2,888 research outputs found
Derivation of the bacterial run-and-tumble kinetic equation from a model with biochemical pathway
Kinetic-transport equations are, by now, standard models to describe the
dynamics of populations of bacteria moving by run-and-tumble. Experimental
observations show that bacteria increase their run duration when encountering
an increasing gradient of chemotactic molecules. This led to a first class of
models which heuristically include tumbling frequencies depending on the
path-wise gradient of chemotactic signal.
More recently, the biochemical pathways regulating the flagellar motors were
uncovered. This knowledge gave rise to a second class of kinetic-transport
equations, that takes into account an intra-cellular molecular content and
which relates the tumbling frequency to this information. It turns out that the
tumbling frequency depends on the chemotactic signal, and not on its gradient.
For these two classes of models, macroscopic equations of Keller-Segel type,
have been derived using diffusion or hyperbolic rescaling. We complete this
program by showing how the first class of equations can be derived from the
second class with molecular content after appropriate rescaling. The main
difficulty is to explain why the path-wise gradient of chemotactic signal can
arise in this asymptotic process.
Randomness of receptor methylation events can be included, and our approach
can be used to compute the tumbling frequency in presence of such a noise
Traveling wave solution of the Hele-Shaw model of tumor growth with nutrient
Several mathematical models of tumor growth are now commonly used to explain
medical observations and predict cancer evolution based on images. These models
incorporate mechanical laws for tissue compression combined with rules for
nutrients availability which can differ depending on the situation under
consideration, in vivo or in vitro. Numerical solutions exhibit, as expected
from medical observations, a proliferative rim and a necrotic core. However,
their precise profiles are rather complex, both in one and two dimensions.Comment: 25 page
Derivation of a Hele-Shaw type system from a cell model with active motion
We formulate a Hele-Shaw type free boundary problem for a tumor growing under
the combined effects of pressure forces, cell multiplication and active motion,
the latter being the novelty of the present paper. This new ingredient is
considered here as a standard diffusion process. The free boundary model is
derived from a description at the cell level using the asymptotic of a stiff
pressure limit.
Compared to the case when active motion is neglected, the pressure satisfies
the same complementarity Hele-Shaw type formula. However, the cell density is
smoother (Lipschitz continuous), while there is a deep change in the free
boundary velocity, which is no longer given by the gradient of the pressure,
because some kind of 'mushy region' prepares the tumor invasion
Process Optimization and Downscaling of a Single Electron Single Dot Memory
This paper presents the process optimization of a single-electron nanoflash
electron memory. Self-aligned single dot memory structures have been fabricated
using a wet anisotropic oxidation of a silicon nanowire. One of the main issue
was to clarify the process conditions for the dot formation. Based on the
process modeling, the influence of various parameters (oxidation temperature,
nanowire shape) has been investigated. The necessity of a sharp compromise
between these different parameters to ensure the presence of the memory dot has
been established. In order to propose an aggressive memory cell, the
downscaling of the device has been carefully studied. Scaling rules show that
the size of the original device could be reduced by a factor of 2. This point
has been previously confirmed by the realization of single-electron memory
devices
An Eco-Friendly Lifestyle In Your Hands
Society has begun to be more conscious about the products they consume on a daily basis. People are changing to plant-based diets or vegan lifestyles. The high demand of this market, motivated the idea to develop an eco-friendly and user-friendly app that will make people’s life easier. By combining two concepts that are popular amongst this generation, an app was developed. This app contains all the sources and information one looks for during the transition into an eco-friendly lifestyle
Low 60Fe abundance in Semarkona and Sahara 99555
Iron-60 (t1/2=2.62 Myr) is a short-lived nuclide that can help constrain the
astrophysical context of solar system formation and date early solar system
events. A high abundance of 60Fe (60Fe/56Fe= 4x10-7) was reported by in situ
techniques in some chondrules from the LL3.00 Semarkona meteorite, which was
taken as evidence that a supernova exploded in the vicinity of the birthplace
of the Sun. However, our previous MC-ICPMS measurements of a wide range of
meteoritic materials, including chondrules, showed that 60Fe was present in the
early solar system at a much lower level (60Fe/56Fe=10-8). The reason for the
discrepancy is unknown but only two Semarkona chondrules were measured by
MC-ICPMS and these had Fe/Ni ratios below ~2x chondritic. Here, we show that
the initial 60Fe/56Fe ratio in Semarkona chondrules with Fe/Ni ratios up to
~24x chondritic is 5.4x10-9. We also establish the initial 60Fe/56Fe ratio at
the time of crystallization of the Sahara 99555 angrite, a chronological
anchor, to be 1.97x10-9. These results demonstrate that the initial abundance
of 60Fe at solar system birth was low, corresponding to an initial 60Fe/56Fe
ratio of 1.01x10-8.Comment: The Astrophysical Journal, in press. 28 pages, 2 tables, 3 figure
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