Numerical Study On The Microdroplet Generation And Manipulation Mechanism In Microfluidics

Micro-nanofluidic technology is widely used in food safety testing,drug screening,new material synthesis and bioengineering.Droplet microfluidic technology is an important branch of micro-nanofluidic technology.Compared with continuous flow technology,droplet microfluidic technology has many unique advantages,such as small droplet size,fast response speed,high size controllability and low cost.At present,there is a lack of comprehensive,quantitative,systematic and universal research work on the formation mechanism of droplets in microchannels and electrokinetic ion enrichment.According to the significant advantages of the micro-nanofluidic chip such as modularization,intelligence,integration,automation and miniaturization,this thesis carried out the research on the generation and manipulation mechanism of microdroplets in microfluidics and basic research on nano-electrokinetic ion enrichment for sample preparation.The specific research contents of this thesis are as follows:(1)Through building a numerical model of microdroplet formation in T-junction microchannel and T-junction microchannel with step structure,the microdroplet generation and the effects of different control parameters on the microdroplet size were systematically studied.As the flow rate ratio and interfacial tension increase,the effective diameter increases.As the continuous phase viscosity and contact angle increase,the effective diameter decreases.The effective diameter decreases as Ca increases,and the generation frequency increases as Ca increases.When the step structure is in the middle,the droplet diameter is the smallest and the frequency is the fastest.(2)Through building a numerical model for microdroplet generation in a flow-focusing microchannel,the key factors that affect the size and frequency of droplet formation are investigated.The formation of droplets is mainly due to the continuous flow focusing effect of the dispersed phase.As the flow rate ratio and the interfacial tension increase,the droplet size increases and the frequency decreases.When the viscosity of the continuous phase increases,the droplet size decreases and the frequency increases.(3)By building a numerical model of microdroplet formation in a double T-junction microchannel,the droplet generation process and the effects of several key factors on droplet size were studied based on the level-set method.As the flow rate ratio and interfacial tension increase,the effective droplet diameter increases.As the continuous phase viscosity and contact angle increase,the effective droplet diameter decreases.In the merged droplet formation,the pressure fluctuation period at the specific position at the junction of the two phases is the same as the droplet generation period.As the continuous flow rate and viscosity increase,the peak and valley pressures increase,and the pressure cycle time decreases.When the interfacial tension increases,the peak pressure value and pressure period increase,while the valley pressure value decreases.In the process of merged droplet formation,there is a critical value for the influence of the geometry of the microchannel on the droplet size.The droplet generated at the critical value has the smallest diameter and the fastest frequency.(4)Based on the ion concentration polarization effect,the effect of nano-electrokinetic ion enrichment of high-viscosity fluids and the width and wall structure of nanochannels on ion enrichment were studied.Ion enrichment is due to the balance between the electrophoresis effect provided by the applied voltage and the repulsive force generated by the surface charge.The nanochannels of the square-wave wall structure have the highest peak concentration.Square nanochannels has the highest enrichment ratio.(5)Through building numerical models of micro-nanofludic preconcentrators with fractal wall structure in the nanochannel,the effect of fractal parameters on ion enrichment was studied.In the micro-nanofludic preconcentrator with Cantor fractal wall structure,reducing the initial length L0,increasing the initial height h0,increasing the fractal time n,and using unstaggered structure can increase the ion concentration and peak voltage.In the micro-nanofludic preconcentrator with Koch fractal surface structure,increasing the unit length L,increasing the fractal time n,increasing the fractal angle ?,and using unstaggered structure can significantly increase the enrichment ratio.Through studying the generation and manipulation mechanism of microdroplets in different microchannels in microfluidics,it is revealed the flow mechanism and control law therein,and it is provided a theoretical basis for meeting the different requirements of microdroplet generation and precise manipulation.Through the research on nano-electrokinetic ion enrichment,the problems of difficult preparation and trace detection of ultra-low concentration samples were solved.The research results provide guiding significance for the optimization and innovation of microfluidic droplet device design and the design of ultra-low concentration molecular detection equipment.