Formation Mechanism And Application In Interfacial Toughening Of Bubble-Electrospun Nanofibers

Nanofibers are defined as ductile materials with diameter less than100nm and length diameter ration more than100. Nowadays, nanofibers could be comprehensively applied to bio-technology, environment, energy, medical treatment, filtration, sensor and other industries. Electrospinning, as one simple and straightforward fabrication process, is widely adopted. Whereas, the drawbacks in fibers output and fineness to some extent impose restrictions on the future development of Electrospinning. To get rid of those problems, based on the mechanism of spider spinning, Bubble-electrospinning applies viscous bubbles into electrospinning process to prepare nanofibers with high quality and efficiency. It is a fantastic technique, which can impove the output and reduce the fibers fineness effectively.In this research, in the beginning we discuss the effects of operating parameters(such as voltage, collecting distance, flow rate) and the solution property(such as viscosity, conductivity, surface tension, solvent volatility) on the bubble deformation process, the movement of jets and the morphology of as-spun nanofibers. The effects of voltage and solution viscosity are focused to be analyzed. The scaling laws about fiber diameter and voltage:d∝V4, and fiber diameter and viscosity:d∝η2/3, are respectively deduced. And both of their accuracies are confirmed by the experiment results.The dynamics characteristics of bubble and charged jets in the electric field are the core content of this thesis. The bubble rupture process is filmed by high speed camera. Accordingly, The half angle of taylor cone measured is33.02°, which is coincided with the theoretical value that is caculated theoretically. We analyze the directional motion of jets using d’Alembert principle, and discussed the reason why the rim of bubble forms neck-like configuration. Then, using dimension analysis, we obtain the relation between the initial velocity of a debris with the film thickness, which reads where FE is the electronic force, ρ is the liquid density and h is the film thickness. The constants λ and β can be determined by experiments. As a result, the initial velocity can be obtained by an energy approach. It can be predicted that smaller thickness results in a larger ejection velocity, this is the reasons why multiple jets from upper half of the bubble, because the upper half has smaller h than that of the bottom half. Through the analysis and discussions in detail, a law on the bubble-electrospun production formation is concluded:In case the ruptured film retracts to be daughter bubbles. The daughter bubbles may continues to be broken to form sub-daughter bubbles, and the process repeats until some a hierarchical ruptured bubble is pulled upwards to form a charged jet, nanofibers are obtained; when the thickness and the width of the jet satisfies h/a=4π,fibers are bubble-electrospun with diameter of4h; and when a�'h, the thickness scales with the width, micro-spheres can be found.Furthermore, oscillation theory is introduced to elaborate the nanofibers shaping mechanism. A short strip sticked to a large piece of a ruptured film is formed, which is regarded as a thin rectangular plate. The plate’s natural frequency fn can be caculated by Rayleigh’s method approximately: In case far from the natural frequency, no pulsation occurs, and continuous strip-like fibers can be obtained; when h/a2≌fn/k, the plate pulsates (that is, they oscillate in size) at its natural frequency Consequently backbone-like fibers are formed. In case fe≌fn, the frequency of external excitation fe is close to fn, the plate resonates at the same time absorbs energy from external surroundings to maximum extent. The plate can’t afford that, and then plate texture is damaged, micro-spheres are likely to be formed.With the aid of Fluent software, the space distribution of electric field is demonstrated vividly. And the jet motion is simulated under the circumstance whether the electric field exsits or not. The velocity contours show the bending instability happens when the jet flies to the collector. And this phenomenon can be verified from the optical photos of spinning process.At last, the application of bubble-electrospun nanofibers is explored. PEI nanofibers mebranes are used to improve the interfacial toughness of the carbon fiber reinforced composites. When the thickness of membrane is0.058±0.007mm, the value of GIC of DCB sample increases by114.55%in contrast to the untoughened, which is attributed to the nail-anchorage action.