23 research outputs found
Probing an nonequilibrium Einstein relation in an aging colloidal glass
We present a direct experimental measurement of an effective temperature in a
colloidal glass of Laponite, using a micrometric bead as a thermometer. The
nonequilibrium fluctuation-dissipation relation, in the particular form of a
modified Einstein relation, is investigated with diffusion and mobility
measurements of the bead embedded in the glass. We observe an unusual
non-monotonic behavior of the effective temperature : starting from the bath
temperature, it is found to increase up to a maximum value, and then decreases
back, as the system ages. We show that the observed deviation from the Einstein
relation is related to the relaxation times previously measured in dynamic
light scattering experiments.Comment: 4 pages, 4 figures, corrected references, published in Phys. Rev.
Lette
Questioning the relationship between the 4 susceptibility and the dynamical correlation length in a glass former
Clusters of fast and slow correlated particles, identified as dynamical
heterogeneities (DHs), con-stitute a central aspect of glassy dynamics. A key
ingredient of the glass transition scenario is asignificant increase of the
cluster size 4 as the transition is approached. In need of
easy-to-computetools to measure 4 , the dynamical susceptibility 4
was introduced recently, and used in various ex-perimental works to probe DHs.
Here, we investigate DHs in dense microgel suspensions using imagecorrelation
analysis, and compute both 4 and the four-point correlation function G4 .
The spatialdecrease of G4 provides a direct access to 4 , which is found
to grow significantly with increasingvolume fraction. However, this increase is
not captured by 4 . We show that the assumptions thatvalidate the
connection between 4 and 4 are not fulfilled in our experiments.Comment: The present version was accepted for publication in Soft Matter
(http://pubs.rsc.org/en/journals/journalissues/sm
Rotational microrheology of Maxwell fluids using micron-sized wires
We demonstrate a simple method for rotational microrheology in complex
fluids, using micrometric wires. The three-dimensional rotational Brownian
motion of the wires suspended in Maxwell fluids is measured from their
projection on the focal plane of a microscope. We analyze the mean-squared
angular displacement of the wires of length between 1 and 40 microns. The
viscoelastic properties of the suspending fluids are extracted from this
analysis and found to be in good agreement with macrorheology data. Viscosities
of simple and complex fluids between 0.01 and 30 Pa.s could be measured. As for
the elastic modulus, values up to ~ 5 Pa could be determined. This simple
technique, allowing for a broad range of probed length scales, opens new
perspectives in microrheology of heterogeneous materials such as gels, glasses
and cells.Comment: to appear in Soft Matte
Activity statistics in a colloidal glass former: experimental evidence for a dynamical transition
In a dense colloidal suspension at a volume fraction slightly lower than that
of its glass transition, we follow the trajectories of an assembly of tracers
over a large time window. We define a local activity, which quantifies the
local tendency of the system to rearrange. We determine the statistics of the
time and space integrated activity, and we argue that it develops a low
activity tail that comes on a par with the onset of glassy behavior and
heterogeneous dynamics. These rare events may be interpreted as the reflection
of an underlying dynamic phase transition.Comment: 20 pages, 16 figure
Generalized Einstein Relation in an aging colloidal glass
20 pages, 6 figures; revised versionInternational audienceWe present an experimental and theoretical investigation of the Generalized Einstein Relation (GER), a particular form of a fluctuation-dissipation relation, in an out-of-equilibrium visco-elastic fluid. Micrometer beads, used as thermometers, are immersed in an aging colloidal glass to provide both fluctuation and dissipation measurements. The deviations from the Generalized Einstein Relation are derived as a function of frequency and aging time. The observed deviations from GER are interpreted as directly related to the change in the glass relaxation times with aging time. In our scenario, deviations are observed in the regime where the observation time scale is of the order of a characteristic relaxation time of the glass
Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition
The microscopic dynamics and aging of a soft thermosensitive suspension was
investigated by looking at the thermal fluctuations of tracers in the
suspension. Below and above the glass transition, the dense microgel particles
suspension was found to develop an heterogeneous dynamics, featured by a non
Gaussian Probability Distribution Function (PDF) of the probes' displacements,
with an exponential tail. We show that non Gaussian shapes are a characteristic
of the ensemble-averaged PDF, while local PDF remain Gaussian. This shows that
the scenario behind the non Gaussian van Hove functions is a spatially
heterogeneous dynamics, characterized by a spatial distribution of locally
homogeneous dynamical environments through the sample, on the considered time
scales. We characterize these statistical distributions of dynamical
environments, in the liquid, supercooled, and glass states, and show that it
can explain the observed exponential tail of the van Hove functions observed in
the concentrated states. The intensity of spatial heterogeneities was found to
amplify with increasing volume fraction. In the aging regime, it tends to
increase as the glass gets more arrested.Comment: 19 pages, 10 figures, Soft Matter accepte
Nonlinear rheology of Laponite suspensions under an external drive
We investigate the nonlinear rheological behavior of colloidal suspensions of
Laponite, a synthetic clay, driven by a steady and homogeneous shear strain. We
show that the external drive leads to a drastic slowing down of the aging
dynamics or even, in some cases, in the rejuvenation of the system. Under
shear, after a surprisingly long time, the spontaneous aging process observed
at rest is suppressed. The system then reaches a non-equilibrium stationary
state, characterised by a complex viscosity depending on the applied shear
rate. In addition, the glass exhibits a non-Newtonian shear-thinning behavior.
These rheological behaviors confirm recent numerical and theoretical
predictions.Comment: 17 pages, Latex, 6 figures, figures included in the body of the text,
to appear in Journal of Rheolog
Craquelures dans les couches picturales des peintures d'art
Reflets de la Physique, vol. 3 (Mars 2007).Les craquelures dans les couches picturales présentent un intérêt nouveau dans l'étude des peintures d'art. Par la complexité de leur morphologie, elles sont depuis longtemps un moyen de juger l'authenticité des peintures. Les figures de craquelures retracent également la manière dont l'artiste a œuvré. En étroite collaboration avec le C2RMF du musée du Louvre, nous avons étudié les morphologies de craquelures dans la série de peintures consacrées aux Apôtres de Georges de La Tour, pour laquelle peintures de l'artiste et copies n'ont pas été clairement dissociées jusqu'à présent. Nous nous baserons sur des expériences modèles de séchage de suspensions colloïdales qui peuvent aider à comprendre les morphologies de craquelures observées sur les œuvres réelles
Extensive collection of femtoliter pad secretion droplets in beetle Leptinotarsa decemlineata allows nanoliter microrheology
Pads of beetles are covered with long, deformable setae, each ending in a
micrometric terminal plate coated with secretory fluid. It was recently shown
that the layer of the pad secretion covering the terminal plates is responsible
for the generation of strong attractive forces. However, less is known about
the fluid itself because it is produced in extremely small quantity. We here
present a first experimental investigation of the rheological properties of the
pad secretion in the Colorado potato beetle {\it Leptinotarsa decemlineata}.
Because the secretion is produced in an extremely small amount at the level of
the terminal plate, we first develop a procedure based on capillary effects to
collect the secretion. We then manage to incorporate micrometric beads,
initially in the form of a dry powder, and record their thermal motion to
determine the mechanical properties of the surrounding medium. We achieve such
a quantitative measurement within the collected volume, much smaller than the
l sample volume usually required for this technique. Surprisingly,
the beetle secretion was found to behave as a purely viscous liquid, of high
viscosity. This suggests that no specific complex fluid behaviour is needed
during beetle locomotion. We build a scenario for the contact formation between
the spatula at the setal tip and a substrate, during the insect walk. We show
that the attachment dynamics of the insect pad computed from the high measured
viscosity is in good agreement with observed insect pace. We finally discuss
the consequences of the secretion viscosity on the insect adhesion
Coping with the climate: cuticular hydrocarbon acclimation of ants under constant and fluctuating conditions.
Terrestrial arthropods achieve waterproofing by a layer of cuticular hydrocarbons (CHCs). At the same time, CHCs also serve as communication signals. To maintain waterproofing under different climate conditions, insects adjust the chemical composition of their CHC layer, but this may affect the communication via CHCs. The detailed acclimatory changes of CHCs and how these influence their physical properties are still unknown. Here, we studied acclimation in two closely related ant species with distinct CHC profiles, Myrmica rubra and Myrmica ruginodis, in response to constant or fluctuating temperature and humidity regimes. We measured how acclimation affected CHC composition and viscosity, and the ants' drought survival. In both species, CHC composition showed strong, predictable responses to temperature regimes. Warm-acclimated individuals had higher proportions of linear alkanes, and less methyl-branched or unsaturated CHCs. These changes coincided with higher solid content and viscosity of CHCs in warm-acclimated ants. Temperature fluctuation caused effects similar to those observed under constant-cool conditions in Mrubra, but led to entirely different profiles in Mruginodis, suggesting that fluctuating and constant conditions pose very different challenges. Acclimation to dry conditions led to higher absolute amounts of CHCs, which increased the ants' drought survival, whereas temperature acclimation did not. Hence, the temperature-induced CHC changes cannot be explained by the need for waterproofing alone. Although these changes could be non-adaptive, we propose that they serve to maintain a constant CHC viscosity, which may be essential for communication and other functions