6,538 research outputs found
Data management study, volume 5. Appendix A - Contractor data package technical description and system engineering /SE/ Final report
Technical description and systems engineering contractor data package for Voyager spacecraf
Islands of conformational stability for Filopodia
Filopodia are long, thin protrusions formed when bundles of fibers grow outwardly from a cell surface while remaining closed in a membrane tube. We study the subtle issue of the mechanical stability of such filopodia and how this depends on the deformation of the membrane that arises when the fiber bundle adopts a helical configuration. We calculate the ground state conformation of such filopodia, taking into account the steric interaction between the membrane and the enclosed semiflexible fiber bundle. For typical filopodia we find that a minimum number of fibers is required for filopodium stability. Our calculation elucidates how experimentally observed filopodia can obviate the classical Euler buckling condition and remain stable up to several tens of . We briefly discuss how experimental observation of the results obtained in this work for the helical-like deformations of enclosing membrane tubes in filopodia could possibly be observed in the acrosomal reactions of the sea cucumber Thyone, and the horseshoe crab Limulus. Any realistic future theories for filopodium stability are likely to rely on an accurate treatment of such steric effects, as analysed in this work
Behavioral responses to “alarm odors” in potentially invasive and non-invasive crayfish species from aquaculture ponds.
Spicules and the effect of rigid rods on enclosing membrane tubes
Membrane tubes (spicules) arise in cells, or artificial membranes, in the
nonlinear deformation regime due to, e.g. the growth of microtubules, actin
filaments or sickle hemoglobin fibers towards a membrane. We calculate the
axial force exerted by the cylindrical membrane tube, and its average radius,
by taking into account steric interactions between the fluctuating membrane and
the enclosed rod. The force required to confine a fluctuating membrane near the
surface of the enclosed rod diverges as the separation approaches zero. This
results in a smooth crossover of the axial force between a square root and a
linear dependence on the membrane tension as the tension increases and the tube
radius shrinks. This crossover can occur at the most physiologically relevant
membrane tensions. Our work may be important in (i) interpreting experiments in
which axial force is related to the tube radius or membrane tension (ii)
dynamical theories for biopolymer growth in narrow tubes where these
fluctuation effects control the tube radius.Comment: 10 pages, 1 figur
Preliminary Solar Sail Design and Fabrication Assessment: Spinning Sail Blade, Square Sail Sheet
Blade design aspects most affecting producibility and means of measurement and control of length, scallop, fullness and straightness requirements and tolerances were extensively considered. Alternate designs of the panel seams and edge reinforcing members are believed to offer advantages of seam integrity, producibility, reliability, cost and weight. Approaches to and requirements for highly specialized metalizing methods, processes and equipment were studied and identified. Alternate methods of sail blade fabrication and related special machinery, tooling, fixtures and trade offs were examined. A preferred and recommended approach is also described. Quality control plans, inspection procedures, flow charts and special test equipment associated with the preferred manufacturing method were analyzed and are discussed
Scaling in the time-dependent failure of a fiber bundle with local load sharing
We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to ,
where is the lifetime of the bundle, and is a quite
universal scaling exponent. The average lifetime of the bundle scales
with the system size as , where depends on the
distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.
Behavior of composite - beam to column joints (manuscript for preprint, ASCE Struct. Eng. Meeting, Louisville, Kentucky, April 14-18, 1969) (-20-9)
Bounds for the time to failure of hierarchical systems of fracture
For years limited Monte Carlo simulations have led to the suspicion that the
time to failure of hierarchically organized load-transfer models of fracture is
non-zero for sets of infinite size. This fact could have a profound
significance in engineering practice and also in geophysics. Here, we develop
an exact algebraic iterative method to compute the successive time intervals
for individual breaking in systems of height in terms of the information
calculated in the previous height . As a byproduct of this method,
rigorous lower and higher bounds for the time to failure of very large systems
are easily obtained. The asymptotic behavior of the resulting lower bound leads
to the evidence that the above mentioned suspicion is actually true.Comment: Final version. To appear in Phys. Rev. E, Feb 199
Probabilistic Approach to Time-Dependent Load-Transfer Models of Fracture
A probabilistic method for solving time-dependent load-transfer models of
fracture is developed. It is applicable to any rule of load redistribution,
i.e, local, hierarchical, etc. In the new method, the fluctuations are
generated during the breaking process (annealed randomness) while in the usual
method, the random lifetimes are fixed at the beginning (quenched disorder).
Both approaches are equivalent.Comment: 13 pages, 4 figures. To appear in Phys.Rev.
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