Formation Mechanism And Filtration Application Of Electrospun Micro-nanofibers

Electrospun fibers exhibit excellent and unique size effect due to their micro-nano diameter and high specific surface area,endowing them with enormous application prospect in numerous fields.To realize the material’s high performance and multifunction,the research on the fiber size control is the basis and key to the material substrate design.However,the precise control of fiber diameter remains a challenge because the electrospinning system exists the problems that the jet motion is multistage,complicated disturbance and instability.Therefore,the electrospun fiber diameter control becomes a hot research direction for the functionalized fiber materials triggered by the scale effect.Firstly,this thesis summarizes the origin and development of the electrospinning technology,introduces the influencing factors of electrospinning and emphatically analyzes the effect of each factor on fiber diameter,and reviews the formation theory of electrospun fibers from three stages:Taylor cone,stable jet and whipping jet.Based on the coupling effect of multiple factors,a method of controlling the fiber diameter by the stable-jet length is proposed,and the mechanism of the stable-jet length controlling the fiber diameter under the effect of surface charge is studied.Based on the fact that the fiber diameter is mainly determined by the stretching of the whipping jet and the first spiral of the whipping jet（FSWJ）plays an important role in the jet stretching,the stretching mechanism of the FSWJ under the effect of surface charge is researched.Based on the reason that the jet velocity can directly control the jet stretching,the mechanism of the whipping-jet velocity is investigated.Subsequently,a method of the whipping-jet velocity controlling the fiber diameter is proposed,and the mechanism of the whipping-jet velocity controlling the fiber diameter under the effect of surface charge is explored.As a result,the efficient control of fiber diameter is achieved,giving the fiber materials with controllable diameter for high-efficiency air filtration.The main research contents of this thesis are as follows:（1）The mechanism of the stable jet controlling the fiber diameter is studied and scaling law models between the stable-jet length and the fiber diameter under the effect of surface charge are constructed.Based on the leaky dielectric theory,the slender-body theory and the three conservation laws of mass,charge and momentum,the scaling relation between the stable-jet length and the stable-jet critical radius obeying a power exponent-1/2 for fully charged jet is established.Confirming the growth rate of the bending perturbation and the forces of the whipping jet,the relationship between the stable-jet critical radius and the fiber diameter is built.The stable-jet critical radius is introduced to create the scaling law between the stable-jet length and the fiber diameter following a power exponent-2/3 for fully charged jet.While the power exponent of the scaling model is a function of the surface charge saturation for partly charged jet,and it increases with the surface charge saturation.Based on the interionic attraction theory,inorganic salt lithium chloride（LiCl）is utilized to tune the surface charge saturation of the poly（methyl methacrylate）（PMMA）fluid jet.It is found that the critical concentration of LiCl is about 1.0 wt%and the surface charge saturation is linear with the salt concentration.The mechanism on the stable-jet length controlling the fiber diameter under the effect of surface charge is verified by using PMMA spinning solutions with different LiCl concentrations.（2）The stretching mechanism of the whipping jet is researched and physical models on the jet radius and stretch rate of the FSWJ under the effect of surface charge are established.Based on electrostatics and hydrodynamics,comprehensive electrohydrodynamic models on the jet radius and stretch rate of the FSWJ for fully charged jet are built.Based on the Navier-Stokes equation and the motion differential equation in stress form,scaling models on the jet radius and stretch rate of the FSWJ along the axial direction for partly charged jet are created.A novel method is proposed to quantitatively measure the dynamic scale change of the stretching and refinement of the micro-nano whipping jet,verifying the stretching and refinement mechanism of the whipping jet.The results show that the spinning parameters have a great impact on the jet radius and stretch rate of the FSWJ,while they have little influence on the axial change rate of the jet radius and stretch rate.The surface charge saturation has a significant effect on the axial change rate of the jet radius and stretch rate.For the fully charged jet,the axial change rate of the jet radius and stretch rate is constant,which means that the power exponents of the jet radius and stretch rate models are-1/4 and-1/2respectively.For the partly charged jet,the axial change rate of the jet radius and stretch rate is variable,which means that the power exponents of the jet radius and stretch rate models are functions of the surface charge saturation.With the increase of the surface charge saturation,the power exponent of the jet radius model decreases（the axial refinement rate of the jet radius increases）and the power exponent of the jet stretch rate model increases（the axial decay rate of the jet stretch rate decreases）.The stretching and refinement mechanism on the whipping jet under the effect of surface charge is verified by using polystyrene（PS）spinning solutions and the PMMA spinning solutions with different LiCl concentrations.（3）The mechanism of the whipping-jet velocity is investigated,scaling models on the dynamic variation of the whipping-jet velocity in time and space are established,and a scaling model on the dynamic variation of the whipping-jet stretch rate in space is built.Based on the Earnshaw’s theorem,it is established that the scaling laws on the variations of the whipping-jet amplitude and velocity in time satisfy power exponents 2/3 and-1/3 respectively.The relationship between the whipping-jet amplitude and the axial distance is confirmed to create the scaling laws on the variations of the whipping-jet velocity and stretch rate in space obeying power exponents-1/2 and-3/2 respectively.The mechanism on the dynamic variation of the whipping-jet velocity is verified by applying the high-speed photography technology for experimental characterization and combining the Lagrange method and the Euler method.The results show that the jet velocity and stretch rate are dominantly determined by the surface charge.The spinning parameters have little influence on the axial change rate of the jet velocity and stretch rate,while the surface charge density has a great effect on that.The highest velocity(about 1.8 m·s^-1)and maximum stretch rate(about 142 s^-1)of the PS whipping jet with low surface charge density are small,while the velocity(about 5.4 m·s^-1)and stretch rate(about 513 s^-1)of the LiCl/PMMA whipping jet with high surface charge density are large.（4）The mechanism of the whipping jet controlling the fiber diameter is explored,and it is embedded to deconstruct the structure regulation of fiber membrane for achieving the fiber diameter control in air filtration field.Based on the jet dynamic theory,a scaling model of the whipping-jet velocity is constructed to control the fiber diameter under the effect of surface charge.The fiber diameter is found to decrease as a power function with increasing the whipping-jet velocity,and the power exponent is closely related to the surface charge density.Poly（vinylidene fluoride）/polyacrylonitrile（PVDF/PAN）spinning solutions with different LiCl concentrations are used to control the formed fiber diameter by regulating the whipping-jet velocity under the effect of surface charge.It realizes the application of fiber material for high-efficiency air filtration.The results show that the increase of the whipping-jet velocity significantly reduces the fiber diameter,which improves the filtration performance of the fiber membrane.When the fiber diameter and the CV value of fiber diameter are decreased to 183 nm and 16.2%respectively,an ultralight membrane(0.521 g·m^-2)possesses high filtration efficiency（99.93%）and low pressure drop（105.2 Pa）against ultrafine aerosol particles（≤0.26μm）under an airflow face velocity of 5.33 cm·s^-1.This thesis systematically studies the mechanism of the electrospinning-jet stretching to control the formed fiber diameter and achieves an efficient control of it.It enriches and expands the formation theory of electrospun fibers,and deepens the understanding of the micro-nanofiber formation process.It provides a theoretical guide for the control of jet stretching and refinement,and a new theoretical tool and experimental evidence for the controllable fiber size.Meanwhile,it contributes to promoting the further development of electrospinning technology,and improving the function development and potential application of high-quality micro-nanofiber materials.