A Scenario of a Fiber Formation Mechanism in Electrospinning: Jet Evolves Assemblies of Phase-Separated Strings That Eventually Split into As-spun Fibers Observed on the Grounded Collector

Electrospinning of polymer solutions involves self-organization of dissipative structures to order through fluctuations, providing one of the fundamental research topics in statistical physics of nonequilibrium systems. In this work, we found that the electrospun poly(vinyl alcohol) aqueous solutions evolve, in cascade, hierarchically self-organized dissipative structures along the electrospinning line as follows: First the flow-induced orientation and concentration fluctuations of polymer chains in the single-phase solution and subsequently a phase-separated structure composed of polymer-rich solution phases dispersed in polymer-poor matrix solution phase; the polymer-rich solution phases evolve assemblies of strings (designated as a "pre-string phase") that are composed of a bundle of chains oriented along the jet axis, which are swollen with solvent prior to crystallization. The pre-string phase eventually orders into "strings"with varying lateral size via intra- A nd inter-hydrogen bonds of highly stretched chains and subsequent solidification. The polymer-poor matrix solution phase itself subsequently evolves in cascade essentially the same dissipative structure as described above, viz., the assemblies of the newly born strings dispersed in the polymer-poor matrix solution phase, induced by a subsequent increase in polymer concentration in the matrix solution phase with increasing curvilinear distance z along the jet line via syneresis and vaporization of water molecules. We discovered that the assemblies of the strings grown in the jet are eventually split from the flowing jet line, through the processes of the jet whipping and jet colliding against the grounded collector, to result in the so-called as-spun fibers on the collector. The as-spun fibers are the stacks of the strings with varying lateral size ordered through solidification. We found that the cross sections of the fractured stacks of the strings were flattened into a ribbon shape due to deformation induced by the lateral pressure imposed on the stacks during their splitting and colliding process. We discovered also the fact that the splitting of the jet into the stacks of the strings and their piling on the already deposited as-spun fibers on the grounded collector is a universal phenomenon for various electrospun fibers, as will be described in the text.