557 research outputs found
Transient modeling of the thermohydraulic behavior of high temperature heat pipes for space reactor applications
Many proposed space reactor designs employ heat pipes as a means of conveying heat. Previous researchers have been concerned with steady state operation, but the transient operation is of interest in space reactor applications due to the necessity of remote startup and shutdown. A model is being developed to study the dynamic behavior of high temperature heat pipes during startup, shutdown and normal operation under space environments. Model development and preliminary results for a hypothetical design of the system are presented
The Dynamic Transition of Protein Hydration Water
Thin layers of water on biomolecular and other nanostructured surfaces can be
supercooled to temperatures not accessible with bulk water. Chen et al. [PNAS
103, 9012 (2006)] suggested that anomalies near 220 K observed by quasi-elastic
neutron scattering can be explained by a hidden critical point of bulk water.
Based on more sensitive measurements of water on perdeuterated phycocyanin,
using the new neutron backscattering spectrometer SPHERES, and an improved data
analysis, we present results that show no sign of such a fragile-to-strong
transition. The inflection of the elastic intensity at 220 K has a dynamic
origin that is compatible with a calorimetric glass transition at 170 K. The
temperature dependence of the relaxation times is highly sensitive to data
evaluation; it can be brought into perfect agreement with the results of other
techniques, without any anomaly.Comment: 4 pages, 3 figures. Phys. Rev. Lett. (in press
The dynamical transition in proteins and non-Gaussian behavior of low frequency modes in Self Consistent Normal Mode Analysis
Self Consistent Normal Mode Analysis (SCNMA) is applied to heme c type
cytochrome f to study temperature dependent protein motion. Classical Normal
Mode Analysis (NMA) assumes harmonic behavior and the protein Mean Square
Displacement (MSD) has a linear dependence on temperature. This is only
consistent with low temperature experimental results. To connect the protein
vibrational motions between low temperature and physiological temperature, we
have incorporated a fitted set of anharmonic potentials into SCNMA. In
addition, Quantum Harmonic Oscillator (QHO) theory has been used to calculate
the displacement distribution for individual vibrational modes. We find that
the modes involving soft bonds exhibit significant non-Gaussian dynamics at
physiological temperature, which suggests it may be the cause of the
non-Gaussian behavior of the protein motions probed by Elastic Incoherent
Neutron Scattering (EINS). The combined theory displays a dynamical transition
caused by the softening of few "torsional" modes in the low frequency regime (<
50cm-1or 0.6ps). These modes change from Gaussian to a classical
distribution upon heating. Our theory provides an alternative way to understand
the microscopic origin of the protein dynamical transition.Comment: 17 pages, 6 figures, 1 tabl
Collective dynamics of strain-coupled nanomechanical pillar resonators
Semiconductur nano- and micropillars represent a promising platform for hybrid nanodevices. Their ability to couple to a broad variety of nanomechanical, acoustic, charge, spin, excitonic, polaritonic, or electromagnetic excitations is utilized in fields as diverse as force sensing or optoelectronics. In order to fully exploit the potential of these versatile systems e.g. for metamaterials, synchronization or topologically protected devices an intrinsic coupling mechanism between individual pillars needs to be established. This can be accomplished by taking advantage of the strain field induced by the flexural modes of the pillars. Here, we demonstrate strain-induced, strong coupling between two adjacent nanomechanical pillar resonators. Both mode hybridization and the formation of an avoided level crossing in the response of the nanopillar pair are experimentally observed. The described coupling mechanism is readily scalable, enabling hybrid nanomechanical resonator networks for the investigation of a broad range of collective dynamical phenomena
Glass transition in biomolecules and the liquid-liquid critical point of water
Using molecular dynamics simulations, we investigate the relation between the
dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and
thermodynamic properties of hydration water. We find that the dynamic
transition of the macromolecules, sometimes called a ``protein glass
transition'', occurs at the temperature of dynamic crossover in the diffusivity
of hydration water, and also coincides with the maxima of the isobaric specific
heat and the temperature derivative of the orientational order parameter.
We relate these findings to the hypothesis of a liquid-liquid critical point in
water. Our simulations are consistent with the possibility that the protein
glass transition results from crossing the Widom line, which is defined as the
locus of correlation length maxima emanating from the hypothesized second
critical point of water.Comment: 10 Pages, 12 figure
Influence of the Environment Fluctuations on Incoherent Neutron Scattering Functions
In extending the conventional dynamic models, we consider a simple model to
account for the environment fluctuations of particle atoms in a protein system
and derive the elastic incoherent structure factor (EISF) and the incoherent
scattering correlation function C(Q,t) for both the jump dynamics between sites
with fluctuating site interspacing and for the diffusion inside a fluctuating
sphere. We find that the EISF of the system (or the normalized elastic
intensity) is equal to that in the absence of fluctuations averaged over the
distribution of site interspacing or sphere radius a. The scattering
correlation function is ,
where the average is taken over the Q-dependent effective distribution of
relaxation rates \lambda_n(a) and \psi(t) is the correlation function of the
length a. When \psi(t)=1, the relaxation of C(Q,t) is exponential for the jump
dynamics between sites (since \lambda_n(a) is independent of a) while it is
nonexponential for diffusion inside a sphere.Comment: 7 pages, 7 eps figure
P16-47. Interleukin (IL)-21 induces cytolytic molecule perforin in CD4 and CD8 T cells without CD4 activation in chronically SIV infected rhesus macaques
A multiscale multilayer vertically integrated model with vertical dynamics for CO<sub>2</sub> sequestration in layered geological formations
First Documentation that Henslow\u27s Sparrow Regularly Occurs During the Breeding and Wintering Seasons in Arkansas
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