Research Of Microfluidic Driving Theory And Method Based On Lattice Boltzmann

The technologies of driving the microfluids is the core of micro analysis systems such as MEMS,microfluidic systems,biochips,micro-sensors.Due to the particularity of micro-scale flow,there are still many uncertain problems in the technologies of driving microfluids,such as the electrowetting-on-dielectric(EWOD)drive of discrete microdroplet,peristaltic drive of continuous microfluids,microfluidic heat transfer and so on.In view of the above problems,this paper proposes a lattice Boltzmann-electrohydrodynamic method to study the motion of microdroplet driven by EWOD,and analyze the liquid microlens based EWOD to explore the application of the microfluidic driving technologies.This method reveals the contact angle saturation phenomenon that cannot be explained by EWOD theory from the perspective of the electric force acting on the droplet.Second,we attempt to apply a modified immersed boundary-lattice Boltzmann method to study the peristaltic flow driven by the deformed tube wall.This method breaks the limitation of the traditional numerical method on the amplitude and wavelength of the peristaltic flow,and overcomes the shortcomings of the traditional immersed boundary-lattice Boltzmann method which does not satisfy the condition of non-slip boundary condition,and avoids the complicated calculation of the force generated by the boundary deformation.Finally,the coupled double-distribution function thermal lattice Boltzmann method is proposed to study the effects of viscous heat dissipation and compression work on Rayleigh-Bénard convection.This method breaks the limitation that the Boltzmann method of thermal lattice is only applicable to the thermal flow satisfying the Boussinesq approximation.The main research contents of this paper are as follows:1)A lattice Boltzmann-electrohydrodynamic method is proposed to study the driving mechanism of EWOD on microdroplets.The method combines the lattice Boltzmann method with the electrohydrodynamics theory,uses the density distribution function to describe the flow field,and introduces a new distribution function to calculate the electric field.The electric force is coupled into the evolution equation of the density distribution function to realize the coupling between the electric field and the flow field.The EWOD effect is analyzed to verify the feasibility and accuracy of the method.The variation of electric field distribution,droplet shape,droplet edge position and contact angle with time during droplet movement are analyzed.The influences of electrode switching frequency,applied voltage and viscosity of liquid on the droplet velocity are studied.This method can not only reveal the phenomenon of contact angle saturation,but also explain the driving the mechanism of droplets formed by conductive liquid,dielectric liquid and low surface tension liquid from the point of view of electric force acting on the droplet,thus perfecting the theory of microhydrodynamics.2)The LB-EHD method is applied to study the zoom liquid microlens.The influences of voltage on the curvature radius,focal length and focal power are analyzed,and the relationship between voltage and focal length is established.The transformation of lens from concave lens to convex lens is studied,and the dynamic evolution process of lens zoom is analyzed.The influence of liquid physical properties on the response time and performance of lens is discussed.3)A modified immersed boundary-lattice Boltzmann method is applied to study the driving mechanism of peristaltic flow.In this method,the velocity of deformed tube wall is introduced into the lattice Boltzmann equation as a velocity source,which avoids the complicated calculation of the force generated by the boundary deformation in the traditional immersed boundary-lattice Boltzmann method.The flow field in the tube is studied,and the influences of the various parameters such as amplitude ratio,frequency,wavenumber and liquid viscosity on driving effect are analyzed.The simulation results obtain a convergent solution even under the condition of large amplitude radio and wavenumber,and it is in good agreement with the existing literature.It breaks the limitation of the traditional numerical method on the amplitude and wavelength of the peristaltic flow and overcomes the difficulties of mesh reconstruction when dealing with moving boundaries in traditional numerical methods.Moreover,it overcomes the shortcomings of the traditional immersed boundary-lattice Boltzmann method which does not satisfy the condition of non-slip boundary condition.4)The effects of viscous heat dissipation and compression power on Rayleigh-Bénard convection are studied by applying a coupled double distribution function thermal lattice Boltzmann method.By introducing the influence of temperature change into momentum evolution equation in the form of momentum source,which realizes the coupling of momentum and energy field,breaking the limitation of the decoupling double distribution function LBM model limited to the Boussinesq flow with small temperature variation.The differences between two Rayleigh-Bénard convection models with viscous heat dissipation and compression work and without viscous heat dissipation and compression work are analyzed under different Rayleigh numbers and aspect rations.The effects of viscous heat dissipation and compression work on micro-scale heat transfer are explored.