Study On The Mechanism And Application Of Microdroplet Generation Based On Co-flowing Step Emulsification

Driven by miniaturization,integration and intelligence,microfluidic technology has gained rapidly development.Droplet microfluidic,which has broad application prospects in the fields of biomedicine,chemistry engineering and materials,is an important branch of microfluidic technology.Microfluidic droplet generation approaches have made a great breakthrough in droplet uniformity,because the generated droplet monodispersities are much higher than the traditional methods.However,with the development of microfluidic technology,high throughput and on-line controllability of microdroplet generation are urgently required by more and more applications.The complex manufacturing process,low throughput and lack of controllability limit the further development and application of these droplet generation methods Therefore,it is of great significance to investigate a droplet generation approach with high droplet monodispersity,high throughput,controllability and manufacturing simplicity.For this purpose,this paper proposed the method of co-flowing step emulsification,and carried out research on the preparation of high-throughput droplets based on this method.The main research contents and results are as follows(1)By combining the co-flow strategy with step emulsification method,a co-flowing step emulsification chip based on micro-capillary film(MCF)was designed and fabricated.Single-component droplet generation experiment was carried out by connecting the chip to the experimental system.The result indicated that the droplets generated by the co-flowing step emulsification chip were highly monodispersed with a deviation coefficient CV of 1.4%Moreover,compared with step emulsification,the droplets generated by the chip were smaller and had higher generation frequency.Based on the experimental results,the droplet generation mechanism of the co-flowing step emulsification was analyzed and the theoretical expression of the pressure difference between inside and outside the step was deduced at the initial stage of droplet generation(2)On the basis of the theoretical analysis,a model of co-flowing step emulsification chip was established,and the detailed process of droplet generation was obtained through CFD simulation,which had been verified by experiment.Using the established model,the effects of technological parameters including interfacial tension and flow velocities of continuous and dispersed phases on droplet generation were simulated,and the influences of these factors on initial pressure difference and droplet generation process were analyzed in detail(3)The effects of chip critical dimensions including microchannel diameter,microchannel center distance,step length and wall contact angle on droplet generation were simulated by the corresponding CFD models.The droplet sizes and generation frequencies which change with the critical parameters of co-flowing step emulsification chip were calculated.The influences of the critical parameters on the initial pressure difference and droplet generation process were analyzed(4)Based on the numerical simulation results,the relationships between the technological and chip critical parameters,and the droplet diameter were fitted.As a result,a droplet diameter prediction model for co-flowing step emulsification was deduced.By comparison,the model predicted results were in good agreement with the numerical simulation results.In addition,a new co-flowing step emulsification chip with different dimensions was fabricated in experiment by an MCF and acrylic plates.By changing the chip and technological parameters,the average droplet diameters under different parameters were obtained.Results showed that the differences between the experimental and the predicted results were less significant,which verified the feasibility of the prediction model.In addition,an expression of the mode indicator factor a and its value range were derived based on the numerical simulation of different droplet generation modes.According to the value of a,the droplet generation mode in co-flowing step emulsification could be predicted(5)An MCF multi-channel chip was fabricated by using the co-flowing step emulsification method proposed in this paper.According to the effect of wall contact angle on droplet coalescence,four-and five-channel co-flowing step emulsifying chips were fabricated by hydrophilic glass capillaries.Through experiments,Janus droplets and three-component droplets were successfully prepared.At last,the on-line control effects of the co-flowing step emulsification method on the Janus and three-component droplets were studied by changing the flow rates of continuous and dispersed phases.