105,860 research outputs found
Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces
Droplets impacting on a superheated surface can either exhibit a contact
boiling regime, in which they make direct contact with the surface and boil
violently, or a film boiling regime, in which they remain separated from the
surface by their own vapor. The transition from the contact to the film boiling
regime depends not only on the temperature of the surface and kinetic energy of
the droplet, but also on the size of the structures fabricated on the surface.
Here we experimentally show that surfaces covered with carbon-nanofibers delay
the transition to film boiling to much higher temperature compared to smooth
surfaces. We present physical arguments showing that, because of the small
scale of the carbon fibers, they are cooled by the vapor flow just before the
liquid impact, thus permitting contact boiling up to much higher temperatures
than on smooth surfaces. We also show that, as long as the impact is in the
film boiling regime, the spreading factor of impacting droplets follows the
same \We^{3/10} scaling (with \We the Weber number) found for smooth
surfaces, which is caused by the vapor flow underneath the droplet.Comment: 10 pages, 6 figure
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
Predictions of pressure-induced transition temperature increase for a variety of high temperature superconductors
A wide variety of superconducting oxides are used to test a general model of
high pressure induced transition temperature (T c) changes. The T c 's vary
from a low of 24 K to a high of 164 K. Although the model is capable of
predicting both increases and decreases in T c with pressure, only
superconductors that exhibit an increase are considered at this time.
Predictions are made of the maximum T^ cP theo for 15 super-conductors as a
function of their compressibilities. The theoretical results generally agree
well with experiment. This model of T c as a function of pressure is derived
from a recent successful phenomenological theory of short coherence length
superconductivity.Comment: 9 pages. 1 table, 0 figure
Physical mechanism of anisotropic sensitivity in pentaerythritol tetranitrate from compressive-shear reaction dynamics simulations
We propose computational protocol (compressive shear reactive dynamics) utilizing the ReaxFF reactive force field to study chemical initiation under combined shear and compressive load. We apply it to predict the anisotropic initiation sensitivity observed experimentally for shocked pentaerythritol tetranitrate single crystals. For crystal directions known to be sensitive we find large stress overshoots and fast temperature increase that result in early bond-breaking processes whereas insensitive directions exhibit small stress overshoot, lower temperature increase, and little bond dissociation. These simulations confirm the model of steric hindrance to shear and capture the thermochemical processes dominating the phenomena of shear-induced chemical initiation
Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials
The fundamental processes in shock-induced instabilities of materials remain obscure, particularly for detonation of energetic materials. We simulated these processes at the atomic scale on a realistic model of a polymer-bonded explosive (3,695,375 atoms/cell) and observed that a hot spot forms at the nonuniform interface, arising from shear relaxation that results in shear along the interface that leads to a large temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that lead to detonation. We show that decreasing the density of the binder eliminates the hot spot
Throttling downlink throughput to mitigate device temperature increase
The temperature of a mobile device can increase due to heavy use, e.g., high-speed downloads, large computational load, etc. Sustained periods of high temperature can damage the mobile device. The techniques of this disclosure reduce downlink throughput upon detection of device temperature that exceeds a threshold. Throughput is reduced, e.g., by signaling the thermal state to the network, by reporting lower channel quality indicator (CQI) values to the network, etc. After the temperature drops to a safe level, throughput is brought back up in a phased manner
- …