Research On Molding And Control Of Millimeter-level PDVB Hollow Spheres By Microfluidic Method

Millimeter-scale poly divinyl benzene(PDVB)foam shells were supposed to be the most suitable cryogenic fuel capsules because it could prevent the reduction of an energy yield caused by oxygen in Inertial Fusion Energy(IFE)experiments.However,there were some challenges to be faced about the instability of fluid mechanics and thermodynamics due to the millimeter-sized characteristic dimension when the double emulsion precursors were formed by microfluidics and when they were gelled in situ later on.For a long time,the double emulsion stability and survival,size control and geometric morphology determined the poor quality of the target spheres which limited their use in IFE experiments.In this work,the double Y-shaped compound microfluidic channel and the simulating microgravity flow field were designed to obtain the millimeter-scale PDVB foam shells with low poly-dispersity,out-of-roundness,non-concentricity and density.The dissertations covered the development of PDVB shells,and,the method,principle and technical parameters for the control of foam forming,which were summarized as follows:First,a novel double Y-shaped compound channel(the combination of co-flowing and flow-focusing geometry)was developed by transforming the channels' geometry.The results of Computational fluid dynamics(CFD)illustrated the stronger lateral shear(shearing)and extrusion(squeezing)due to the symmetric and nonorthogonal contact between the three fluid phases.Meanwhile,the more stable hydraulics focusing and the surrounding by three phase interface could be obtained to avoid the fluid mechanics stagnation and the wettability which usually occurred in orthogonal channel.The forming patterns were transformed from jetting,dripping into squeezing with the decrease of capillary number(Caouter).Perfect dripping occurred when both capillary number(Caouter?(0.018,0.09))and Webber number(Wedrop?(8.7×10-4,9.96×10-3))were simultaneously low enough.Furthermore,the great velocity fluctuations,the variations in drop size and the metastability of the produced emulsion were also not observed during the regular dripping.Efficient encapsulation(100%)and high monodispersity were afforded by one-step dual emulsification.Second,various factors were systematically investigated for precise size-control of the dripping in partly confined channel geometry,such as the outlet channel size,the fluid properties and the flow rates.Experiments showed both the diameter and thickness of the compound droplet increased dramatically with the increases of channel size.When the emulgator concentration was less than the critical micelle concentration(0.5wt%SPAN80),the larger spherical emulsions could be formed with uniform geometry.The size of the droplets changed greatly with the viscosity and the flow rate of the outer water phase.The parameters,such as a comparable outlet channel,a 0.4wt%SPAN80 in the middle phase and a viscosity ratio of 3:1 between the outer water phase and the middle oil phase,were ascertained as the optimized approach.Moreover,the stable flow of three fluids and the good-matching fluid flow rates could be maintained to afford the low coefficient of variation(CV<0.2%)of the adjustable double-emulsion dimensions(ddrop:0.5?5.4 mm,tshell:50?350 ?m).Third,the mechanism of millimeters sized double emulsion formation was discussed quantitatively by the means of experimental results.The calculated results showed that the relevant nondimensional numbers for a typically regular dripping were similar in value to those in the general microfluidic devices.This kind of millifluidic flows were converted to microfluidic hydrodynamic features due to the technique of two angled junctions,coaxial capillaries and compound channels geometry.Furthermore,the effects of the rival forces due to the fluid factors on droplet formation were clarified in a quantitative way to explain the dynamic behaviors of double-emulsion formation and to establish the mechanism of droplet breakup.It could be seen that the actual viscous shearing force was magnified by the shear-driven model due to the geometric restriction while the pressure drop along the growing droplet could not be neglected.The results showed that this kind of dripping did not entirely undergo shear-driven breakup but a dynamics mechanism combining squeezing and shearing due to the confinement of the outlet channel.Fourth,based on the full-filled laminar flow field and the density matching approach,the rotary curing reaction system was designed to afford the millimeter-scale PDVB foam shells by the in situ curing of double emulsions.The flow field characteristics in the rotating cylindrical bottle filling cavity was investigated by the means of experiments and computational fluid dynamics(CFD)simulation method.Simulating microgravity flow field was set up,in which double-emulsion drops were suspended with spontaneous activity.It has overcome the difficulties caused by the attenuation of large size emulsion interface,gravity gradient and curing process density mismatch.The inner water spherical drop was centered in the oil phase liquid film,which provided a natural conditions and a good security for in situ polymerization with a 100%survival rate.There were no gas-liquid interface,supersaturated air and strong shearing,millimeter-scale double emulsion could be gelled in situ with a spherical shape and concentric geometry(respectively,sphericity>99%and concentricity>99%).Finally,the kinetics of the polymerization of divinyl benzene(DVB)were discussed before the effects of temperature,monomer and initiator on the gelation process were systematically investigated.And then,the optimized approach for the gelling process of the millimeter-scale W1/O/W2 double emulsions was used to fabricate the predetermined-size foam shells to satisfy the IFE experiments.The oil phase(O)containing 4?17vol%DVB80,3wt%BAPO and 3wt%AIBN,and,0.4wt%SPAN80 in DBP solution were introdued to obtain foam shells.The double emulsions precursors were exposed to a UV light and heated up by elevating temperature 5? every 5 minutes for polymerization.After 45 minutes,temperature was fixed at 70 ? until polymerization finished.Two weeks later,the intact PDVB foam shells with an adjustable 1?5 mm diameter,a 100?300 ?m wall thickness and a 40?160 mg/cm3 density were obtained by CO2 supercritical dryer.Among of them,the percentage of intact shells was more than 65%with a less than 0.6%poly-dispersity,0.5%OOR and 1.0%NC,respectively.The results indicated that a combination of photo-and thermo-polymerization improved cross-linking reaction process of the low monomer concentration emulsion.This work provided a practical guideline for mass production in creating size-desired,geometrically uniform and low density polymer hollow spheres from comparable double emulsions.