| The droplet microfluidic technology has been widely used in biology,chemical industry,medicine,environment and other fields.Compared with the traditional methods,it has many advantages,such as small droplet with good monodispersity,high throughput,and fast reaction speed.The application of small droplets(10-15?10-9 L)has been widely reported.With the deepening of research,the application value of larger droplets(10-8?10-6 L)has gradually attracted researchers'attention.This work mainly studies the formation and counting method of large-size droplets based on droplet microfluidic technology.Through a combination of numerical simulation and experiment,the inherent laws of the method are revealed and the experimental conditions are optimized,which provides a reference and support for the subsequent practical application of large-size droplets.This work first uses the two-phase flow model with Level Set method to numerically simulate the formation of slug droplets in the T-shaped channel,which explores the key factors that affect the size of droplets.Through the analysis of the pressure changes in the horizontal channel,it can be seen that the continuous phases'squeezing and shearing force is the key force promoting the formation of droplets.The force is dominated by the pressure difference between the upstream and downstream of the horizontal channel and the viscous force of continuous phase.Interfacial tension and wall wettability hinder the formation of droplets.Therefore,the parameters related to the above-mentioned force are the key factors affecting the diameter of the generated droplets.Numerical simulation and experimental results show that increasing the flow ratio of two-phase and liquid-liquid interfacial tension coefficient,or reducing the viscosity of the continuous phase and the contact angle between the dispersed phase and the hydrophobic wall can effectively increase the size of the droplet.The two-phase flow ratio is the most important factor.When the flow ratio of two-phase Qd/Qc increases from 0.2 to 2,the diameter of droplets increases by 55%.Besides,when the two-phase flow ratio is constant,the size of the droplet is determined by the relative value between the continuous phase viscosity and the interfacial tension coefficient which can be measured by the capillary number Ca.The size of the droplet decreases as Ca decreases.When using liquid paraffin and3%(w/w)ABIL EM 90 as the continuous phase and pure water as the dispersed phase,the experiment shows that the size of droplets generated in the T-shaped channel range from 300 to 620?m with the relative standard deviation less than 5%.In order to increase the size of generated droplets,this work continuously do some research on generating droplets method in the cross channel.The two-phase flow model with Phase Field method is used to study the formation process of slug droplets in the cross channel,and the key factors affecting the size of the droplets are analyzed.In the cross channel,the force that the droplet suffers during generation process is similar to the force in the T-shaped channel,so the key factors(flow rate of two-phase,viscosity of continuous phase,interfacial tension)have similar influence on the droplet size as the T-shaped channel.When the flow ratio of two-phase Qd/Qc increases from 0.25 to 2,the diameter of the droplet increases by 68%.The difference is that in the cross channel,the adhesion force of the wall to the droplet is the driving force for the formation of the droplet,so the larger the contact angle between the dispersed phase and the hydrophobic wall,the larger the size of the generated droplet.Experimental results show that using the same solution,the cross-channel method can generate droplets with a wider range of sizes,ranging from 300to 750?m.Presumably,the reason is that before and after the neck of the droplet breaks,the pressure difference between the upstream and downstream of the main channel in the cross channel changes by 7 Pa,which is much smaller than the pressure difference change in the T-shaped channel(52 Pa).So that the large-size droplets with smaller spacing have a lower chance of coalescence in the cross channel due to pressure fluctuations.In order to improve the controllability and practicability of the experiment,it is necessary to accurately count the droplets.This article studies the droplet counting method based on capacitively coupled contactless conductivity detection technology.The key factors affecting the detection signal are discussed through numerical simulation,namely,electrode parameters,channel solution parameters and detection channel parameters.It is determined by numerical simulation that the width of the excitation electrode is 0.017 mm,the width of the detection electrode is 0.035 mm,the distance between the two electrodes is 0.6 mm,the wall thickness of the detection channel is 0.3 mm,and the dielectric constant of the detection channel is 2.1,which can improve the accuracy of counting.It is found through experiments that the thickness of the detection channels'wall is the main factor affecting the counting accuracy.The results show that with the decrease of the tube wall thickness,which means,when the outer diameter of the channel remains unchanged,as the inner diameter of the detection channel increases,the detection range of this method increases.At the same time,the size of the generated droplets increases and the frequency of droplet generation decreases.When using a detection channel with an inner diameter of 1 mm,the maximum flow rate of the dispersed phase is 200?L/min and the maximum flow rate of the continuous phase is 500?L/min.The diameter of the droplets can reach 1736?m.In addition,experiments have found that the change in the detection signal directly reflects the change in the thickness of the adherent oil film layer,rather than the change in the two-phase solution in the channel. |
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