54 research outputs found
Surface wave excitations and backflow effect over dense polymer brushes
Polymer brushes are being increasingly used to tailor surface
physicochemistry for diverse applications such as wetting, adhesion of
biological objects, implantable devices and much more. Here we perform
Dissipative Particle Dynamics simulations to study the behaviour of
dense polymer brushes under flow in a slit-pore channel. We discover
that the system displays flow inversion at the brush interface for
several disconnected ranges of the imposed flow. We associate such
phenomenon to collective polymer dynamics: a wave propagating on the
brush surface. The relation between the wavelength, the amplitude and
the propagation speed of the flow-generated wave is consistent with the
solution of the Stokes equations when an imposed traveling wave is
assumed as the boundary condition (the famous Taylor's swimmer)
Surface wave excitations and backflow effect over dense polymer brushes
Polymer brushes are increasingly used to tailor surface physicochemistry for
various applications such as wetting, adhesion of biological objects,
implantable devices, etc. We perform Dissipative Particle Dynamics simulations
to study the behavior of dense polymer brushes under flow in a slit-pore
channel. We discover that the system displays flow inversion at the brush
interface for several disconnected ranges of the imposed flow. We associate
such phenomenon to collective polymer dynamics: a wave propagating on the brush
surface. The relation between the wavelength, the amplitude and the propagation
speed of the flow-generated wave is consistent with the solution of the Stokes
equations when an imposed traveling wave is assumed as boundary condition (the
famous Taylor's swimmer).Comment: 5 pages, 6 figures, supplemental material (5 pages, 4 figures)
attached below the letter and before bibliography, videos available on
request to the corresponding author, submitted to SciRe
Coarsening scenarios in unstable crystal growth
Crystal surfaces may undergo thermodynamical as well kinetic,
out-of-equilibrium instabilities. We consider the case of mound and pyramid
formation, a common phenomenon in crystal growth and a long-standing problem in
the field of pattern formation and coarsening dynamics. We are finally able to
attack the problem analytically and get rigorous results. Three dynamical
scenarios are possible: perpetual coarsening, interrupted coarsening, and no
coarsening. In the perpetual coarsening scenario, mound size increases in time
as L=t^n, where the coasening exponent is n=1/3 when faceting occurs, otherwise
n=1/4.Comment: Changes in the final part. Accepted for publication in Phys. Rev.
Let
Etude mésoscopique du glycocalyx endothélial et de son interaction avec le sang
Polymer brushes are dense matrices of grafted macromolecules. In addition to brushes finely designed in laboratory, various examples are offered by Nature, as the endothelial glycocalyx, decorating the lumen of mammalian blood vessels. The interaction of such network with the flowing plasma and cells is still partially unknown.The present thesis, by mean of Dissipative Particle Dynamics simulations, proposes a coarse-grained model for the self-consistent analysis of a dense polymer brush under parabolic flow. Our mesoscale investigation highlights the relevance of collective effects, driven by hydrodynamics, and proposes novel interpretations regarding the rich phenomenology of the brush-flow system.Preliminary results are also provided for the interplay between a mesoscopic deformable flowing object (prototype of a red blood cell) and the grafted polymers.Une brosse de polymères est une matrice dense de macromolécules greffées à une surface donnée. Au-delà des brosses synthétiques réalisées en laboratoire, on trouve des exemples très variés dans la nature: un exemple emblématique est le glycocalyx endothélial, décorant la surface interne des vaisseaux sanguins des mammifères. L'interaction de cette structure avec le plasma et les cellules sous écoulement n'est encore que très partiellement explorée. La présente thèse propose, grâce à des simulations de "Dissipative Particle Dynamics", un modèle coarse-grained pour une analyse auto-cohérente d'une brosse polymérique dense sous écoulement parabolique. Cette étude mésoscopique met en évidence l'importance des effets collectifs entre molécules, entraînée par l'hydrodynamique, et propose des nouvelles interprétations à la phénoménologie du système brosse-écoulement. Des résultats préliminaires sont également produits pour l'interaction sous écoulement entre un objet mésoscopique déformable (prototype d'un globule rouge) et les polymères greffés
BOLD Response Selective to Flow-Motion in Very Young Infants.
In adults, motion perception is mediated by an extensive network of occipital, parietal, temporal, and insular cortical areas. Little is known about the neural substrate of visual motion in infants, although behavioural studies suggest that motion perception is rudimentary at birth and matures steadily over the first few years. Here, by measuring Blood Oxygenated Level Dependent (BOLD) responses to flow versus random-motion stimuli, we demonstrate that the major cortical areas serving motion processing in adults are operative by 7 wk of age. Resting-state correlations demonstrate adult-like functional connectivity between the motion-selective associative areas, but not between primary cortex and temporo-occipital and posterior-insular cortices. Taken together, the results suggest that the development of motion perception may be limited by slow maturation of the subcortical input and of the cortico-cortical connections. In addition they support the existence of independent input to primary (V1) and temporo-occipital (V5/MT+) cortices very early in life
Hydrodynamic instability and flow reduction in polymer brush coated channels
International audienceA polymer brush is a passive medium but, exposed to fluid flow, it displays a surprising dynamics, for which filamentous protrusions and their interactions via hydrodynamics come heavily into play
Spatiotopic coding of BOLD signal in human visual cortex depends on spatial attention.
The neural substrate of the phenomenological experience of a stable visual world remains obscure. One possible mechanism would be to construct spatiotopic neural maps where the response is selective to the position of the stimulus in external space, rather than to retinal eccentricities, but evidence for these maps has been inconsistent. Here we show, with fMRI, that when human subjects perform concomitantly a demanding attentive task on stimuli displayed at the fovea, BOLD responses evoked by moving stimuli irrelevant to the task were mostly tuned in retinotopic coordinates. However, under more unconstrained conditions, where subjects could attend easily to the motion stimuli, BOLD responses were tuned not in retinal but in external coordinates (spatiotopic selectivity) in many visual areas, including MT, MST, LO and V6, agreeing with our previous fMRI study. These results indicate that spatial attention may play an important role in mediating spatiotopic selectivity
Spatiotopic Coding of BOLD Signal in Human Visual Cortex Depends on Spatial Attention
The neural substrate of the phenomenological experience of a stable visual world remains obscure. One possible mechanism would be to construct spatiotopic neural maps where the response is selective to the position of the stimulus in external space, rather than to retinal eccentricities, but evidence for these maps has been inconsistent. Here we show, with fMRI, that when human subjects perform concomitantly a demanding attentive task on stimuli displayed at the fovea, BOLD responses evoked by moving stimuli irrelevant to the task were mostly tuned in retinotopic coordinates. However, under more unconstrained conditions, where subjects could attend easily to the motion stimuli, BOLD responses were tuned not in retinal but in external coordinates (spatiotopic selectivity) in many visual areas, including MT, MST, LO and V6, agreeing with our previous fMRI study. These results indicate that spatial attention may play an important role in mediating spatiotopic selectivity
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