretical analyzes and numerical modeling, conducted the transport law of the pressure-driven flow in the microchannels as system research.The main research task of this paper is as follows:Firstly, the paper studied the history and study situation of the microfluidic chips at home and abroad, introduced the classification and control mechanism of the electrokinetic phenomena and microfluid existed in microfluidics, and the basic issues of microfluid mechanics, moreover, it deduced the research details based on the features of the multiphysics coupling.Secondly, under the microscale, this paper focused on the electrokinetic effect in the flow channel and based on the theory of the macroscopy hydromechanics, it analyzed the mathematical model in the microfluidics, deduced the P-B Equation of the potential in the electrical double layer, induced electric field potential and the amended N-S potential, these work provided the theoretical basis for the whole text.Thirdly, it had built on a 2D physical model of smooth parallel plate microchannels, induced the linear analytical solution and nonlinear complete numerical solution of distribution of the potential in single-dimensional electrical double layer, the induced electric field and the distribution of the flow field speed, and applied the result of the simulated pressure-driven flow based on the COMSOL Software data of FEM in slick microchannels to be compared with linear analytical solution to verify the validity and accuracy of the COMSOL software. By means of COMSOL, this paper made a separate simulated calculation of the distribution of the potential in electrical double layer, the average flow speed, the ratio of bulk viscosity to apparent viscosity, the friction coefficient and the streaming potential of the fluid in the microchannels, which was made of hydrophilic material or hydrophobic material. Researches showed that the distribution of the potential in electrical double layer went from plug to parabola as the diminution of the characteristic length and the zeta potential; considering the situation of electrokinetic effect, all the cases could be presented below:the average flow speed of microfluid was lower than that of macrofluid, the flow speed in the centre of the flow channel became slower, the electro-viscous effect enhanced, the surface viscosity of microfluid increased, and the friction coefficient was larger than that of macrofluid, additionally, along with the characteristic length and the impressed pressure, the streaming potential increased, and decreased when the concentration of electrolyte solution went up; otherwise, microchannels, made of hydrophobic material, had slip speed on the border, compared with hydrophilic material microchannels, the average flow speed and streaming potential of fluid increased, however, hydrophobic material had the indubitable effect of coefficient, the apparent viscosity could be alleviated.The research above had reference value of reducing the scale of microfluidic chips further more.Fourthly, this paper also established a 3D physical model of rectangle slick microchannels, induced the linear analytical solution and nonlinear complete numerical solution of distribution of the potential in two dimension electrical double layer, the induced electric field and the distribution of the flow field speed, and applied MATLAB software to compile P-B equation, induced electric field equation and the discretization grid code of N-S equation to solve the complete numerical solution of the equations above. The research showed that, because of the singularity of the boundary of the rectangle microchannels, the potential distribution on the boundary changed little, and the backflow were very obvious; when the hydraulic diameter was fixed, accompanied with the depth-to-width ratio magnified, the average flow speed and streaming potential would go up while the ratio of bulk viscosity to apparent viscosity and friction coefficient would be dropped, and they could reach the limit when the depth-to-width ratio was one; in electrokinetic effect, the hydraulic diameter, the concentration of electrolyte solution, and the impressed pressure were important factors, together with which the everage flow speed, the ratio of bulk viscosity to apparent viscosity, the streaming potential, friction coefficient and other fluid charaeteristic were similar to the relationship of the factors in electrokinetic effect and the fluid characteristic date of the parallel plate microchannels; the fluid friction coefficient in rectangle microchannels was greater than that of the parallel-plate microchannels. All of these researches provided the reference for achieving the goal of controlling the flow speed of fluid in microfluidic chips accurately and quantificationally.Fifthly, the paper also researched the split-flow transmission of the Y type microchannel and the sample transmission model of the complicated U type microchannnel. It has showed that, the eletrokinetic effect blocked the split-flow transmission of the Y type microchannel, lead to the deduce of the ratio of bulk viscosity to apparent viscosity and the increase of the friction coefficient; in the sample transmission of the complicated U type microchannnel, backflow phenomenon deduced the convection and proliferation of the solution greatly, which could make the concentration asymmetrical and various during the process of transmission more serious, so backflow phenomenon should be avoided as much as possible.