| The microfluidic chips,also known as Lab-On-a-Chip(LOC),have received increasing attentions in the scientific and technological areas,such as chemistry,biology,medicine,micro/nano material synthesis.The developments in microfluidic devices include reduced sample and reagent size,shortened reaction and analysis time,increased efficiency and throughput,multifunction,integration,automation and miniaturization for portability and point-of-care(POC)operation.To drive well and manipulate precisely the microscale fluids and particles in the microfluidic channel,various pumps have been explored to promote the progress of microfluidic chips.Among these,AC electrokinetic(ACEK),including the AC electroosmosis(ACEO),AC electrothermal(ACET)and dielectrophoresis(DEP)effects,have shown the great potential for pumping the microfluidic flow,mixing the samples,and trapping,separating,sorting,concentrating the micro/nano particles in the microchannel with an array of asymmetric electrode pairs via AC signal of a few volts.ACEK techniques are rapidly developing owing to their basic implementation efficiency,reliability due to their lack of moving parts,simplicity in design and fabrication,minimized electrode degradation,etc.To strengthen the flow rate and mixing efficiency by the ACEO effect,we numerically studied a 3D ring ACEO micropump with an array of asymmetric ring electrodes in the cylindrical microchannel.The ACEO mechanism is based on the interaction between the nonuniform AC electric field and ions of electric double layer(EDL)on the electrode surface.To establish the ring ACEO model,we firstly proposed the equivalent hollow cylinder capacitance of electrical double layer(EDL)on the ring electrode surface.Then,the 3D linear Poisson-Boltzmann model of ring ACEO micropump was established for solving the electric field and fluidic flow field with the charge conservation and slip velocity boundary conditions.In the simulation,we utilized the modified Kohlrausch's dilution empirical equation to provide the function of dilute strong electrolyte solution and the conductivity.The dependencies of pumping flow rate on the AC frequencies,the solution conductivity,electrode geometric sizes are discussed in detail.The numerical results show that the flow rate of ring ACEO pump is higher than the planar ACEO micropump,which agreed well with the experiment.We also achieved an optimal velocity with the proper geometric parameters.To study the mixing efficiency by the ring ACEO micropump,we solved the linear PB model with the convection-diffusion equation.The vortices on top of electrodes induced by the ACEO flow can stir the flow to mix.When the samples are converged into the microfluidic channel from the inner and outer pipe areas of inlet respectively,the mixing efficiency can be enhanced.The reason is that the two different samples can be exchanged from the outer pipe areas to the inner pipe,so that is can strengthen the mixing by the vortices.The microfluidic velocity field,vorticity field and concentration field are investigated in detail to explain the mechanism of pumping and mixing.We also designed combination sequences PnMm of ring electrode pairs based on the pumping mode of “forward driving of electrode pair” and the mixing mode of “reversal driving of electrode pair” to further improve the mixing efficiency.The simulation results show that the ring micropump with inner and outer pipe inlet can pump the microflows rapidly and mix the samples well simultaneously.To pump the flow rate and mix the samples simultaneously by the ACET effect,we proposed a multifuctional ACET micropump embedded with an asymmetric spiral microelectrode pair in a cylindrical microchannel with a high-conductivity fluid,which can be used for biofluid applications.We established the decouple electrictemperature-fluid fields model with the convection-diffusion equation to study the microfluid flow and concentration field distributions.The vorticity field is inclined against the microchannel direction,and the vortices on top of the spiral electrodes can affect the ACET flow in two aspects at the same time: one is pumping the flow in the microchannel direction,and the other is mixing the samples by stirring the flow.The geometric ratios of the electrode width to the gap or slant angel of spiral electrode can control the relative strength of the simultaneous pumping and mixing ability,and we achieve an optimal geometric design.This study shows that the spiral ACET micropump design can rapidly drive the high-conductivity fluids and efficiently mix samples simultaneously.To study the manipulation of micro/nano particles by ring ACEO or ACET micropump in the cylindrical microchannel,we combined the ACEO/DEP and the ACET/DEP using the established ring ACEO and ACET models.The movement equations of particles satisfy the Langevin equation for single particles and the FokkerPlanck equation for particle concentration or probability density,respectively.The simulation results show that both the ring ACEO/nDEP and ACET/nDEP micropump can pump the particle movement and focus the particles near the central axis of the cylindrical microchannel simultaneously.The mechanism is that at the electrode edges the negative DEP force can repel the particles away from the electrode,and at the electrode center,the ACEO flow or ACET flow can drag the particles away from the electrode where the nDEP is small.The AC signal,the fluid properties,the particle size and density,the fluid and particle conductivity and permittivity,and the electrode geometric sizes will influenced the relative strength of the forces acting on the particles,and manipulating the particle movement.The numerical simulation of the ring ACEK techniques,including the ACEO,ring ACET and spiral ACET micropumps,are of significant importance for pumping fluid and mixing samples,transporting and focusing particles simultaneously in the practical chemical and biological applications,and feasible fabrication techniques should be experimentally investigated in future studies. |
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