Theories And Experiment Studies On Droplet Formation And Mixing In Microchannel

With the rapid development of micro-electro-mechanical systems,the miniaturization and integration of fluid control device have become an important research direction.Microfluidic chip is mainly used in life sciences,analytical chemistry and other fields.In these research areas,reaction is a very important operational unit,and the thorough mixing is the important condition for realizing the function of microfluidic device.Rapid and uniform mixing has important significance for chemical synthesis,biochemical analysis,drug delivery,nucleic acids sequencing or synthesis and other fields.Microfluidics have the main flow characteristics with laminar flow or low Reynolds number,and the unmixed layered flowing caused the difficulty of flow mixing.The introduction of the micromixer and micromixing technology for fast and efficient hybrid brings breakthrough in microfluidic.In particular,the mixing technology based on mirco droplet is employed the dispersed droplet unit as airtight micro-reactor,which is rapid,efficient,easy to control and integration.It has become one of the important research for micro fluid mixing technology.To solve the difficult mixing of microfludics,this paper presents the mechanism of rapid and uniform mixing based on microdroplet in non-uniform sinusoidal microchannel by numerical simulations and experiments methods.Due to the structure of non-uniform sinusoidal microchannel,the interaction of vortices in the different dimensions is achieved.Hence,it can strengthen the disturbance of the fluid in the microchannel to achieve fully mixing.The PDMS(Polydimethylsiloxane)micromixer which is used to format the microdroplet and mix experiments is developed and packaged with a self built microfluidic chip processing system.The experimental results show good agreements with numerical simulation results,which further verify the feasibility of development on efficient micromixer.The main research work of this dissertation is summarized as follows:(1)The two-dimensional model of T shape microchannel is established.The two phase flow field(Naiver-Stokes equations of incompressible fluid)and temperature field(energy conservation equation)are solved by coupling solution.The droplet formation mechanism for T shape microchannel is revealed,and the influence of continuous phase velocity,and viscosity on microdroplet formation are studied.(2)The concentration transport equation of the micro mixing is established,then the mechanism of diffusion mixing and convective mixing is revealed,and the effective ways to enhance the micromixing are presented.Chaos convection mixing is the best method for enhancing the mixing effect for the dissolved two-phase fluid.Meanwhile,an evaluating index is established for a micromixer performance which laid the foundation on the subsequent mixing performance of quantitative evaluation.(3)The convection diffusion equations and two-phase flow level set equation for the two-dimensional microchannel model are solved by coupling on the basis of droplet formation mathematical model.Then,concentration distribution inside microdroplet in non-uniform sinusoidal microchannel are obtained.With the aid of the twirling effect,nonuniform cross-section effect,and asymmetrica vortices,the mixing effect in the microdroplet is significantly enhanced.The mixing index achieves 0.9 or so within 20-30 seconds,so the high efficiency and fast mixing is realized.(4)The influences of structure parameters of the curved microchannel with nonuniform cross-section on the mixing index on the microdroplet are analyzed.These structure parameters include the continuous phase velocity,the length of the droplet,the ratio and product of wavelength to amplitude etc.Then the effecting law of structure parameters on the mixing effect is revealed.(5)Based on independent development and packaged PDMS micrmixer chip,the uniform and stable liquid droplets are generated with T shape channel.Based on the above experiment studies,rapid mixing experiments in micromixer are studied.The influences of the entry form,the ratio of two phase flow rate,the droplet size,the product of the amplitude and wavelength,and the wave number on the mixing efficiency are analyzed.The experimental results show good agreements with numerical simulation results,whichverifies the validity of the rapid mixing mechanism and the simulation model based on microdroplet.Based on the analysis of rapid and uniform mixing mechanism in non-uniform sinusoidal microchannel,simulation and experimental studies,we conclude that the rapid mixing of different fluid based on microdroplet can be finished in few seconds with the aid of droplet deformation,the asymmetrical vortices inside the droplet,and the twirling effect.