58,841 research outputs found
Shear Force Fiber Spinning: Process Parameter and Polymer Solution Property Considerations
For application of polymer nanofibers (e.g., sensors, and scaffolds to study cell behavior) it is important to control the spatial orientation of the fibers. We compare the ability to align and pattern fibers using shear force fiber spinning, i.e. contacting a drop of polymer solution with a rotating collector to mechanically draw a fiber, with electrospinning onto a rotating drum. Using polystyrene as a model system, we observe that the fiber spacing using shear force fiber spinning was more uniform than electrospinning with the rotating drum with relative standard deviations of 18% and 39%, respectively. Importantly, the approaches are complementary as the fiber spacing achieved using electrospinning with the rotating drum was ~10 microns while fiber spacing achieved using shear force fiber spinning was ~250 microns. To expand to additional polymer systems, we use polymer entanglement and capillary number. Solution properties that favor large capillary numbers (\u3e50) prevent droplet breakup to facilitate fiber formation. Draw-down ratio was useful for determining appropriate process conditions (flow rate, rotational speed of the collector) to achieve continuous formation of fibers. These rules of thumb for considering the polymer solution properties and process parameters are expected to expand use of this platform for creating hierarchical structures of multiple fiber layers for cell scaffolds and additional applications
Nucleation of quark matter in neutron stars cores
We consider the general conditions of quark droplets formation in high
density neutron matter. The growth of the quark bubble (assumed to contain a
sufficiently large number of particles) can be described by means of a
Fokker-Planck equation. The dynamics of the nucleation essentially depends on
the physical properties of the medium it takes place. The conditions for quark
bubble formation are analyzed within the frameworks of both dissipative and
non-dissipative (with zero bulk and shear viscosity coefficients) approaches.
The conversion time of the neutron star to a quark star is obtained as a
function of the equation of state of the neutron matter and of the microscopic
parameters of the quark nuclei. As an application of the obtained formalism we
analyze the first order phase transition from neutron matter to quark matter in
rapidly rotating neutron stars cores, triggered by the gravitational energy
released during the spinning down of the neutron star. The endothermic
conversion process, via gravitational energy absorption, could take place, in a
very short time interval, of the order of few tens seconds, in a class of dense
compact objects, with very high magnetic fields, called magnetars.Comment: 31 pages, 2 figures, to appear in Ap
Novel Precursors for Boron Nanotubes: The Competition of Two-Center and Three-Center Bonding in Boron Sheets
We present a new class of boron sheets, composed of triangular and hexagonal
motifs, that are more stable than structures considered to date and thus are
likely to be the precursors of boron nanotubes. We describe a simple and clear
picture of electronic bonding in boron sheets and highlight the importance of
three-center bonding and its competition with two-center bonding, which can
also explain the stability of recently discovered boron fullerenes. Our
findings call for reconsideration of the literature on boron sheets, nanotubes,
and clusters.Comment: 4 pages, 4 figures, 1 tabl
Geometry dependence of the clogging transition in tilted hoppers
We report the effect of system geometry on the clogging of granular material
flowing out of flat-bottomed hoppers with variable aperture size D. For such
systems, there exists a critical aperture size Dc at which there is a
divergence in the time for a flow to clog. To better understand the origins of
Dc, we perturb the system by tilting the hopper an angle Q and mapping out a
clogging phase diagram as a function of Q and D. The clogging transition
demarcates the boundary between the freely-flowing (large D, small Q) and
clogging (small D, large Q) regimes. We investigate how the system geometry
affects Dc by mapping out this phase diagram for hoppers with either a circular
hole or a rectangular narrow slit. Additionally, we vary the grain shape,
investigating smooth spheres (glass beads), compact angular grains (beach
sand), disk-like grains (lentils), and rod-like grains (rice). We find that the
value of Dc grows with increasing Q, diverging at pi-Qr where Qr is the angle
of repose. For circular apertures, the shape of the clogging transition is the
same for all grain types. However, this is not the case for the narrow slit
apertures, where the rate of growth of the critical hole size with tilt angle
depends on the material
Diving Deep into Sentiment: Understanding Fine-tuned CNNs for Visual Sentiment Prediction
Visual media are powerful means of expressing emotions and sentiments. The
constant generation of new content in social networks highlights the need of
automated visual sentiment analysis tools. While Convolutional Neural Networks
(CNNs) have established a new state-of-the-art in several vision problems,
their application to the task of sentiment analysis is mostly unexplored and
there are few studies regarding how to design CNNs for this purpose. In this
work, we study the suitability of fine-tuning a CNN for visual sentiment
prediction as well as explore performance boosting techniques within this deep
learning setting. Finally, we provide a deep-dive analysis into a benchmark,
state-of-the-art network architecture to gain insight about how to design
patterns for CNNs on the task of visual sentiment prediction.Comment: Preprint of the paper accepted at the 1st Workshop on Affect and
Sentiment in Multimedia (ASM), in ACM MultiMedia 2015. Brisbane, Australi
Self-organized criticality and directed percolation
A sandpile model with stochastic toppling rule is studied. The control
parameters and the phase diagram are determined through a MF approach, the
subcritical and critical regions are analyzed. The model is found to have some
similarities with directed percolation, but the existence of different boundary
conditions and conservation law leads to a different universality class, where
the critical state is extended to a line segment due to self-organization.
These results are supported with numerical simulations in one dimension. The
present model constitute a simple model which capture the essential difference
between ordinary nonequilibrium critical phenomena, like DP, and self-organized
criticality.Comment: 9 pages, 10 eps figs, revtex, submitted to J. Phys.
Development of hot drawing process for nitinol tube
In recent years, Nitinol, near-equiatomic nickel-titanium alloys, have found growing applications in medical technology and joining technology, due to their special characteristics such as shape memory, superplasticity and biocompatibility. The production of Nitinol tube cost-effectively remains a technical challenge. In this paper, we describe a hot drawing process for Nitinol tube production. A Nitinol tube blank and a metal core are assembled together. The assembly is hot drawn for several passes to a final diameter. The metal core is then plastically stretched to reduce its diameter and removed from the tube. Hot drawing process has been applied to Ni50.7Ti and Ni47Ti44Nb9 alloys. Nitinol tubes of 13.6 mm outer diameter and 1 mm wall thickness have been successfully produced from a tube blank of 20 mm outer diameter and 3.5 mm thickness
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