Mechanism Of Single Bubble Electrospinning For Nanofiber Fabrication

Electrospinning has now become one of the most important and basic methods for fabricating nanofibers.Because of their excellent properties,such as high specific surface area,unique netted or porous structure,the electrospun nanofibers have been widely applied to many fields,for examples,textile and clothing industry,environment engineering,bioscience and biotechnology,medicine and health,energy storage,military and anti-terrorism security.This thesis begins with a brief introduction to the history,development and current research focus of the electrospinning technology,then elucidates the properties of nano-materials and illustrates various attractive applications.Though the technology is promising and challenging,there are some shortcomings which have to be solved before mass-production.To this end,bubble electrospinning is invested,and its principle is analyzed.During bubble electrospinning,bubbles are produced on the polymer solution surface by air flow,and bubble surfaces are charged under a high DC voltage,which induces electric force acting on the bubble surfaces,as a result Taylor-like cones emerge on the bubble surfaces.When the DC voltage is high than a threshold,multiple charged jets are ejected and collected on the collector as nanofibers.Because of the large surface area of the bubbles,many Taylor-like cones emerge on the surface of each bubble simultaneously,which can significantly improve the throughout of nanofibers.The formation of the nanofibers in bubble electrospinning process and the advantages of the bubble electrospinning method over the common electrospinning are described in Chapter 2.In order to investigate the controllability of the bubble electrospinning,formation and interaction of multi-bubble is exhibited and analyzed theoretically.Due to the inherent instability of the multi-bubble,it is difficult to study the effects in the bubble electrospinning process on the morphology of the obtained nanofibers,the thesis,therefore,invests a novel device called the single bubble electrospinning,where only one bubble is produced on the surface of the polymer solution.Chapter 3 aims electrically at theoretical study of the single bubble electrospinning.Firstly,the method and whole process of the single bubble electrospinning is described systematically,including formation of the single bubble and the Taylor-like cones,motion of the jets,and solidification and deposit of the nanofibers onto the grounded plane.This kind of the method not only overcomes the multi-bubble instability,but also simplifies electrospinning environments,where a single bubble can produce multiple jets solidified on the collector.This kind of device is very much suitable for both theoretical analysis and experimental verification.Secondly,according to the characteristics of electrostatic charge distributed in the bubble electrospinning system,electric field distribution is evaluated by the electric image method.Solving the governing equations for the electric field distribution reveals that the electric field is extremely non-uniform,that will greatly affect the spinning process.The formations of the Taylor-like cones and jets,the stretch of the jets,and the solidification of the nanofibers take the advantages of the non-uniform electric field.Thirdly,the electric current in each charged jet,consisting of conduction current and displacement current,is studied,which induces a magnetic field in the electrospinning space,and as a result a magnetic force is excited acting on the jets during the electrospinning procedure.A reasonable explanation of the initial instability of the jet motion is discovered through analysis of the magnetic effects.It is argued that the jet instability results in a longer way of each jet to the collector,and it is helpful for complete stretching and solidifying process of the electrospun nanofibers.Chapter 4 gives a quantitative analysis of mechanism of the laminar flow in each charged jet,and elucidates that the flow pattern pushes polymer molecules together into a parallel arrangement,which affects crystallization of the electrospun nanofibers greatly.According to the interaction of the polymer molecule chains and the flow layers in the jets,an allometric scaling law between average molecular weight and solution viscosity is obtained,and the scaling relationship between the diameter of the electrospun nanofibers and molecular weight is also derived,which agrees well with the experimental data from the open literature.Chapter 5 focuses itself on study of the effects of the solution properties,processing parameters and temperature on morphology of the electrospum nanofibers.The study reveals that polymer concentration plays an important role for nanofiber morphology.When the concentration of the polymer solution is low,it is difficulty to obtain continuous fibers but beads,however,when the concentration becomes higher,beads disappear gradually,and nanofibers are obtained when the voltage arrives at the threshold value.The effect of the concentration on nanofiber diameter is studied when the voltage is above its threshold:it is shown that diameter of the nanofibers increases with the solution concentration as the approximate relationship d?C⁴,where d is the diameter of the nanofibers,and C is the concentration of the polymer solution.Bubble size is included in the effects of processing parameters on the diameter of the nanofibers.The experimental observation shows that larger bubbles results in the smaller nanofibers,which can be approximated using the following relationship d????,where d is the diameter of the nanofibers,and D₀ is the diameter ofbubble-making tube.Furthermore,the effect of temperature is studied experimentally.Temperature effects on the morphology of the nanofibers through viscocity,surface tension,and electric conductibility of the polymer solution,which bring favorable conditions for small diameter of the nanofibers when the temperature increases.