Study On Preparation Of Phase Change Functional Materials By Multiphase Microfluidic Encapsulation

Phase change material(PCM)uses the material phase change process to absorb and release heat to achieve energy storage.It has the advantages of high energy storage density,small temperature variation ranges during phase change,stable energy storage,etc.The encapsulation treatment of Phase change material can effectively prevent the PCM from reacting with the external environment,thereby improving the stability of the use of the phase change material.The microencapsulation/microfibrillation of PCM can significantly improve the heat transfer area of PCM itself and thus increase the thermal conductivity.In addition,the encapsulated PCM is more convenient in storage,transportation and use than the non encapsulated PCM.In conclusion,the encapsulation PCM has a wide range of applications in energy saving buildings,thermal regulation textiles,solar energy utilization and waste heat recoveryHowever,the traditional microencapsulation and microfibrillation methods of PCM have the disadvantages of uncontrollable size,irregular shape,poor monodispersity and low encapsulation efficiency,which make the energy storage density and stability of phase change functional materials decrease in practical production and application,and bring difficulties to further improve the comprehensive properties of phase change functional materials.In order to solve the above problems,this paper builds multiple emulsion templates and laminar flow line templates based on the unique advantages of microfluidic technology that can precisely control the flow of multiphase liquid in microchannels,and on this basis,the microencapsulation/microfibrillation encapsulation of phase change materials are realized.The method of combining numerical simulation and experimental observation is used to conduct in-depth research on the construction of multiphase flow templates in micro fluidic devices and the preparation of phase change functional materials:An unsteady theoretical model for the preparation of phase change functional material templates by multiphase microfluidic encapsulation considering phase interface evolution is established to numerically the hydrodynamic behavior of typical emulsification process under different flow conditions and fluid physical properties in a typical flow-focusing microchannel;In addition,a visual experimental system for the preparation of phase change functional materials by multi-phase microfluidic encapsulation is designed and built,and phase change microcapsule and phase change microfiber samples are successfully prepared and their morphology and thermal properties and thermal regulation ability are characterized.The main research conclusions are as follows:(1)Based on the VOF phase interface tracing method,a theoretical model of the hydrodynamic behavior characteristics of the emulsification process in the flow-focusing microchannel was established.Based on the numerical model,the flow evolution process of typical emulsification modes in microchannels under different fluid physical properties and different flow conditions was numerically studied,for subsequent preparation phase change functional materials based on the typical emulsion model provided a theoretical guidance,the research results showed that there are two typical double emulsion generation flow patterns in the flow-focusing microfluidic device:dropping regime and jetting regime,and the two show different hydrodynamic behaviors in the device to different interface driving forces;The transitions of emulsification modes from dripping to jetting are observed with increasing the flow rate of the outer phase.When the flow rate of the outer phase exceeds a certain value,the stable laminar flow line can be generated continuously in the microchannel.The double emulsion wall thickness and the number of cores are determined by the flow rate of the middle fluid.Increasing the flow rate of the middle phase is expected to form a jet with uniform distribution of internal droplets.The slow transitions of generated flow patterns from dripping to jetting are realized with increasing the viscosity ratio.The formation rate of the double emulsion of dropping with low intermediate viscosity is much faster than that of jetting with large viscosity.The droplet size and formation time are not sensitive to the change of intermediate viscosity near μmo=1.With the decrease of interfacial tension ratio σoi,the internal phase jet is not easy to form,and the injection section in the device channel keeps retracting,adjusting the interfacial tension value has no obvious effect on changing the structural parameters of the double emulsion(2)A three-dimensional microfluidic device with matching and nesting capillaries was developed,and a visual experimental system for the preparation of phase change microcapsules by multi-phase microfluidic encapsulation was designed and built.The phase change microcapsule with the inner phase of RT25 and the outer phase of calcium alginate was prepared based on the precise regulation of multiple emulsion templates generated in the channel of microfluidic device,and the core-shell structure of phase change microcapsule was characterized by a scanning electron microscope.The regulation law of internal phase,the important thermophysical properties and the thermal regulation properties of phase change microcapsules were discussed in detail.The results indicated that the phase change microcapsules prepared based on microfluidic device have good monodispersity and the variation coefficient of contour size is less than 5%.The external size of phase change microcapsule and the content of inner phase core material can be accurately adjusted by varying the flow rate of each phase,and the effect of changing the flow rate of the internal phase on the above two is more effective than changing the flow rate of the external phase.The microcapsules with a large latent heat value and good thermal stability are indicated by the latent heat characteristics and thermal stability test.The thermal regulation performance of phase change microcapsule was proved to be good by comparing the thermal regulation performance of gypsum model houses with different FR values.In addition,the thermal conductivity of phase change materials RT25 is enhanced by adding multilayer graphene,and the results of thermophysical property test show that the addition of multilayer graphene can effectively improve the thermal conductivity while ensuring the energy storage density of phase change microcapsule(3)A three-dimensional microfluidic device with capillary matching and nesting was developed,and a visual experimental system for the preparation of phase change microfibers by multi-phase microfluidic encapsulation was designed and built.Based on the precise regulation of laminar liquid line templates generated in the channel of microfluidic device,phase change micro fibers with inner phase of RT25 and outer phase of P VB were prepared.The core-shell structure of the phase change microfiber was characterized,and the regulation law of the internal phase,the important thermophysical properties and thermal regulation properties of phase change microfibers were discussed in detail.The experimental results showed that phase change microfiber with complete core-shell structure and dense shell layer can be prepared successfully by multi-phase microfluidic encapsulation.The formation process of laminar fluid line template in microfluidic device is stable and continuous,so that the phase change microfibers obtained after the shell solidification have uniform cylindrical structure.The content of RT25 in phase change microfiber can be adjusted quickly by varying the velocity of internal phase flow.The content of phase change microfiber core material can reach 61.73%by increasing the internal phase flow rate,and the latent heat value of phase change is 122.7J/g.The results of thermophysical properties indicate that phase change microfibers can exhibit satisfactory thermal stability at ambient temperatures below 120℃.In addition,the phase change microfiber prepared in this paper has shown the advantage of slowing down the temperature change on the surface and inside of the measured object in the thermal regulation performance characterization experiment,which indicates that it has a satisfactory temperature regulation effect and has excellent stability in the use process.The relevant research results in this paper not only provide a new idea for the refined production of encapsulated phase change functional materials but also provide a reliable theoretical prediction tool for the microfluidic construction of phase change material templates.