4,902 research outputs found
Laminar free convection heat transfer in a vertical channel with linear wall temperatures
Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1961.MIT copy bound with: Transient responses in gaseous fractionation by adsorption - analogue computer study / Donald D. Easson. 1961.Includes bibliographical references (leaves 48-49).by Nelson E. Stefany, Paul H. Fricke.B.S
A generalized theory of semiflexible polymers
DNA bending on length scales shorter than a persistence length plays an
integral role in the translation of genetic information from DNA to cellular
function. Quantitative experimental studies of these biological systems have
led to a renewed interest in the polymer mechanics relevant for describing the
conformational free energy of DNA bending induced by protein-DNA complexes.
Recent experimental results from DNA cyclization studies have cast doubt on the
applicability of the canonical semiflexible polymer theory, the wormlike chain
(WLC) model, to DNA bending on biological length scales. This paper develops a
theory of the chain statistics of a class of generalized semiflexible polymer
models. Our focus is on the theoretical development of these models and the
calculation of experimental observables. To illustrate our methods, we focus on
a specific toy model of DNA bending. We show that the WLC model generically
describes the long-length-scale chain statistics of semiflexible polymers, as
predicted by the Renormalization Group. In particular, we show that either the
WLC or our new model adequate describes force-extension, solution scattering,
and long-contour-length cyclization experiments, regardless of the details of
DNA bend elasticity. In contrast, experiments sensitive to short-length-scale
chain behavior can in principle reveal dramatic departures from the linear
elastic behavior assumed in the WLC model. We demonstrate this explicitly by
showing that our toy model can reproduce the anomalously large
short-contour-length cyclization J factors observed by Cloutier and Widom.
Finally, we discuss the applicability of these models to DNA chain statistics
in the context of future experiments
Goldstone-type fluctuations and their implications for the amorphous solid state
In sufficiently high spatial dimensions, the formation of the amorphous (i.e.
random) solid state of matter, e.g., upon sufficent crosslinking of a
macromolecular fluid, involves particle localization and, concommitantly, the
spontaneous breakdown of the (global, continuous) symmetry of translations.
Correspondingly, the state supports Goldstone-type low energy, long wave-length
fluctuations, the structure and implications of which are identified and
explored from the perspective of an appropriate replica field theory. In terms
of this replica perspective, the lost symmetry is that of relative translations
of the replicas; common translations remain as intact symmetries, reflecting
the statistical homogeneity of the amorphous solid state. What emerges is a
picture of the Goldstone-type fluctuations of the amorphous solid state as
shear deformations of an elastic medium, along with a derivation of the shear
modulus and the elastic free energy of the state. The consequences of these
fluctuations -- which dominate deep inside the amorphous solid state -- for the
order parameter of the amorphous solid state are ascertained and interpreted in
terms of their impact on the statistical distribution of localization lengths,
a central diagnostic of the the state. The correlations of these order
parameter fluctuations are also determined, and are shown to contain
information concerning further diagnostics of the amorphous solid state, such
as spatial correlations in the statistics of the localization characteristics.
Special attention is paid to the properties of the amorphous solid state in two
spatial dimensions, for which it is shown that Goldstone-type fluctuations
destroy particle localization, the order parameter is driven to zero, and
power-law order-parameter correlations hold.Comment: 20 pages, 3 figure
Dynamical Arrest in Attractive Colloids: The Effect of Long-Range Repulsion
We study gelation in suspensions of model colloidal particles with
short-ranged attractive and long-ranged repulsive interactions by means of
three-dimensional fluorescence confocal microscopy. At low packing fractions,
particles form stable equilibrium clusters. Upon increasing the packing
fraction the clusters grow in size and become increasingly anisotropic until
finally associating into a fully connected network at gelation. We find a
surprising order in the gel structure. Analysis of spatial and orientational
correlations reveals that the gel is composed of dense chains of particles
constructed from face-sharing tetrahedral clusters. Our findings imply that
dynamical arrest occurs via cluster growth and association.Comment: Final version: Phys. Rev. Lett. 94, 208301 (2005
Transitions of tethered polymer chains: A simulation study with the bond fluctuation lattice model
A polymer chain tethered to a surface may be compact or extended, adsorbed or
desorbed, depending on interactions with the surface and the surrounding
solvent. This leads to a rich phase diagram with a variety of transitions. To
investigate these transitions we have performed Monte Carlo simulations of a
bond-fluctuation model with Wang-Landau and umbrella sampling algorithms in a
two-dimensional state space. The simulations' density of states results have
been evaluated for interaction parameters spanning the range from good to poor
solvent conditions and from repulsive to strongly attractive surfaces. In this
work, we describe the simulation method and present results for the overall
phase behavior and for some of the transitions. For adsorption in good solvent,
we compare with Metropolis Monte Carlo data for the same model and find good
agreement between the results. For the collapse transition, which occurs when
the solvent quality changes from good to poor, we consider two situations
corresponding to three-dimensional (hard surface) and two-dimensional (very
attractive surface) chain conformations, respectively. For the hard surface, we
compare tethered chains with free chains and find very similar behavior for
both types of chains. For the very attractive surface, we find the
two-dimensional chain collapse to be a two-step transition with the same
sequence of transitions that is observed for three-dimensional chains: a
coil-globule transition that changes the overall chain size is followed by a
local rearrangement of chain segments.Comment: 17 pages, 12 figures, to appear in J. Chem. Phy
Secure Network-Centric Aviation Communication (SNAC)
The existing National Airspace System (NAS) communications capabilities are largely unsecured, are not designed for efficient use of spectrum and collectively are not capable of servicing the future needs of the NAS with the inclusion of new operators in Unmanned Aviation Systems (UAS) or On Demand Mobility (ODM). SNAC will provide a ubiquitous secure, network-based communications architecture that will provide new service capabilities and allow for the migration of current communications to SNAC over time. The necessary change in communication technologies to digital domains will allow for the adoption of security mechanisms, sharing of link technologies, large increase in spectrum utilization, new forms of resilience and redundancy and the possibly of spectrum reuse. SNAC consists of a long term open architectural approach with increasingly capable designs used to steer research and development and enable operating capabilities that run in parallel with current NAS systems
The Ethics of Eliminating Harmful Species: The Case of the Tsetse Fly
Wildlife species harmful to humans are often targets of control and elimination programs. A contemporary example is the tsetse fly, a vector of sleeping sickness and African animal trypanosomosis. Tsetse flies have recently been targeted by a pan-African eradication campaign. If it is successful, the campaign could push the entire tsetse family to extinction. With the emergence of effective and efficient elimination technologies, ethical assessment of proposed elimination campaigns is urgently needed. We examine the ethics of tsetse fly elimination by considering arguments predicated on both the instrumental and the intrinsic values of the species at local and global scales. We conclude that, although global eradication of tsetse flies is not ethically justified, localized elimination campaigns targeting isolated populations are ethically defensible. We urge assessments of this kind be conducted regularly and in context, so that all relevant factors underlying decisions on species elimination are routinely laid bare for evaluation
Feasibility of self-structured current accessed bubble devices in spacecraft recording systems
The self-structured, current aperture approach to magnetic bubble memory is described. Key results include: (1) demonstration that self-structured bubbles (a lattice of strongly interacting bubbles) will slip by one another in a storage loop at spacings of 2.5 bubble diameters, (2) the ability of self-structured bubbles to move past international fabrication defects (missing apertures) in the propagation conductors (defeat tolerance), and (3) moving bubbles at mobility limited speeds. Milled barriers in the epitaxial garnet are discussed for containment of the bubble lattice. Experimental work on input/output tracks, storage loops, gates, generators, and magneto-resistive detectors for a prototype device are discussed. Potential final device architectures are described with modeling of power consumption, data rates, and access times. Appendices compare the self-structured bubble memory from the device and system perspectives with other non-volatile memory technologies
Exact theory of kinkable elastic polymers
The importance of nonlinearities in material constitutive relations has long
been appreciated in the continuum mechanics of macroscopic rods. Although the
moment (torque) response to bending is almost universally linear for small
deflection angles, many rod systems exhibit a high-curvature softening. The
signature behavior of these rod systems is a kinking transition in which the
bending is localized. Recent DNA cyclization experiments by Cloutier and Widom
have offered evidence that the linear-elastic bending theory fails to describe
the high-curvature mechanics of DNA. Motivated by this recent experimental
work, we develop a simple and exact theory of the statistical mechanics of
linear-elastic polymer chains that can undergo a kinking transition. We
characterize the kinking behavior with a single parameter and show that the
resulting theory reproduces both the low-curvature linear-elastic behavior
which is already well described by the Wormlike Chain model, as well as the
high-curvature softening observed in recent cyclization experiments.Comment: Revised for PRE. 40 pages, 12 figure
A parameterization of flow separation over subaqueous dunes
Flow separation plays a key role in the development of dunes, and modeling the complicated flow behavior inside the flow separation zone requires much computational effort. To make a first step toward modeling dune development at reasonable temporal and spatial scales, a parameterization of the shape of the flow separation zone over two-dimensional dunes is proposed herein, in order to avoid modeling the complex flow inside the flow separation zone. Flow separation behind dunes, with an angle-of-repose slip face, is characterized by a large circulating leeside eddy, where a separation streamline forms the upper boundary of the recirculating eddy. Experimental data of turbulent flow over two-dimensional subaqueous bed forms are used to parameterize this separation streamline. The bed forms have various heights and height to length ratios, and a wide range of flow conditions is analyzed. This paper shows that the shape of the flow separation zone can be approximated by a third-order polynomial as a function of the distance away from the flow separation point. The coefficients of the polynomial can be estimated, independent of flow conditions, on the basis of bed form shape at the flow separation point and a constant angle of the separation streamline at the flow reattachment point. \ud
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