Multi-fields Interaction Study Of Flow-Electric-Ion Transport In Microfluidic System

The fluid flows are primary means to implement analysis functions of microfluidic chips. It is very important to make a good understanding of the liquid flow behavior of microfluidics in order to guide the microfluidic design and optimization. Because of electric double layer, the liquid flows in microchannel show a number of new characteristics, electrokinetic effects are the primary flow phenomena. This work focus on theory and numerical analysis of flow electrokinetic effects in microfluidics, in order to reveale the liquid flow nature and provide useful instruction for microfluidic design and development.This work analyzes the flow electrokinetic effects in microfluidics in detail based on electric double layer theory and hydrodynamics theory. The results show that three solution methods of Poisson-Boltzmann equation describing the electric double layer in microchannel have their valid application range. The Debye-Huckel linearization is valid in cases of low surface electric potential condition, the isolated flat approximation is valid in small electrokinetic diameter condition, the weak double layer overlap approximation is suitable to almost all the conditions. The analysis results reveal that in steady pressure-driven flow, both the surface electric potential and electrokinetic diameter have significant influence on the electro-viscous effects. The electro-viscous effects increase as the increasing of surface electric potential, and decreasing of electrokinetic diameter. There exists a critical electrokinetic diameter, around which the ratio of apparent viscosity and liquid viscosity reach a maximum.The analysis of steady pressure-driven flow in a micro-diffuse channel reveals that the total flow rate through the micro-diffuse channel decreases because of the electro-viscous effects, and the flow rate loss in micro-nozzle channel flow is higher than that in micro-diffuse channel flow. The streaming potential in micro-diffuse channel is quite different from that in regular channel. Streaming potantial nonlinearly increases in the axial direction, increases rapidly in the section of small area of micro-diffuser. Second, there is radial streaming potential due to a radial flow velocity. Third, the radial electro-viscous force in micro-nozzle channel flow increases the total flow resistance, but the radial electro-viscous force in micro-diffuse channel flow decreases the total flow resistance. As results, the flow rate lost due to electro-viscous effects in nozzle channel is larger than that in diffuse channel. The difference of flow rate lost of between nozzle channel and diffuse channel increases when the diffuse angle increases.The analysis of periodic pressure-driven flow in microchannel reveals that periodic velocity is a function of three independent dimensionless parameters: periodical Reynolds number, electro-viscous parameter and electrokinetic diameter. Periodic velocity amplitude decreases as periodical Reynolds number or electro-viscous parameter increases. As electrokinetic diameter increases, periodic velocity amplitude increases. There exists a critical periodical Reynolds number. The streaming potential behaviors similarly as in steady flow when Reynolds number is less than one,and then decreases rapidly when Reynolds number is greater than one. Analytical results also indicate that electro-viscous force depends on three parameters: (1) Electro-viscous parameter representing the ratio of maximum electro-viscous force to the pressure gradient in steady flow (2) Profile parameter describing the distribution behavior of electro-viscous force in channel section (3) Coupling coefficient denoting amplitude attenuation and phase offset of the electro-viscous force, it is a function of Reynolds number and electro-viscous parameter.This work also analyzes the electroosmosis flow control methods by applying an external transverse bias voltage on the wall surface. When a bias voltage is applied, the surface zeta potential is modified, and depended on the ratio of the liquid dielectric permittivity and wall dielectric permittivity. Furthermore, the ratio of the electric double layer thickness and wall thickness has more significant influence on surface zeta potential. In initial stage of the applied bias voltage, the surface zeta potential increases quickly at first, then increases slowly down. The surface zeta potential does not increase infinitely with increase of the applied external bias voltage, but a limitation. Liquid transportation and circumfluence can be achieved by means of applying a wave external bias voltage. When different external bias voltages are applied on different walls, two layer fluid flows in microchannel can also be generated.