Polyethylene Oxide Nanofiber Production by Electrospinning

Electrospinning is an inexpensive technique that is used to produce nanofibers for a variety of applications. In electrospinning, a polymer solution is dispensed from a hypodermic-like syringe where an intense electric field attracts the solution to a collector while drawing the polymer into a very thin fiber. The diameter of the fiber can be controlled by tuning the process parameters such as the applied electric field, solution flow rate, distance between syringe tip and collector, and the collector geometry. In this paper we describe results from electrospinning poly(ethylene oxide) (PEO), a likely candidate for applications involving scaffolding for tissue engineering. The PEO nanofibers were fabricated from different polymer solution concentrations ranging from 14% - 22% (by weight). Each sample was then imaged using a scanning electron microscope. The morphology of the fibers produced from varying solution concentrations is discussed

Eutectic Colony Formation: A Stability Analysis

Experiments have widely shown that a steady-state lamellar eutectic solidification front is destabilized on a scale much larger than the lamellar spacing by the rejection of a dilute ternary impurity and forms two-phase cells commonly referred to as `eutectic colonies'. We extend the stability analysis of Datye and Langer for a binary eutectic to include the effect of a ternary impurity. We find that the expressions for the critical onset velocity and morphological instability wavelength are analogous to those for the classic Mullins-Sekerka instability of a monophase planar interface, albeit with an effective surface tension that depends on the geometry of the lamellar interface and, non-trivially, on interlamellar diffusion. A qualitatively new aspect of this instability is the occurence of oscillatory modes due to the interplay between the destabilizing effect of the ternary impurity and the dynamical feedback of the local change in lamellar spacing on the front motion. In a transient regime, these modes lead to the formation of large scale oscillatory microstructures for which there is recent experimental evidence in a transparent organic system. Moreover, it is shown that the eutectic front dynamics on a scale larger than the lamellar spacing can be formulated as an effective monophase interface free boundary problem with a modified Gibbs-Thomson condition that is coupled to a slow evolution equation for the lamellar spacing. This formulation provides additional physical insights into the nature of the instability and a simple means to calculate an approximate stability spectrum. Finally, we investigate the influence of the ternary impurity on a short wavelength oscillatory instability that is already present at off-eutectic compositions in binary eutectics.Comment: 26 pages RevTex, 14 figures (28 EPS files); some minor changes; references adde

Cosmological constraints on primordial black holes produced in the near-critical gravitational collapse

The mass function of primordial black holes created through the near-critical gravitational collapse is calculated in a manner fairly independent of the statistical distribution of underlying density fluctuation, assuming that it has a sharp peak on a specific scale. Comparing it with various cosmological constraints on their mass spectrum, some newly excluded range is found in the volume fraction of the region collapsing into black holes as a function of the horizon mass.Comment: 9 pages. Typos corrected. To appear in Physical Review

Attentive Learning of Sequential Handwriting Movements: A Neural Network Model

Defense Advanced research Projects Agency and the Office of Naval Research (N00014-95-1-0409, N00014-92-J-1309); National Science Foundation (IRI-97-20333); National Institutes of Health (I-R29-DC02952-01)