Study On The Breakup Characteristics Of Droplets In The Fractal Tree-like Microchannels

Droplet-based microfluidics have been widely used in scientific frontiers such as biochemistry,advanced material preparation and other application fields because it can generate tiny droplets reliably and stably,and also control their size precisely.However,since the channel size of the microfluidic device is micron-sized,the flow rate of the working fluid is restricted by the high flow resistance of the microchannel,resulting in the droplet production still not meeting the industrialization needs.The fractal tree-like structure,as a kind of geometric structure widely existed in nature(such as mountains,river networks,human vascular systems,etc.),provides the space optimal solution for mass and energy transfer from point to surface(body)or from surface(body)to point no matter in nature systems or engineering transport systems.Inspired by this,designing the droplet microfluidic chip with reference to the fractal tree-like structure,to realizes the high-throughput preparation of droplets in fractal tree-like microchannels by its excellent uniform distribution and transport characteristics.Therefore,It is of great significance to research on the mechanism of droplet breakup in fractal tree-like microchannels.The visualization experiment on the fractal tree-like microfluidic chip with cross-focus structure is performed to study on droplet formation and splitting characteristics,especially focusing on the mechanism of stable droplet formation and droplet rupture in fractal tree-like microchannels.In order to obtain the fractal tree-like microchannel optimization design scheme and the optimal working condition for droplet generation.The main research contents and research conclusions are summarized as follows:(1)Generating droplets based on a cross-focus structure microchannel,visualization of droplet formation mechanism in fractal tree microfluidic chips,especially focusing on the influence of two-phase flow parameters,physical parameters and channel geometry parameters on droplet formation characteristics in the microchannel.The results indicate that four different flow patterns are found in the experiment,including squeezing,dripping,threading,tubing and viscous displacement.The droplet size decreases as the continuous phase flow rate increases when keeping the discrete phase volume flow rate constant.and the droplet size maintains a good linear relationship with continuous phase flow rate when the droplet size is reduced to a certain extent(<1.25 mm).While controlling the volumetric flow rate of the continuous phase,the size of the droplets gradually increase with the increase of the discrete phase flow rate.The flow pattern threading and tubing are formed as the viscous shear force of the continuous phase is insufficient to cause the droplets to breakup.Continue to increase the discrete phase,threading gradually transitions to tubing,the viscous displacement flow occurs when the viscous shear force of the continuous phase cannot overcome the pressure of the discrete phase.The physical parameters are also affect the two-phase flow in the microchannel.Constant viscosity of the continuous phase,m_c,the droplet size is observed to increase slightly when increase the discrete phase viscosity,m_d,and the droplet size decrease with the increase ofm_c.The formula for generating the droplet length can be predicted as l~*=(ε+wQ)Ca_c~m,linear regression of experimental data to obtaine=0.248,w=0.55 and m=-0.203.Comparing the predicted droplet size with the experimental values,the results show that the maximum relative error does not exceed 15%.(2)Visualization of droplet rupture in fractal tree-like microfluidic chips was carried out to observed the typical flow patterns of droplet breakage at the bifurcations in the fractal tree microchannel and the typical flow conditions of droplet rupture in the entire microchannel.And analysis the droplets breakup conditions with different droplet dimensionless size(l~*)and capillary number(Ca)through the control variable method.The results show that there are three typical flow patterns at each fork,including squeezing breakup,tunnel breakup and none-breakup.And four main types of typical flow conditions for droplet breakage in the entire channel,including none-breakup condition(#00),primary breakup condition(#01),secondary breakup condition(#12)and tertiary breakup condition(#23).The bigger the velocity disparity is,the more obvious the asymmetric breakup,contributing to bigger special coefficient of variation.These uncertain factors such as chip process deviation and droplets outflow lead to the pressure fluctuation at the outlets of the fractal tree microchannels,resulting in asymmetric breakup,for the asymmetrical cases,the higher branch level channels have more obvious asymmetry effects.Incomplete breakup conditions will gradually shift to complete breakup conditions with the increases of the capillary number.While under the same condition,the size distribution range of droplets gradually decreases with the increase of capillary number,and the coefficient of variation also decreases.When the capillary number Ca is constant,complete breakup conditions will gradually shift to incomplete breakup conditions as the droplet size decreases,and the size distribution range of the generated droplets will increase,and the coefficient of variation will also gradually increase.In this paper,the experimental studies on the breakup characteristics of droplets in the fractal tree-like microchannels are systematically carried out.It reveals the formation mechanism of droplets in the cross-focusing microchannel and the breakup characteristics in the fractal tree-like microchannel.The research work provides theoretical support for the high-throughput production of droplets.