Hydrodynamics Mechanism And Application Of Droplet Microfluidic Emulsification

As a bottom-up droplet preparation method,droplet microfluidic technology has shown great potential in the preparation of highly sphericity and highly monodisperse droplet.The droplet can be widely used as a template in the biological,pharmaceutical,chemical and other industries.With the continuous progress of microfluidic technology,droplet microfluidic has expanded from twophase to multiphase flow,two-dimensional to three-dimensional,Newtonian fluid system to nonNewtonian fluid system.There are still some deficiencies in the relevant theoretical researches.On the one hand,the researches on non-Newtonian fluids only consider single emulsion.On the other hand,the step emulsification channel is limited to visualization and rarely applied.In this paper,an unsteady theoretical model of multiphase microfluidic emulsification process with shear thinning effect is established to further enrich the existing theoretical system of droplet microfluidic.The mechanism of droplet emulsification in step emulsion microchannels was elucidated,and the influence of physical parameters on droplet preparation process was comprehensively and quantitatively analyzed,which provided a new theoretical basis for channel design.Based on the step emulsification channel,the visualization experiment system of carbon capture microspheres prepared by microfluidic method was designed and built.This provides a new idea for the preparation of carbon capture microspheres.The main conclusions are as follows:(1)A theoretical analysis model of hydrodynamic behavior during the flow-focused microfluidic emulsion formation of two-dimensional shear thinned composite droplets was established based on the VOF method.The Carreau-Cross model was introduced to consider nonNewtonian effects of shear thinned fluids.The emulsification processes in two systems of Newtonian fluid and Newtonian fluid-shear thinning fluid-Newtonian fluid were numerically simulated.The fluid dynamical behavior characteristics of typical emulsification models were given.The similarities and differences between the two systems of typical emulsification models were compared.This provided theoretical guidance for the template encapsulation of double emulsion droplets.The results show that there are two typical emulsification models in both systems,namely,dripping and jetting type.The shear thinning effect has no obviously effect on most emulsion process.The shear thinning effect plays a role in delaying the rupture when the neck is strongly stretched.There is also a difference in the transition of flow pattern between the two systems.It requires a larger external fluid capillary number for the non-Newtonian fluid system to transform from dripping to jetting type than that of the Newtonian fluid system.The increase of the middle phase fluid concentration will also cause the transition from dripping to jetting type.The transition is mainly due to the increase of viscosity.The shear thinning characteristics of the middle phase can reduce the probability of double-core emulsion and are conducive to the preparation of single-core double emulsion.(2)A theoretical analysis model of the three-dimensional triangular and rectangular microchannels fluid dynamic behavior characteristics emulsification process was established based on the VOF method.Numerical simulations of droplet generation process in triangle channel under typical dripping flow and jetting flow were carried out.The generation process was compared with droplet generation process in straight channel.The effects of various physical parameters on the preparation process were analyzed.The results show that the generation of droplets in step emulsification channel is mainly affected by the Laplace pressure difference.The generation process also affected by the continuous phase fluid.The continuous phase fluid affects the triangle nozzle microchannel more.This due to the promotion of droplet generation.The droplet generated under the jetting flow is much larger than that under the dripping flow.The droplet size is inversely proportional to the contact angle and interfacial tension coefficient at the wall of the triangular channel.It should be noted that droplet generation cannot occur normally when the contact angle on the outlet wall is less than 115°.The gradual expansion angle of the triangular channel also affects the droplet preparation.It was found that an opening angle greater than 35° is more conducive to the preparation of highly monodisperse droplets.(3)A visual experimental system for the preparation of carbon capture microspheres by microfluidic method was constructed.The microfluidic chip was used triangle step emulsification microfluidic device.Based on the regulation of various parameters in the device,the precursor of porous carbon capture microspheres formed by chitosan and formaldehyde was prepared.Carbon capture microspheres were formed by freeze-drying and carbonization.The surface morphology,sample composition,etc.were characterized.The carbon dioxide adsorption experiment was also carried out.The following conclusions are obtained:The precursor prepared by the triangle step emulsification microfluidic chip has good monodispersity.The size of the precursor can be accurately adjusted by the flow rate.The octane droplets obtained by homogenizer stirring can reach the nanoscale and successfully form porous structures on the surface and inside of the carbon capture microsphere.The main component of the carbon capture microspheres formed by carbonization of the precursor is graphite.The influence of nano-octane droplets on the structure and chemical composition of the microspheres can be ignored.5wt%nano-octane droplets added to the chitosan aqueous solution is most suitable.Too many octane droplets will agglomerate and produce more mesopores.The mesopores will result in decreased adsorption performance.