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N. Olivi-Tran

Publications and source records attributed to N. Olivi-Tran.

18 recordsLinked to original sources
arXiv2009

Analysis of a Lennard-Jones fcc structure melting to the corresponding frozen liquid: differences between the bulk and the surface

We computed a Lennard Jones frozen liquid with a free surface using classical molecular dynamics. The structure factor curves on the free surface of this sample was calculated for different depths knowing that we have periodic boundary conditions on the other parts of the sample. The resulting structure factor curves show an horizontal shift of their first peak depending on how deep in the sample the curves are computed. We analyze our resulting curves in the light of spatial correlation functions during melting and at when the liquid is frozen. The conclusion is that near the free surface the sample is less dense than in the bulk and that the frozen liquid surface has a spatial correlation which does not differ very much from that of the bulk. This result is intrinsic to the melting of the Lennard Jones liquid and does not depend on any other parameter.

cond-mat.soft↗
arXiv2009

Sol-gel obtained YSZ nanocrystals on a substrate: a Monte Carlo approach of the crystallographic orientation

Sol gel obtained YZS (Yttria Stabilized Zirconia) nanocrystals have different morphologies when they grow on an $α$-alumina substrate after thermal treatment. When the substrate has planar defects, the nanocrystals grow in height and are narrow with a crystallographic orientation [111] in the vertical direction, while when the substrate is a perfect plane at the nanometric scale, the nanocrystals are rather extended over the substrate and do not grow in height, with a crystallo- graphic orientation [100] in the vertical direction. We present here a Monte Carlo approach which computes the actions of the substrate on the nanocrystals during thermal treatment: one action is the change in crystallographic orientation depending on the presence of defects and the other is the action on the morphology of the nanocrystals. The equivalent of thermal treatment is obtained after applying the Metropolis algorithm with adequate expressions of the energy depending on the inter-plane spacings and surface diffusion. Our numerical approach is in good agreement with the experimental results on the orientations and morphologies of the YSZ nanocrystals growing on a $α$-alumina substrate with planar defects

cond-mat.mtrl-sci↗
arXiv2008

Total electronic energy by tight binding approximation and experimental toughness of three different hybrid polymers

We computed by a modified tight binding approximation, the total electronic energy of three different hybrid polymers: $H-SiO_2$, $CH_3-SiO_2$ and $C_6H_5-SiO_2$. We made the hypothesis that the structures of these polymers are amorphous. Computational results regarding the total electronic energy and experimental data \cite{ferchichi} on the toughness of these three hybrid polymers were compared. A good qualitative agreement was found between computations and experiments.

cond-mat.mtrl-sci↗
arXiv2008

Structure of hybrid polymer by numerical simulation: example of $(Si(CH_3)_2)_n(OSiO)_p(OH)_q$

The structure depends on the mechanism of hydrolyse and polycondensation in the case sol-gel process used to obtain hybrid polymers containing uniquely covalent bonds. We computed by a tight binding calculation taking into account hybridization, the total electronic energy of different structures (amorphous, fractal and linear) of $(Si-(CH_3)_2)_n-(OSiO)_p(OH)_q$. We found that the total electronic energy of the amorphous structure was the smallest as a function of the number of atoms contained by the structure. As the total electronic energy is linked to the toughness of the structure, we may say that the structure of such hybrid polymer has the highest toughness for the amorphous structure.

cond-mat.mtrl-sci↗
arXiv2008

The role of the extra cellular matrix on memory

We expose first a biological model of memory based on one hand of the mechanical oscillations of axons during action potential and on the other hand on the changes in the extra cellular matrix composition when a mechanical strain is applied on it. Due to these changes, the stiffness of the extra cellular matrix along the most excited neurons will increase close to these neurons due to the growth of astrocytes around them and to the elastoplastic behavior of collagen. This will create preferential paths linked to a memory effect. In a second part, we expose a physical model based on random walk of the action potential on the array composed of dendrites and axons. This last model shows that repetition of the same event leads to long time memory of this event and that paradoxical sleep leads to the linking of different events put into memory.

cond-mat.soft↗
arXiv2008

Comparison between models of insulator and semiconductor thin films islanding

The synthesis of self-organized quantum dots (QD's) can be achieved through bottom up layer by layer deposition processes as chemical vapor deposition (CVD) or physical vapor deposition (PVD). However, QD's may also be synthesized via sol-gel route, which involves a spontaneous evolution from thin films to discrete QD's without further deposition. The aim of the paper is to discuss and compare the physical phenomena involved in QD's formation which initiate from thin film surface roughening between PVD-CVD and sol-gel synthesis approaches. We propose two simple physical models which are relevant to explain the fundamental differences between those methods.

cond-mat.mtrl-sci↗
arXiv2008

Interface energies of (100)_{YSZ} and (111)_{YSZ} epitaxial islands on (0001)_{alpha-Al_2O_3} substrates from first principles

We present an ab initio study of the interface energies of cubic yttria-stabilized zirconia (YSZ) epitaxial layers on a (0001)_{alpha-Al_2O_3} substrate. The interfaces are modelled using a supercell geometry and the calculations are carried out in the framework of density-functional theory (DFT) and the local-density approximation (LDA) using the projector-augmented-wave (PAW) pseudopotential approach. Our calculations clearly demonstrate that the (111)_{YSZ} || (0001)_{alpha-Al_2O_3} interface energy is lower than that of (100)_{YSZ} || (0001)_{alpha-Al_2O_3}. This result is central to understanding the behaviour of YSZ thin solid film islanding on (0001)_{alpha-Al_2O_3} substrates, either flat or in presence of defects.

cond-mat.mtrl-sci↗
arXiv2007

Two dimensional curved disks on a sphere: the evolution of kinetic energy

We put disks on a sphere between two parallels of this sphere. The disks have the curvature of the sphere and interact via a simplified Hertz law. We analyze the behavior of the total kinetic energy of the whole assembly of disks with Riemanian geometry and contrarily to flat spaces, this kinetic energy increases. This is due to a non radial component of the resulting velocity vector at each interaction between two disks. The study of granular matter in twodimensional curved spaces is therefore not trivial.

cond-mat.soft↗
arXiv2007

A link between ECM plasticity and synaptic morphological evolution

We made a link between Extra Cellular Matrix (ECM) plasticity and the morphologi cal changes in synapses after synaptic excitation. A recent study by Zhang et al \cite{zhang} showed tha t transmembrane voltage causes movement of the cell membrane. Here we will study the relation between th e mechanical properties of collagen which is the major component of the ECM and synaptic morphological c hanges in relation with the theory of DO Hebb \cite{hebb}

cond-mat.soft↗
arXiv2006

Micrometric particles twodimensional self-assembly during drying of liquid film

We computed the self-organisation process of a monodisperse collection of spherical micrometric particles trapped in a two-dimensional (2D) thin liquid film isothermally dried on a chemically inert substrate. The substrate is either flat or indented to create linear stripes on its surface. The numerical results are illustrated and discussed in the light of experimental ones obtained from the drying of diamond particles water based suspension ($d_{50} = 10 μm$) on a glass substrate. The drying of the suspension on a flat substrate leads to the formation of linear patterns and small clusters of micrometric particles distributed over the whole surface of the substrate, whereas the drying of the suspension on a indented substrate leads to the aggregation of the particles along one side of the stripe which has a higher roughness than the other side of the stripe. This is an easy experimental way to obtain colloidal selforganized patterns.

cond-mat.soft↗
arXiv2006

Effects of elasticity on the shape of measured shear signal in two-dimensional assembly of disks

A Molecular Dynamics approach has been used to compute the shear force resulting from the shearing of disks. Two-dimensional monodisperse disks have been put in an horizontal and rectangular shearing cell with periodic boundary conditions on right and left hand sides. The shear is applied by pulling the cover of the cell either at a constant rate or by pulling a spring, linked to the cover, with a constant force. Depending on the rate of shearing and on the elasticity of the whole set up, we showed that the measured shear force signal is either irregular in time, regular in time but not in shape, or regular in shape.

cond-mat.soft↗
arXiv2005

Monte Carlo approach of the islanding of polycrystalline thin films

We computed by a Monte Carlo method derived from the Solid on Solid model, the evolution of a polycrystalline thin film deposited on a substrate during thermal treatment. Two types of substrates have been studied: a single crystalline substrate with no defects and a single crystalline substrate with defects. We obtain islands which are either flat (i.e. with a height which does not overcome a given value) or grow in height like narrow towers. A good agreement was found regarding the morphology of numerical nanoislands at equilibrium, deduced from our model, and experimental nanoislands resulting from the fragmentation of YSZ thin films after thermal treatment.

cond-mat.mtrl-sci↗
arXiv2005

Stick slip motion in grain grain friction in a humid atmosphere

We set up an original apparatus to measure the grain grain friction stress inside a granular medium composed of sodo-silicate-glass beads surrounded by a water vapor atmosphere.We analyze here the influence of the physico chemistry of water on our glass beads and its consequences on our shear experiment. We found two scales in the analysis of the shear stress signal. On the microscopic scale of one bead, the experimental results show a dependence on the size of beads, on the shear rate and on humidity for the resulting stick slip signal. On the macroscopic scale of the whole assembly of beads, the behavior of the total amplitude of the shear stress depends on the size of the beads and is humidity dependent only for relative humidity larger than 80%. For high degrees of humidity, on the microscopic scale, water lubricates the surface of the beads leading to a decrease in the microscopic resistance to shear while on the macroscopic scale the resistance to shear is increased: the assembly of very humid grains behaves as a non Newtonian fluid.

cond-mat.soft↗
arXiv2005

Constant rate shearing on two dimensional cohesive disks

We performed two-dimensional Molecular Dynamics simulations of cohesive disks under shear. The cohesion between the disks is added by the action of springs between very next neighbouring disks, modelling capillary forces. The geometry of the cell allows disk-disk shearing and not disk-cell wall shearing as it is commonly found in literature. Does a stick-slip phenomenon happen though the upper cover moves at a constant velocity, i.e. with an infinite shearing force? We measured the forces acted by the disks on the upper cover for different shearing rates, as well as the disk velocities as a function of the distance to the bottom of the cell. It appears that the forces measured versus time present a periodic behavior,very close to a stick slip phenomenon, for shearing rates larger than a given threshold. The disks' collective displacements in the shearing cell (back and ahead) is the counterpart of the constant velocity of the upper cover leading to a periodic behavior of the shear stress.

cond-mat.soft↗
arXiv2005

Numerical study of the antiferromagnetic Ising model on a hypersphere

We built a model where all spins are in interaction with each other via an antiferromagnetic Ising Hamiltonian. The geometry of such a model is a tetrahedron placed on a hypersphere in spaces of dimensions enclosed between 1 and 9. Due to confinement and to the fact that all spins interact which each other, our spin system exhibit frustration. The temperatures of the observed antiferro-paramagnetic transitions are equal for all space dimensions to one of two given values depending on the parity of the space dimension. Moreover, the order parameter $ $, i.e. the magnetization of the system, has been also studied.

physics.comp-ph↗
arXiv2005

Morphological instabilities of a thin film on a Penrose lattice: a Monte Carlo study

We computed by a Monte Carlo method the thermal relaxation of a polycrystalline thin film deposited on a Penrose lattice. The thin film was modelled by a 2 dimensional array of elementary domains, which have each a given height. During the Monte Carlo process, the height of each of these elementary domains is allowed to change as well as their crystallographic orientation. After equilibrium is reached at a given numerical temperature, all elementary domains have changed their orientation into the same one and small islands appear, preferentially on the domains of the Penrose lattice located in the center of heptagons. This method is a new numerical approach to study the influence of the substrate and its defects on the islanding process of polycrystalline films.

cond-mat.mtrl-sci↗
arXiv2002

Membrane bound protein diffusion viewed by fluorescence recovery after bleaching experiments : models analysis

Diffusion processes in biological membranes are of interest to understand the macromolecular organisation and function of several molecules. Fluorescence Recovery After Photobleaching (FRAP) has been widely used as a method to analyse this processes using classical Brownian diffusion model. In the first part of this work, the analytical expression of the fluorescence recovery as a function of time has been established for anomalous diffusion due to long waiting times. Then, experimental fluorescence recoveries recorded in living cells on a membrane-bound protein have been analysed using three different models : normal Brownian diffusion, Brownian diffusion with an immobile fraction and anomalous diffusion due to long waiting times.

cond-mat.stat-mech↗