Generation And Hydrodynamics Of Gas-core Double Emulsions In A Microchannel

Gas-core double emulsions refer to the droplets containing gas bubbles inside dispersed in another immiscible fluid.The Gas-core double emulsions have a wide range of applications invloving extraction enhancement,process intensification,core-shell material preparation,and biomedical materials such as ultrasound contrast agents and drug delivery vesicles.The preparation of gas-core double emulsions with controllable morphology using microchannel devices has attracted extensive attention in recent years.The researches on gas-core double emulsions in microchannels are still carried using experimental methods,and there is no research report using numerical simulation so far.Numerical simulation is another alternative tool to study structured microemulsions,especially the hydrodynamics of the emulsions in micro/nano scales,where the experimental techniques could not be used.Here,a numerical simulation,volume of fluid(VOF)technique,was adopted to study the generation and hydrodynamics of gas-core double emulsions in a coaxial flow-focusing microchannel.The contents of this study are as follows:(1)The generation process of gas/liquid/liquid(G/L/L)double emulsions was studied numerically.The generation of G/L/L double emulsions can be divided into dripping and jetting models.Due to the low viscosity of the gas,the gas stream inside the liquid stream broke quickly and retracted back toward upstream.The pressure fields and velocity fields of double emulsions changed continually after the generation of double emulsions.(2)The effects of different parameters on the size and deformation indices of the G/L/L double emulsion were investigated.The results showed that the effects of the three-phase flow rates on the size and the deformation index of the internal bubbles and the deformation were insignificant,while the effects on the size and the deformation index of the external droplets are remarkable.The intrinsic relation between the gas-liquid phase flow and the G/L/L double emulsion was investigated by analyzing the dimensionless numbers and the velocity fields in the channel.(3)The preparation of microdroplets in gases in a flow focusing microchannel was investigated by numerical simulations.The droplet size,generation frequency,deformation index and droplet generation mechanism under different flow states were investigated by varying the parameters such as the flow rates of the dispersed and continuous phases,the viscosity of the dispersed phase,the surface tension between the two phases,and the flow rate ratio of the two phases.The simulations revealed that the droplet generation process in this system was different from the previous droplet generation process in a liquid-liquid two-phase system,and some different flow regimes were observed as well.The results of the numerical simulations were adopted to guide the experiments in which continuous controlled droplets were generated in gases in microchannels,and further used to prepare polymer nanoparticles.