34,618 research outputs found
Rocket motor system Patent
Igniter capsule for chemical ignition of liquid rocket propellant
Undulated cylinders of charged diblock copolymers
We study the cylinder to sphere morphological transition of diblock
copolymers in aqueous solution with a hydrophobic block and a charged block. We
find a metastable undulated cylinder configuration for a range of charge and
salt concentrations which, nevertheless, occurs above the threshold where
spheres are thermodynamically favorable. By modeling the shape of the cylinder
ends, we find that the free energy barrier for the transition from cylinders to
spheres is quite large and that this barrier falls significantly in the limit
of high polymer charge and low solution salinity. This suggests that observed
undulated cylinder phases are kinetically trapped structures
Planar cell polarity genes Frizzled3a, Vangl2, and Scribble are required for spinal commissural axon guidance
Background A fundamental feature of early nervous system development is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. Spinal commissural neurons are an attractive model to investigate the multiple guidance cues that control growth cone navigation both pre- and post-midline crossing, as well as along both the dorsal–ventral (D–V) and anterior–posterior (A–P) axes. Accumulating evidence suggests that guidance of spinal commissural axons along the A–P axis is dependent on components of the planar cell polarity (PCP) signaling pathway. In the zebrafish, the earliest born spinal commissural neuron to navigate the midline and turn rostrally is termed commissural primary ascending (CoPA). Unlike mammalian systems, CoPA axons cross the midline as a single axon and allow an analysis of the role of PCP components in anterior pathfinding in single pioneering axons. Results Here, we establish CoPA cells in the zebrafish spinal cord as a model system for investigating the molecular function of planar cell polarity signaling in axon guidance. Using mutant analysis, we show that the functions of Fzd3a and Vangl2 in the anterior turning of commissural axons are evolutionarily conserved in teleosts. We extend our findings to reveal a role for the PCP gene scribble in the anterior guidance of CoPA axons. Analysis of single CoPA axons reveals that these commissural axons become responsive to PCP-dependent anterior guidance cues even prior to midline crossing. When midline crossing is prevented by dcc gene knockdown, ipsilateral CoPA axons still extend axons anteriorly in response to A–P guidance cues. We show that this ipsilateral anterior pathfinding that occurs in the absence of midline crossing is dependent on PCP signaling. Conclusion Our results demonstrate that anterior guidance decisions by CoPA axons are dependent on the function of planar cell polarity genes both prior to and after midline crossing
Frustrated order on extrinsic geometries
We study, analytically and theoretically, defects in a nematically-ordered
surface that couple to the extrinsic geometry of a surface. Though the
intrinsic geometry tends to confine topological defects to regions of large
Gaussian curvature, extrinsic couplings tend to orient the nematic in the local
direction of maximum or minimum bending. This additional frustration is
unavoidable and most important on surfaces of negative Gaussian curvature,
where it leads to a complex ground state thermodynamics. We show, in
contradistinction to the well-known effects of intrinsic geometry, that
extrinsic curvature expels disclinations from the region of maximum curvature
above a critical coupling threshold. On catenoids lacking an "inside-outside"
symmetry, defects are expelled altogether.Comment: 4 pages, 3 figure
Near-field coupling of gold plasmonic antennas for sub-100 nm magneto-thermal microscopy
The development of spintronic technology with increasingly dense, high-speed,
and complex devices will be accelerated by accessible microscopy techniques
capable of probing magnetic phenomena on picosecond time scales and at deeply
sub-micron length scales. A recently developed time-resolved magneto-thermal
microscope provides a path towards this goal if it is augmented with a
picosecond, nanoscale heat source. We theoretically study adiabatic
nanofocusing and near-field heat induction using conical gold plasmonic
antennas to generate sub-100 nm thermal gradients for time-resolved
magneto-thermal imaging. Finite element calculations of antenna-sample
interactions reveal focused electromagnetic loss profiles that are either
peaked directly under the antenna or are annular, depending on the sample's
conductivity, the antenna's apex radius, and the tip-sample separation. We find
that the thermal gradient is confined to 40 nm to 60 nm full width at half
maximum for realistic ranges of sample conductivity and apex radius. To
mitigate this variation, which is undesirable for microscopy, we investigate
the use of a platinum capping layer on top of the sample as a thermal
transduction layer to produce heat uniformly across different sample materials.
After determining the optimal capping layer thickness, we simulate the
evolution of the thermal gradient in the underlying sample layer, and find that
the temporal width is below 10 ps. These results lay a theoretical foundation
for nanoscale, time-resolved magneto-thermal imaging.Comment: 24 pages including Supporting Information, 6 figures in the main
text, 4 supporting figure
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