Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition

As the glass (in molecular fluids\cite{Donth}) or the jamming (in colloids and grains\cite{LiuNature1998}) transitions are approached, the dynamics slow down dramatically with no marked structural changes. Dynamical heterogeneity (DH) plays a crucial role: structural relaxation occurs through correlated rearrangements of particle ``blobs'' of size $\xi$\cite{WeeksScience2000,DauchotPRL2005,Glotzer,Ediger}. On approaching these transitions, $\xi$ grows in glass-formers\cite{Glotzer,Ediger}, colloids\cite{WeeksScience2000,BerthierScience2005}, and driven granular materials\cite{KeysNaturePhys2007} alike, strengthening the analogies between the glass and the jamming transitions. However, little is known yet on the behavior of DH very close to dynamical arrest. Here, we measure in colloids the maximum of a ``dynamical susceptibility'', $\chi^*$, whose growth is usually associated to that of $\xi$\cite{LacevicPRE}. $\chi^*$ initially increases with volume fraction $\phi$, as in\cite{KeysNaturePhys2007}, but strikingly drops dramatically very close to jamming. We show that this unexpected behavior results from the competition between the growth of $\xi$ and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.Comment: 1st version originally submitted to Nature Physics. See the Nature Physics website fro the final, published versio

Void Fraction Influence Over Aqueous Foam Flow: Wall Shear Stress and Core Shear Evolution

International audienceIn this study, the two main transport characterization problems of the foam flow are studied: foam flow stability, through the evolution of the velocity at the core of the foam, and rheology, with the study of the wall shear stress over the lateral walls, for different void fractions. The same velocity profile (block flow, mean velocity 1.75 cm/s) is imposed to the foam flow, at the inlet of the channel, for several void fractions (air/water relation) going from 55 to 85 %. Later on these ones are passed through a singularity (fence) to study the different behaviours induced by the particular properties of each case. The velocity fields, the lateral liquid film thickness and the lateral wall shear stress fields are obtained and compared with one another to comprehend and remark the difference in such a complex flow. The results show that as we move closer to very dry foams the shear at the foam core increases and its velocity becomes higher. However, the wall shear stress at the lateral wall does not present big deviations from one void fraction to the other

Structure-property relationships from universal signatures of plasticity in disordered solids

When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, "softness," designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively

Resonance-driven random lasing

4 pages, 4 figures.-- Supplementary material available at http://dx.doi.org/10.1038/nphoton.2008.102: Fig. 1: Ohm's law fit for photonic glass, Fig. 2: Two dyes photonic glass reference sample.A random laser is a system formed by a random assembly of elastic scatterers dispersed into an optical gain medium. The multiple light scattering replaces the standard optical cavity of traditional lasers and the interplay between gain and scattering determines the lasing properties. All random lasers studied to date have consisted of irregularly shaped or polydisperse scatterers, with a certain average scattering strength that was constant over the frequency window of the laser. In this letter we consider the case where the scattering is resonant. We demonstrate that randomly assembled monodisperse spheres can sustain scattering resonances over the gain frequency window, and that the lasing wavelength can therefore be controlled by means of the diameter and refractive index of the spheres. The system is therefore a random laser with an a priori designed lasing peak within the gain curve.The work was financially supported by the European Commission (EC) (LENS) under contract number RII3-CT-2003-506350, by the European Union (EU) through the Network of Excellence IST-2-511616-NOE (PHOREMOST), CICyT NAN2004-08843-C05, MAT2006-09062, the Spanish MEC Consolider-QOIT CSD2006-0019 and the Comunidad de Madrid S-0505/ESP-0200.Peer reviewe