Study On The Factors Of Preparing Monodisperse Micro-droplets In Step-emulsification

Microdroplets are usually defined as droplets with particle size ranging from 1 μm to 1000 μm.Due to their small size,large specific surface area and high chemical stability,microdroplets with good monodispersity can be widely used in biomedical,food and spices,chemical engineering,pharmaceutical science and cosmetics industries.In recent years,the development of microfluidic technology has provided a convenient environment for the preparation of monodisperse microdroplets.However,the commonly used droplet preparation methods,such as cross-channel,coaxial-flow and focused-flow,all rely on the shear force of continuous phase,and the droplets will be affected by the flow fluctuation of each phase,resulting in poor monodispersity of droplets.In contrast,step-emulsification is a microfluidic method for droplet spontaneous fracture under the difference of Laplace pressure.This fracture principle makes droplet size less affected by flow rate of dispersed phase,so it has an obvious development prospect in the preparation of highly monodispersed microdroplets.In this paper,an experimental platform for the study of step-emulsification parameters is designed and constructed.Combined with the numerical simulation method,the influencing factors in the process of micro-droplets generation based on stepemulsification are analyzed and studied synthetically.The main contents are as follows:(1)A method of manufacturing chip was designed,and the preparation of monodisperse droplet based on step emulsification was finished on a droplet emulsification experimental platform.The influence of structural parameters,fluid properties and interface parameters in the emulsification process during droplet preparation was studied through experiments.The effects of step-length,step-height and channel-width on the size and frequency of the droplets obtained at different flow rates were analyzed.At the same time,the role of the viscosity and interfacial tension of the dispersed phase in the droplet fracture process was studied,and the experimental results can be used to guide the selection and design of the simulated parameters.(2)Combined with the VOF interface tracking method,the simulation model of step-emulsification was established to reproduce the droplet generation process during emulsification.The validity of the model and calculation method was verified by comparing the results of both experiments and simulation.By appling the model to the study of step structure size,the qualitative relationship between droplet size and steplength,step-height,or channel-width was established,and the internal mechanism of droplet size changes was explained by comparing the fluid pressure contours and velocity analysis.Besides,a section ratio of the step was proposed to explain the relationship between droplet fracture mode with the step structure.(3)The influence of fluid physical parameters on droplet emulsification was studied.The capillary number Ca was introduced to study the influence of interfacial tension and viscosity on droplets under the method of single variable.The simulation results were compared with the experimental phenomena,which revealed the phenomenon that the addition of glycerin changed the viscosity and interfacial tension of the system at the same time,and put forward the view that the change of fluid composition would play a role in multiple factors of the system at the same time.(4)The influence of wall contact angle on droplet emulsification was studied.Through simulation,the phenomenon of step wetting was found,as well as the rules of droplet diameter decreased with the increase of contact angle.Combining the theoretical analyse and calculation of the pressure change curves at central axis and fracture point of the model,the mechanism of different contact angle on the Laplace pressure inside the step was revealed.The role of the change of contact angle in the step-emulsification was clarified.In this paper,the influence of equipment structures,fluid physical parameters and interface factors on the formation of monodisperse droplets generated through stepemulsification was systematically studied,and the transfomation law of three kinds of droplet fracture modes was analyzed.The related results can provide theoretical and experimental basis for further research of step-emulsification,which can be used for droplets prdiction as well as the preparation of custom size droplets.