Study On Mechanisms And Methods Of Flow Field Control For Curing Compound Droplet

Hollow microspheres have extensive potential in a wide range of applications,such as biomedical detection,drug release control,and atomic energy utilization,etc.In some special applications,it is very strict for the size,sphericity and wall thickness uniformity of hollow microspheres.Hollow microspheres are usually prepared by emulsion microencaulation,including the generation of compound droplets and the curing process of compound droplets for removing organic solvents.In order to improve the quality of microspheres,it is necessary to fully understand the mechanisms and methods of quantitative control on the size of compound droplets and the regulation of flow field during the curing process of compound droplets.In this paper,the quantitative construction mechanisms of compound droplets generated in coaxial microchannels are studied experimentally and the effects of different flow conditions on the quality of hollow microspheres are discussed.In addition,based on the volume of fluid(VOF)interface tracing method,a theoretical model of the dispersion of compound droplets in traditional solidification processes is established and numerically analyzed,the results of which are verified by visual experiments,and the control of external flow field is realized.In this paper,a visualized experimental system for preparation of compound droplets is built,based on whichthe formation processes of compound droplets under different flow conditions are observed and the influence of working conditions on the size of compound droplet is analyzed.The results show that with the increase of inner phase flow rate,the external diameters of compound droplets decrease and the inner diameters increase.By increasing the flow rate of middle phase,the external diameters of compound droplets gradually increase and the growing rates of which gradually decrease,the inner diameters almost remain unchanged.The internal and external diameters of the compound droplets all decrease with the increase of external phase flow rate.An experimental study on the effects of different parameters(liquid level,rotation rate and density difference)on the sphericity and wall thickness uniformity of hollow microspheres in traditional solidification process is carried out.A new type of random rotary solidification device is developed,and based on which,the regulation of the external flow field is realized.Subsequently,the optimization effect of random rotary solidification device on the quality of hollow microspheres is verified by comparison experiment.The results show that in the curing flow field of traditional solidification process,the quality of hollow microspheres first increase and then decrease with the increase of rotational speed at the same liquid level.An optimal rotation rate which increases with the increase of liquid level is found.Compared with the traditional solidification process,the curing flow field created by random rotary device achieves three-dimensional irregular circumferential shear effect of external flow field on compound droplets.The eccentricity and demulsification of compound droplets due to the difference of buoyancy caused by density mismatch are overcome.Accordingly,an optimal rotation rate for random rotary device is discovered,which has the best survival rate and the quality of hollow microspheres.A theoretical model of compound droplets dispersing in the curing flow field of the traditional experimental platform is established and numerically studied based on the volume of fluid method.From which,the distribution of the compound droplets,velocity field and shear strength in flow field of traditional solidification process is given.Combined with visualization experiment,the rotation rate,filling rate and the density difference are taken into consideration to investigate the influence on the sphericity and the uniformity of wall thickness of hollow microspheres.The results of simulation and experiment both show that optimizing the speed,the filling rate during the curing process of compound droplets and reducing the density difference between the outflows and compound droplets can improve the quality of hollow microspheres.The research above has systematically revealed the effect of different experimental parameters on the generation process of compound droplets and the effect of the flow field on the quality of microspheres during the curing process.The relevant research results provide a powerful theoretical support for the preparation of hollow microspheres.