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Equilibrium concentration profiles and sedimentation kinetics of colloidal gels under gravitational stress 
Auteur(s): Buzzaccaro Stefano, Secchi Eleonora, Brambilla G., Piazza R., Cipelletti L.
Conference: Eight Liquid Matter Conference (Vienna, AT, 2011-09-06)
Actes de conférence: NA, vol. 24 p.284103 (2012)
Texte intégral en Openaccess : 
Ref HAL: hal-00675086_v1
Ref Arxiv: 1202.6474
DOI: 10.1088/0953-8984/24/28/284103
WoS: 000305786400005
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
13 Citations
Résumé: We study the sedimentation of colloidal gels by using a combination of light scattering, polarimetry and video imaging. The asymptotic concentration profiles $\varphi(z,t\rightarrow \infty)$ exhibit remarkable scaling properties: profiles for gels prepared at different initial volume fractions and particle interactions can be superimposed onto a single master curve by using suitable reduced variables. We show theoretically that this behavior stems from a power law dependence of the compressive elastic modulus \textit{vs} $\varphi$, which we directly test experimentally. The sedimentation kinetics comprises an initial latency stage, followed by a rapid collapse where the gel height $h$ decreases at constant velocity, and a final compaction stage characterized by a stretched exponential relaxation of $h$ towards a plateau. Analogies and differences with previous works are briefly discussed.
Commentaires: This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher-authenticated version will be available online
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Self-controlled confinement of nanoparticles in the web of grain boundaries of a colloidal polycrystal
Auteur(s): Ghofraniha N., Tamborini E., Oberdisse J., Cipelletti L., Ramos L.
Conference: 8th Liquid Matter Conference 2011 (Vienne, AT, 2011-09-06)
Ref HAL: hal-00637578_v1
Exporter : BibTex | endNote
Résumé: Composites materials comprising nanoparticles dispersed in a matrix are of great scientific and technological interest, since nanoparticles can enhance dramatically the matrix properties or even impart new functionalities, and because the matrix can act as a template that structures the particles at the nanoscopic level. However, controlling the three-dimensional spatial distribution of nanoparticles in a molecular or macromolecular matrix is a challenging task, as particle segregation usually depends crucially on the surface chemistry of the particles. Here, we present a model hybrid material, obtained by dispersing nanoparticles in a colloidal crystalline matrix, composed of thermoresponsive micelles. Using confocal microscopy, we show that the nanoparticles segregate in a network of thin sheets, in analogy to impurities confined in the grain boundaries of atomic polycrystals. We demonstrate that the size of the colloidal crystallites is tuned by varying independently the nanoparticle concentration (regardless of their composition and surface chemistry) and the crystallization rate, because they both determine the number of critical nuclei during the nucleation process and we quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is dictated solely by the typical size of the crystalline grains, due to the fact that the larger a grain can grow, the higher the concentration of the impurities progressively expelled from the crystallites during their growth and eventually trapped in the grain boundary, as we clearly show. Our method provides a general approach for confining nanoparticles in absence of any external field and in a controlled and tunable fashion in a three-dimensional soft colloidal matrix.
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