34 research outputs found
A novel particle tracking method with individual particle size measurement and its application to ordering in glassy hard sphere colloids
Particle tracking is a key to single-particle-level confocal microscopy
observation of colloidal suspensions, emulsions, and granular matter. The
conventional tracking method has not been able to provide accurate information
on the size of individual particle. Here we propose a novel method to localise
spherical particles of arbitrary relative sizes from either 2D or 3D (confocal)
images either in dilute or crowded environment. Moreover this method allows us
to estimate the size of each particle reliably. We use this method to analyse
local bond orientational ordering in a supercooled polydisperse colloidal
suspension as well as the heterogeneous crystallisation induced by a substrate.
For the former, we reveal non-trivial couplings of crystal-like bond
orientational order and local icosahedral order with the spatial distribution
of particle sizes: Crystal-like order tends to form in regions where very small
particles are depleted and the slightly smaller size of the central particle
stabilizes icosahedral order. For the latter, on the other hand, we found that
very small particles are expelled from crystals and accumulated on the growth
front of crystals. We emphasize that such information has not been accessible
by conventional tracking methods
Importance of many-body correlations in glass transition: an example from polydisperse hard spheres
Most of the liquid-state theories, including glass-transition theories, are
constructed on the basis of two-body density correlations. However, we have
recently shown that many-body correlations, in particular bond orientational
correlations, play a key role in both the glass transition and the
crystallization transition. Here we show, with numerical simulations of
supercooled polydisperse hard spheres systems, that the lengthscale associated
with any two-point spatial correlation function does not increase toward the
glass transition. A growing lengthscale is instead revealed by considering
many-body correlation functions, such as correlators of orientational order,
which follows the lengthscale of the dynamic heterogeneities. Despite the
growing of crystal-like bond orientational order, we reveal that the stability
against crystallization with increasing polydispersity is due to an increasing
population of icosahedral arrangements of particles. Our results suggest that,
for this type of systems, many-body correlations are a manifestation of the
link between the vitrification and the crystallization phenomena. Whether a
system is vitrified or crystallized can be controlled by the degree of
frustration against crystallization, polydispersity in this case.Comment: To appear in J. Chem. Phys. for a special issue on the Glass
Transitio
Local Oscillatory Rheology from Echography
Local Oscillatory Rheology from Echography (LORE) consists in a traditional
rheology experiment synchronized with high-frequency ultrasonic imaging which
gives access to the local material response to oscillatory shear. Besides
classical global rheological quantities, this method provides quantitative
time-resolved information on the local displacement across the entire gap of
the rheometer. From the local displacement response, we compute and decompose
the local strain in its Fourier components and measure the spatially-resolved
viscoelastic moduli. After benchmarking our method on homogeneous Newtonian
fluids and soft solids, we demonstrate that this technique is well suited to
characterize spatially heterogeneous samples, wall slip, and the emergence of
nonlinearity under large amplitude oscillatory stress in soft materials.Comment: 10 pages, 5 figures, submitted to Phys. Rev. Applie
コロイド過冷却液体における構造的不均一性と動的不均一性:共焦点顕微鏡による研究
The glass transition is often thought as decoupled from any structural change. I show in this thesis that two types of local order can be detected in a simple experimental glass former. This order increases when approaching the glass transition and is spatially correlated with the dynamic heterogeneities of the supercooled liquid.On envisage souvent la transition vitreuse comme découplée de tout changement structurel. Dans cette thèse, je montre que deux types d'ordre local peuvent être détectée dans un système vitreux expérimental simple. Cet ordre croit à l'approche de la transition vitreuse et est corrélé spatialement avec les hétérogénéités dynamiques du liquide surfondu
Creep and Fracture of a Protein Gel under Stress
International audienceBiomaterials such as protein or polysaccharide gels are known to behave qualitatively as soft solids and to rupture under an external load. Combining optical and ultrasonic imaging to shear rheology we show that the failure scenario of a protein gel is reminiscent of brittle solids: after a primary creep regime characterized by a power-law behavior whose exponent is fully accounted for by linear viscoelasticity, fractures nucleate and grow logarithmically perpendicularly to shear, up to the sudden rupture of the gel. A single equation accounting for those two successive processes nicely captures the full rheological response. The failure time follows a decreasing power law with the applied shear stress, similar to the Basquin law offatigue for solids. These results are in excellent agreement with recent fiber-bundle models that include damage accumulation on elastic fibers and exemplify protein gels as model, brittlelike soft solids
Active glass: ergodicity breaking dramatically affects response to self-propulsion
We study experimentally the response of a dense sediment of Brownian
particles to self-propulsion. We observe that the ergodic supercooled liquid
relaxation is monotonically enhanced by activity. By contrast the nonergodic
glass shows an order of magnitude slowdown at low activities with respect to
passive case, followed by fluidization at higher activities. Our results
contrast with theoretical predictions of the ergodic approach to glass
transition summing up to a shift of the glass line. We propose that
nonmonotonicity is due to competing effects of activity: (i) extra energy that
helps breaking cages (ii) directionality that hinders cage exploration. We call
it "Deadlock from the Emergence of Active Directionality" (DEAD). It suggests
further theoretical works should include thermal motion.Comment: 5 pages, 3 figures + supplementary materials (3 pages, 5 figures