Résumé:

We report on experiments probing the interface between two miscible fluids, an aqueous suspension of submicron colloidal particles and water. The suspension is confined between two plates and water is injected through a small hole in the top plate. As water is pushed through the more viscous colloidal suspension, the interface between the two fluids is destabilized, forming distinctive patterns similar to those observed for immiscible fluids (see figure below). We show that the interface shape is ruled by an off-equilibrium surface tension and validate quantitatively the one-century-old predictions by Korteweg. These findings open the way to a better understanding of a wealth of phenomena, from crystal nucleation and growth to shaping and forming in material science.

Résumé:

We study the fingering instability of the interface between two miscible fluids, a colloidal suspension and its own solvent [1]. The temporal evolution of the interface in a Hele-Shaw cell is found to be governed by the competition between the nonlinear viscosity of the suspension and an off-equilibrium, effective surface tension $\Gamme_e$. By studying suspensions in a wide range of volume fractions, $\Phi_C$, we show that $\Gamma_e ∼ \Phi_C^2$, in agreement with Korteweg’s theory for miscible fluids. The surface tension exhibits an anomalous increase with particle size, which we account for using entropy arguments. [1] Truzzolillo et al, Phys. Rev. Lett 112, 128303 (2014).

Résumé:

We describe an extension to current data analysis schemes of Taylor dispersion data that allows size polydispersity to be quantified for an arbitrary sample [1], thereby significantly enhancing the potentiality of this technique. The method is based on a cumulant development similar to that used for the analysis of dynamic light scattering data. Specific challenges posed by the cumulant analysis of Taylor dispersion data are discussed, and practical ways to address them are proposed. We successfully test this new method by analyzing both simulated and real experimental data for solutions of moderately polydisperse polymers and polymer mixtures. [1] L. Cipelletti, J.-P. Biron, M. Martin, H. Cottet, Anal. Chem 2014.

Résumé:

We use a custom shear cell coupled to an optical microscope to investigate at the particle level the yielding transition in concentrated emulsions subjected to an oscillatory shear deformation [1]. By performing experiments lasting thousands of cycles on samples at several volume fractions and for a variety of applied strain amplitudes, we obtain a comprehensive, microscopic picture of the yielding transition. We find that irreversible particle motion sharply increases beyond a volume-fraction dependent critical strain, which is found to be in close agreement with the strain beyond which the stress-strain relation probed in rheology experiments significantly departs from linearity. The shear-induced dynamics are very heterogenous: quiescent particles coexist with two distinct populations of mobile and ‘supermobile’ particles. Dynamic activity exhibits spatial and temporal correlations, with rearrangements events organized in bursts of motion affecting localized regions of the sample.

Ref HAL: hal-01058850_v1
Ref Arxiv: 1311.1996
DOI: 10.1103/PhysRevLett.113.078301
WoS: 000341115700027
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
25 Citations
Résumé:

We use confocal microscopy and time-resolved light scattering to investigate plasticity in a colloidal polycrystal, following the evolution of the network of grain boundaries as the sample is submitted to thousands of shear deformation cycles. The grain boundary motion is found to be ballistic, with a velocity distribution function exhibiting non-trivial power law tails. The shear- induced dynamics initially slow down, similarly to the aging of the spontaneous dynamics in glassy materials, but eventually reach a steady state. Surprisingly, the cross-over time between the ini- tial aging regime and the steady state decreases with increasing probed length scale, hinting at a hierarchical organization of the grain boundary dynamics.

Commentaires: main paper + supplementary materials Journal: Phys. Rev. Lett., 113, 078301 (2014)

Ref HAL: hal-01058494_v1
Ref Arxiv: 1408.6085
DOI: 10.1021/ac501115y
WoS: 000338488800043
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
23 Citations
Résumé:

Taylor dispersion analysis is an increasingly popular characterization method that measures the diffusion coefficient, and hence the hydrodynamic radius, of (bio)polymers, nanoparticles or even small molecules. In this work, we describe an extension to current data analysis schemes that allows size polydispersity to be quantified for an arbitrary sample, thereby significantly enhancing the potentiality of Taylor dispersion analysis. The method is based on a cumulant development similar to that used for the analysis of dynamic light scattering data. Specific challenges posed by the cumulant analysis of Taylor dispersion data are discussed, and practical ways to address them are proposed. We successfully test this new method by analyzing both simulated and experimental data for solutions of moderately polydisperse polymers and polymer mixtures.

Commentaires: 41 pages (including Supporting Information), 5 figures (+ 7 figs. in the Supporting Information)

Ref HAL: hal-01058242_v1
PMID 24920407
Ref Arxiv: 1403.4433
DOI: 10.1039/c4sm00531g
WoS: 000341025700007
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
91 Citations
Résumé:

We use a custom shear cell coupled to an optical microscope to investigate at the particle level the yielding transition in concentrated emulsions subjected to an oscillatory shear deformation. By performing experiments lasting thousands of cycles on samples at several volume fractions and for a variety of applied strain amplitudes, we obtain a comprehensive, microscopic picture of the yielding transition. We find that irreversible particle motion sharply increases beyond a volume-fraction dependent critical strain, which is found to be in close agreement with the strain beyond which the stress-strain relation probed in rheology experiments significantly departs from linearity. The shear-induced dynamics are very heterogenous: quiescent particles coexist with two distinct populations of mobile and 'supermobile' particles. Dynamic activity exhibits spatial and temporal correlations, with rearrangements events organized in bursts of motion affecting localized regions of the sample. Analogies with other sheared soft materials and with recent work on the transition to irreversibility in sheared complex fluids are briefly discussed.

Commentaires: 11 pages, 10 figures. Submitted to Soft Matter