Numerical Study Of Modulated Electroosmosis And Joule Heating Effects In Microfluidics

Flow control is a key technique for implementing analysis functions of microfluidic systems. The liquid flows in microfluidics show many different behaviors from macroflows because of flow scale effect, electric force on liquid-solid interface and multi-physics interaction of flow-electric-temperature-ion transport. Electric double layer and electroosmosis are important fundamental problems in a microfluidic system. This study focus on numerical analysis of electroosmosis control and Joule heating effects in microfluidics, in order to reveale the micro/nanoscopic liquid flow nature and provide theoretical guidance for optimal design of microfluidic chips.A numerical analysis of Joule heating effect of electroosmosis in a finite-length microchannel made of glass and PDMS polymer is presented. Poisson-Boltzmann equation of electric double layer, Navier-Stokes equation of liquid flow and liquid-solid coupled heat transfer equation are solved to investigate temperature behavior of electroosmosis in two-dimensional microchannel. The feedback effect of temperature variation on liquid properties (dielectric constant, viscosity, thermal and electric conductivities) is taken into account. Numerical results indicate that there exists a heat developing length near channel inlet. The flow velocity, electric field and temperature approach to a steady state after heat developing length. Liquid temperature of steady state increases with increase of applied electric field, channel thickness and chip thickness. The temperature on PDMS wall is higher than that on glass wall due to difference of heat conductivities. Temperature variations are found in both longitudinal and transverse derictions of the microchannel. Temperature increase on wall decreases charge density of the electric double layer. Longitudinal temperature variation induces a pressure gradient and changes behavior of electric field in microchannel. Inflow liquid temperature does not change liquid temperature of steady state and heat developing length.An electric field is applied perpendicularly to the channel wall to modulate wall zeta potential and charge density of electric double layer (EDL). Based on liquid-solid coupled Poisson equation for electric potential, Nernst-Planck equation for ion transport and Navier-Stokes equation for liquid flow a numerical analysis is carried out to investigate complex electroosmotic flow in a microchannel with continuous or discrete electrodes. The relationship of induced wall zeta potential, flow velocity and the modulating voltage is presented. Typically modulated electro-osmotic flows are realized by simply changing the externally modulating voltage bias on the channel wall. A number of numerical examples of modulated electroosmotic flows in microchannels with discrete electrodes are presented, including single electrodes, symmetric/asymmetric double electrodes, and triple electrodes. Numerical results indicate that chaotic circulation flows, micro-vortices, and effective fluid mixing can be realized in microchannels by applying modulating electric fields with various electrode configurations. The interaction of a modulating field with an applied field along the channel is also discussed.A symmetric electrode arrays configuration of AC electroosmotic micropump is studied numerically, The numerical results indicate that it is possible to control the flow direction in the microchannel by switching phase of the AC signal on the adjacent electrodes. By solving Poisson-Boltzmann equation for electric double layer (EDL), Navier-Stokes equation for liquid flow, a numerical analysis based on the steric effects of ions in EDL is carried out to investigate symmetric electrode arrays of AC electroosmotic pumping. The relationship between the unidirectional velocity and the AC voltage amplitude, frequency are also presented, and compared with that based on the EDL Debye-Hiickel linearization. Numerical results indicate that steric effects are consistent with Debye-Hiickel linearization at low voltage, but induce reversal flow at large voltage with high-frequency.A microfluidic temperature controller for accurate controlling temperature variation in electronic encapsulation process is studied. Numerical simulation of liquid-solid-air coupled flow-heat transfer of the controller is also carried out. The heat transfer efficiency, the ratio of temperature variation amplitude of solid working surface and inlet liquid, is presented. The working parameter effect on controller behavior is also studied, including controller thickness, inlet liquid temperature, velocity, temperature variation shape and period. Numerical solutions indicate that the heat transfer efficiency increases as the controller thickness decreases, the inlet flow velocity increases, and period of temperature variation increases. It is also found that the heat transfer efficiency with square-shape temperature variation is higher than that with triangle-shape variation, and the temperature variation period of controller working surface is the same as the inlet liquid, but slight phase difference.