17,155 research outputs found
In silico estimates of the free energy rates in growing tumor spheroids
The physics of solid tumor growth can be considered at three distinct size
scales: the tumor scale, the cell-extracellular matrix (ECM) scale and the
sub-cellular scale. In this paper we consider the tumor scale in the interest
of eventually developing a system-level understanding of the progression of
cancer. At this scale, cell populations and chemical species are best treated
as concentration fields that vary with time and space. The cells have
chemo-mechanical interactions with each other and with the ECM, consume glucose
and oxygen that are transported through the tumor, and create chemical
byproducts. We present a continuum mathematical model for the biochemical
dynamics and mechanics that govern tumor growth. The biochemical dynamics and
mechanics also engender free energy changes that serve as universal measures
for comparison of these processes. Within our mathematical framework we
therefore consider the free energy inequality, which arises from the first and
second laws of thermodynamics. With the model we compute preliminary estimates
of the free energy rates of a growing tumor in its pre-vascular stage by using
currently available data from single cells and multicellular tumor spheroids.Comment: 27 pages with 5 figures and 2 tables. Figures and tables appear at
the end of the pape
Macrospin Models of Spin Transfer Dynamics
The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment.Comment: 14 pages, 14 figures. Email [email protected] for a
pdf with higher quality figure
Causal vs. Noncausal Description of Nonlinear Wave Mixing; Resolving the Damping-Sign Controversy
Frequency-domain nonlinear wave mixing processes may be described either
using response functions whereby the signal is generated after all interactions
with the incoming fields, or in terms of scattering amplitudes where all fields
are treated symetrically with no specific time ordering. Closed Green's
function expressions derived for the two types of signals have different
analytical properties. The recent controversy regarding the sign of radiative
damping in the linear (Kramers Heisenberg) formula is put in a broader context
How has the Louisiana Scholarship Program Affected Students? A Comprehensive Summary of Effects after Four Years
School choice has long been a subject of robust debate. Private school vouchers—programs providing public funds for students to attend K-12 private schools—tend to be the most contentious form of school choice. Over the past three years, our research team has released a series of reports examining how the LSP has affected key student and community conditions
Comprehensive HIV care and Anti-Retroviral Therapy in a conflict setting-outcomes, experiences, and lessons learned from Bukavu, Democratic Republic of Congo
2006 AIDS Conference in Toront
Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas
We have studied the temperature dependent phonon emission rate () of a
strongly interacting (22) dilute 2D GaAs hole system using a standard
carrier heating technique. In the still poorly understood metallic state, we
observe that () changes from () to ()
above 100mK, indicating a crossover from screened piezoelectric(PZ) coupling to
screened deformation potential(DP) coupling for hole-phonon scattering.
Quantitative comparison with theory shows that the long range PZ coupling
between holes and phonons has the expected magnitude; however, in the metallic
state, the short range DP coupling between holes and phonons is {\it almost
twenty times stronger} than expected from theory. The density dependence of
() shows that it is {\it easier} to cool low density 2D holes in GaAs
than higher density 2D hole systems.Comment: To appear in Phys. Rev. Let
An approach for collection of nearfield groundwater samples in submerged limestone caverns
Walls of submerged caves feeding Florida springs are often lined with a heavy mat of filamentous bacteria, many of which are able to oxidize reduced sulfur in groundwater migrating from the porous bedrock into the cave conduit. To determine changes in water chemistry as water passes through the microbial mat, a simple device made from standard well screen and sealed with a rubber stopper and controllable vents was installed in a hole drilled in the wall of the cave passage. The sampler was sealed in place with marine epoxy. We measured anions in water from the sampler and from the water-filled conduit taken just outside the sampler. Most anions measured viz., Cl−, NO3−, and PO43−, increased slightly between the matrix and conduit waters. However, traces of sulfide were measured in the water from the rock matrix, but not in the conduit. SO42− concentrations in the conduit were about twice that measured in the water from the sampler, about 22 and 11 mg SO42− L−1, respectively, providing further evidence that sulfur oxidation is an important process in the bacterial mats attached to the limestone surfaces in these caves. An additional use of the sampling device is to measure discharge from the local bedrock into the cave conduit.Keywords: sulfur oxidation, bacteria, acid dissolution, groundwater chemistry.DOI:10.3986/ac.v42i2-3.664
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