Bubble Electrospinning For Fabrication Of Nanoporous Fibers And Beaded Nanofibers And Their Mathematical Analysis

There have been significant advancements in the area of electrospinning and a variety of polymers are electrospun to form fibers on the nanoscale. Past research has focused on fabrication of these fibers from different polymeric solutions but the increasing need for a cheaper, more environmental friendly and safer alternative increases the demand to understand the feasibility of electrospinning from a polymer solution. Many researchers have proposed different methods which are available in the literatures. The main focus was to increase the mass production of nanofibers and also to get the smallest diameter of nanofibers, so it was very urgent to develop a technique which can produce mass production while reducing the size of the diameter. Most of the time, increasing the mass production and reducing the diameter of nanofibers affects the morphology of nanofibers. Furthermore, the formation of beaded nanofibers has been observed widely. Huge number of experimental works on the study of beaded nanofibers is available in the open literatures but theoretical analysis is rare or rather incomplete.Bubble electrospinning comes from spider spinning, so the relationship between spider spinning and bubble electrospinning has been explained here. The basic apparatus of bubble electrospinning is also discussed. Explanation of the bubble electrospinning method has also been hashed out. There are two major shortcomings of bubble electrospinning which have been pointed out. To overcome the first shortcoming of electrospinning and bubble electrospinning, a modified bubble electrospinning device has been suggested. The proposed bubble electrospinning system equipped with a tub containing hot water to increase the temperature of the polymer solution. This mechanism of bubble electrospinning consists of a solution reservoir with a gas tube feeding from the left/right side, and a metal electrode fixed along the centerline of the tube and a grounded collector over the reservoir. It has been observed that many small bubbles with different sizes were produced on the solution surface.To overcome the second shortcoming of electrospinning and bubble electrospinning, a new device named "single/multiple bubble electrospinning" was suggested.This device is mainly consists of a syringe, a battery and a tube filled with a solution.A syringe needle is inserted in the tube through the bottom.The tube is filled with the polymeric solution.The number of bubbles formed on the solution surface depends upon the diameter of the tube. In order to produce a single bubble, the tube with the smallest diameter was used and the diameter of the tube can be changed according to our requirement. Bubbles can be produced on the surface of solution by pressing the syringe pump.Furthermore these devices were used to study the effects of various electrospinning parameters such as applied voltage, distance between solution surface and collector, temperature difference and solution concentration on the products. Later, the effects of comparatively high voltage ranging from30kV to45kV which produces nonporous materials was studied. Before experimental work, three-dimensional thermo-electro-hydrodynamic model was developed. It was reduced to one-dimensional model to give simple mathematical models for studying the effects of applied voltage, distance between solution surface and collector, temperature difference and solution concentration on the diameter of nanofibers. To verify theoretical prediction, the comparison between experimental results and theoretical results was made, which shows good agreement.For all sets of experiments, polyvinylpyrrolidone (PVP-k30, Mw=40, OOOg/mol), and ethanol as a solvent was used. The polymer was dissolved into an ethanol at room temperature, then the mixture was stirred under magnetic stirring for24hours to get a homogeneous and transparent solution with the weight ratio9:1,and then the solution was cooled down at room temperature before use.During the experimental work, it was observed that the voltage supplied and the resultant electric field had an influence in the stretching and on the velocity of the jet. If we keep increasing the voltage, velocity will increase but velocity cannot be infinitely large. So there must exist, a critical velocity. Furthermore, the velocity can exceed this critical value ucr, if a higher voltage is applied. In that case jet dilates by decreasing its density, leading to the porosity of the electrospun fibers. This phenomenon was predicted theoretically and verified experimentally.In the last, theoretical formation of beaded nanofibers was discussed.A huge literature is available on the formation of beaded nanofibers. Although, lots of discussions have been made in the open literature about merits and demerits of beaded nanofibers yet theoretical analysis was rare or rather incomplete. The last chapter is related to theoretical study of the formation of beads. A detailed theoretical analysis has been made, both in the case of Newtonian and non-Newtonian polymer solutions.