Study On The Drop Generation Methods And Mechanism Of Microfluidic Step Emulsification

Drop microfluidic is an emerging technology involving generating and manipulating micro-sized drops by the highly controllable multiphase flow in microchannels.By virtue of the fast indrop mixing,heat transferring and low cross contamination,drop microfluidic is widely used in analytical chemistry,tissue engineering and single cell analysis,etc.With the development of the drop microfluidic,drop generation,as the fundmental technique,is still chanllenging in some aspects,including high-throughput production monodispersed drops and functional drops.Among the mostly used drop generation methods,step emulsification is a low shear method by spontaneous breakup of dispersed fluids under geometry-induced Laplace pressure difference,which is promising in scaling up of drop generation?In this paper,to address the problems in step emulsification,we design and fabricate the multi-channel step emulsifier,based on which the interactions between neighboring emulsifing units are studied.Further,we propose the electro-coalescence method in step emulsification and give a mathematical model to describle the dynamics.At last,we study the step emulsification in portable rectangular microtubes and investigate the effect of nonuniformity of channel height on the drop generation.The major contents and conclusions are as followig:(1)Multi-channel step emulsifier is designed and fabricated based on the microcapillary film.A numerical model is setup for simulation of the step emulsification.The reliability of the numerical simulation is validated by comparing the drop size evolving with experimental results.(2)Two forms of interaction between different units in multi-channel step emulsification are found,including alternative sychronization and in-phase sychronization.The alternative sychronization happens when neighboring dispersed streams do not contact and then drip into drops alternatively.The alternative sychronization locks the frequency of drop generation,which is able to resist 25% increase of flow rate.The in-phase sychronization is triggered when pancake-like flow tips coalesce in the terrace under low surfactant concentration condition.It is able to resist 50% increase of flow rate to maintain the frequency lock between two neighboring channels and the monodispersity of drops.The transformation between alternative sychronization,in-phase sychronization and jetting are mainly determined by the geometrical parameter S/ H and the capillary number Ca.(3)An electro-coalescence method is proposed in step emulsification.By studying the effect of the direct current field,we find the length of the contact line decrease with and only with the increase of voltage,validating that the mechanism of the electro-coalescence is the electro-hydrodynamic instability of the oil film.When alternative current field is applied,the coalescence also slows dow when the frequency increases.Combing the electro-coalescence results and the fluid mechanism of step emulsification,a mathematical model is developed to describle the electro-coalescence step emulsification,which show good accuracy.The size prediction based on necking time is able to predict drop sizes at high voltage area while the other model based on direct fitting is able to calculate drop sizes in a wide parameter range.Further,a series of applications are developed based on the electro-coalescence step emulsification,including regulating the dependence of the drop sizes on flow rate,controlling the coalescence ratio,preparing multi-component droplets and used as clog-free drop reactors.(4)Step emulsification is achieved in portable rectangular microtubes.The double-channel step emulsification in dumbbell-like channels shows alternative sychronization.To address the problem that the cross-sections of the channels in the microtubes are not strictly rectangular,simulations are carried out and it is found that the dispersed fluid cannot occupy the whole channel.The width ratio decreases and the drop size increases with the increase of the nonuniformity of channel height.Furthermore,the step emulsification in rectangular channels can also be combined with gradient generator and produce drops with concentration gradients.