39 research outputs found
Fish evacuate smoothly respecting a social bubble
Crowd movements are observed among different species and on different scales,
from insects to mammals, as well as in non-cognitive systems, such as motile
cells. When forced to escape through a narrow opening, most terrestrial animals
behave like granular materials and clogging events decrease the efficiency of
the evacuation. Here, we explore the evacuation behavior of macroscopic,
aquatic agents, neon fish, and challenge their gregarious behavior by forcing
the school through a constricted passage. Using a statistical analysis method
developed for granular matter and applied to crowd evacuation, our results
clearly show that, unlike crowds of people or herds of sheep, no clogging
occurs at the bottleneck. The fish do not collide and wait for a minimum
waiting time between two successive exits, while respecting a social distance.
When the constriction becomes similar to or smaller than their social distance,
the individual domains defined by this cognitive distance are deformed and fish
density increases. We show that the current of escaping fish behaves like a set
of deformable 2D-bubbles, their 2D domain, passing through a constriction.
Schools of fish show that, by respecting social rules, a crowd of individuals
can evacuate without clogging, even in an emergency situation.Comment: 7 pages, 4 figure
Amoeboid motion in confined geometry
International audienceMany eukaryotic cells undergo frequent shape changes (described as amoeboid motion) that enable them to move forward. We investigate the effect of confinement on a minimal model of amoeboid swimmer. A complex picture emerges: (i) The swimmer's nature (i.e., either pusher or puller) can be modified by confinement, thus suggesting that this is not an intrinsic property of the swimmer. This swimming nature transition stems from intricate internal degrees of freedom of membrane deformation. (ii) The swimming speed might increase with increasing confinement before decreasing again for stronger confinements. (iii) A straight amoeoboid swimmer's trajectory in the channel can become unstable, and ample lateral excursions of the swimmer prevail. This happens for both pusher- and puller-type swimmers. For weak confinement, these excursions are symmetric, while they become asymmetric at stronger confinement, whereby the swimmer is located closer to one of the two walls. In this study, we combine numerical and theoretical analyses
Acid-induced gelation of carboxymethylcellulose solutions
The present work offers a comprehensive description of the acid-induced
gelation of carboxymethylcellulose (CMC), a water-soluble derivative of
cellulose broadly used in numerous applications ranging from food packaging to
biomedical engineering. Linear viscoelastic properties measured at various pH
and CMC contents allow us to build a sol-gel phase diagram, and show that CMC
gels exhibit broad power-law viscoelastic spectra that can be rescaled onto a
master curve following a time-composition superposition principle. These
results demonstrate the microstructural self-similarity of CMC gels, and
inspire a mean-field model based on hydrophobic inter-chain association that
accounts for the sol-gel boundary over the entire range of CMC content under
study. Neutron scattering experiments further confirm this picture and suggest
that CMC gels comprise a fibrous network crosslinked by aggregates. Finally,
low-field NMR measurements offer an original signature of acid-induced gelation
from the solvent perspective. Altogether, these results open avenues for
precise manipulation and control of CMC-based hydrogels.Comment: 7 pages, 4 figures and 4 figures as S
Unconventional MBE Strategies from Computer Simulations for Optimized Growth Conditions
We investigate the influence of step edge diffusion (SED) and desorption on
Molecular Beam Epitaxy (MBE) using kinetic Monte-Carlo simulations of the
solid-on-solid (SOS) model. Based on these investigations we propose two
strategies to optimize MBE growth. The strategies are applicable in different
growth regimes: During layer-by-layer growth one can exploit the presence of
desorption in order to achieve smooth surfaces. By additional short high flux
pulses of particles one can increase the growth rate and assist layer-by-layer
growth. If, however, mounds are formed (non-layer-by-layer growth) the SED can
be used to control size and shape of the three-dimensional structures. By
controlled reduction of the flux with time we achieve a fast coarsening
together with smooth step edges.Comment: 19 pages, 7 figures, submitted to Phys. Rev.
Highly anisotropic g-factor of two-dimensional hole systems
Coupling the spin degree of freedom to the anisotropic orbital motion of
two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman
splitting with respect to different orientations of an in-plane magnetic field
B relative to the crystal axes. This mechanism has no analogue in the bulk band
structure. We obtain good, qualitative agreement between theory and
experimental data, taken in GaAs 2D hole systems grown on (113) substrates,
showing the anisotropic depopulation of the upper spin subband as a function of
in-plane B.Comment: 4 pages, 3 figure
Ripple Texturing of Suspended Graphene Atomic Membranes
Graphene is the nature's thinnest elastic membrane, with exceptional
mechanical and electrical properties. We report the direct observation and
creation of one-dimensional (1D) and 2D periodic ripples in suspended graphene
sheets, using spontaneously and thermally induced longitudinal strains on
patterned substrates, with control over their orientations and wavelengths. We
also provide the first measurement of graphene's thermal expansion coefficient,
which is anomalously large and negative, ~ -7x10^-6 K^-1 at 300K. Our work
enables novel strain-based engineering of graphene devices.Comment: 15 pages, 4 figure
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
Buckling of a compressed elastic membrane: a simple model
The buckling of a folded membrane submitted to a bi-axial compression is studied in the framework of the continuum non-linear elasticity theory. We show that the formation of the fold patterning can be quantitatively well described with a simple non-linear model.
As a matter of fact, with this model, we recover the experimental phase diagram of a secondary buckling instability with a very good precision. In addition, depending on the anisotropy of the applied compressive stress, we find that the buckling coarsening dynamics can be described as a 1D spinodal decomposition (for a uni-axial stress) or as a 2DÂ XY model (for an isotropic bi-axial stress) with an irrotational non-scalar order parameter. For an isotropic bi-axial stress, we indeed recover the famous coarsening exponent: n=1/4. This exponent has to be confirmed
experimentally
Rheology and dynamics of a deformable object in a microfluidic configuration: A numerical study
A dilute suspension is studied in a confined geometry with a 3D
numerical simulation. The suspended element is a non-Brownian elastic dumbbell. The suspension is confined between two walls in a shear flow.
The dynamics of the dumbbell as well as the associated rheology are presented. Despite its simplicity, the system exhibits generic microscopic behaviours of real deformable objects such as vesicles,
biological cells or capsules like tumbling or vacillating-breathing. It also reproduces macroscopic behaviours like a shear thinning viscosity characteristic of
complex fluids rheology. In addition, the model predicts a confinement law where the intrinsic viscosity varies as
for sufficiently strong confinements (c is the
ratio of dumbbell size over the wall-to-wall distance). A transition from a tumbling regime towards a vacillating-breathing one is found.
This mode is promoted further by confinement