Study On The Asymmetric Breakup Characteristics Of Droplets (Bubbles) In The T-Shaped Microchannels

In recent years,microfluidic technology has been widely used in the fields of biology,chemistry,medicine,micro-electromechanics,etc.,which can achieve precise control of its basic carrier micro droplets and micro bubbles.Due to the advantages of high specific surface area and high heat and mass transfer performance caused by the tiny size effect,micro droplets and micro bubbles have received more and more attention.Controlling the production of droplets（bubbles）of different volumes has also become an urgent subject to be studied.The T-shaped microchannels with branches of different length can utilize the asymmetric splitting of droplets（bubbles）to obtain sub-droplets（bubbles）of different volumes.The structure is simple and the operation is convenient.It is the optimal method for producing micro-droplets（bubbles）of different volumes.At present,the study on the asymmetric breakup of droplets（bubbles）in the T-shaped microchannels is insufficient to discuss and analyze the breakup mechanism,and there is a lack of systematic research.Therefore,carrying out the experimental research on the asymmetric breakup of droplets（bubbles）in the T-shaped microchannels is of great application prospects and scientific significance.In this thesis,a visual experiment method is applied to carry out the research on the asymmetric breakup of droplets（bubbles）in the T-shaped microchannels,and the T-shaped microchannel chip with branches of different length is designed,and an experimental platform for asymmetric breakup is established to study the droplets（bubbles）asymmetric breakup flow pattern and formation mechanism systematically,providing a theoretical basis for precise control of size.The main research content and research conclusions are as follows:（1）Study the morphological evolution characteristics of the droplet breakup process quantitatively,analyze the dynamic behavior characteristics of the droplet breakup,reveal the formation mechanism of different breakup flow patterns,and focus on the influencing factors of the droplet breakup ratio.The results show that:1)Four flow patterns of asymmetric breakup are observed:blocking-tunnel breakup,double tunnel breakup（flowing in the opposite direction）,double tunnel breakup（flowing in the same direction）and non-breakup.Among them,the droplet breakup process can be divided into three stages:the entering stage,the squeezing-breakup stage and the post-breakup stage.In the squeezing and breakup stage,the contraction rate of droplet neck first decelerates and then accelerates.On the eve of the breakup,the contraction mode of droplet neck changes from 2D to 3D.The centripetal interfacial tension promotes the accelerated collapse of the neck;for asymmetric T-shaped microchannels which sub-channels with different flow resistance,the droplet will move towards the channel with a lower flow resistance until the neck of the droplet breaks;the emergence of tunnels will slow down the forward movement of the droplets in the sub-channels,and ultimately affect the breakup ratio V_large/V_small.2)When the capillary number remains unchanged,as the initial length of the droplet increases,the flow pattern of droplet breakup transitions from double tunnel breakup to blocking-tunnel breakup.At this time,the droplet breakup ratio V_large/V_small presents a V-shape tendency that first decreases and then increases.The inflection point of the tendency curve is about the critical point of the two-flow pattern;this is because in the blocking-tunnel breakup flow pattern,with the increase of the droplet length,more of the droplet enter the sub-channel,and the droplet with lower viscosity makes the overall viscosity of the branch channel decreases,and the flow resistance decreases overall.For short channel,the flow resistance decreases more obviously,so more of the droplet enters short channel,and V_large/V_small gradually increases;in the double-tunnel split flow pattern,as the length of the droplet increases,the width of the tunnel decreases.The width of the tunnel in the long branch channel decreases significantly,and the flow resistance weakens,and the volume of the droplet entering the long branch channel increases significantly.However,the width of the tunnel in the short branch channel changes little,and the increased volume of the droplet entering the short branch channel is tiny,so the droplet splitting ratio V_large/V_small decreases with the increase of the droplet length.（2）Study the morphological evolution characteristics of the bubble breakup process quantitatively,reveal the formation mechanism of different breakup flow patterns,discuss the influence factors of the bubble breakup ratio,and compare and analyze the difference between asymmetric breakup of bubbles and droplets.The results show that:1)Four flow patterns in asymmetric breakup are observed:blocking-tunnel breakup,double tunnel breakup（flowing in the opposite direction）,double tunnel breakup（flowing in the same direction）and non-breakup;due to the strong gas-liquid interface tension,the bubble non-breakup flow pattern is more common than the droplet,and the instability of bubble breakup leads to indistinct conversion conditions between non-breakup,double tunnel breakup（flowing in the opposite direction）,double tunnel breakup（flowing in the same direction）.2)The asymmetric breakup process of the bubble is also divided into three stages:the entering stage,the squeezing-breakup stage and the post-breakup stage.In the entering phase,the squeezing force of liquid phase dominates.The width of the bubble neck decreases monotonously during the squeezing-breakup stage.Before breakup,the bubble shrinks sharply due to the centripetal interface tension,and the neck width decreases in a cliff-like manner;due to the different flow resistances of the branch channels,The bubble gradually deviates to the channel with a lower flow resistance during the breakup;similar to droplet breakup,the appearance of the tunnel slows down the velocity of the sub bubble head,and then affects the bubble distribution ratio;3)When the capillary number is constant,with the decrease of the initial bubble length,the typical flow pattern gradually changes from blocking-tunnel breakup to double tunnel breakup,and the total time of squeezing-breakup stage increases gradually.Due to the significant expansion of the tension of the gas-liquid interfacial tension of the bubble in the case of the double tunnel breakup（flowing in the same direction）flow pattern,the bubble distribution ratio of the double tunnel breakup（flowing in the same direction）changes exponentially with time.Compared with the existing research on the asymmetric breakup of droplets（bubbles）,this thesis studies the asymmetric breakup process of droplets（bubbles）in the T-shaped microchannels with branches of different length comprehensively and systematically,and discusses the breakup mechanism in detail.It provides strong support for the theoretical analysis and engineering application of asymmetrical breakup of droplets（bubbles）.