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Ref Arxiv: 1509.07987
DOI: 10.1103/PhysRevE.92.032307
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6 Citations
Résumé:

We investigate how the microstructure of a colloidal polycrystal influences its linear viscoelasticity. We use thermosensitive copolymer micelles that arrange in water in a cubic crystalline lattice, yielding a colloidal polycrystal. The polycrystal is doped with a small amount of nanoparticles, of size comparable to that of the micelles, which behave as impurities and thus partially segregate in the grain boundaries. We show that the shear elastic modulus only depends on the packing of the micelles and does not vary neither with the presence of nanoparticles nor with the crystal microstructure. By contrast, we find that the loss modulus is strongly affected by the presence of nanoparticles. A comparison between rheology data and small-angle neutron scattering data suggests that the loss modulus is dictated by the total amount of nanoparticles in the grain boundaries, which in turn depends on the sample microstructure.

Commentaires: . Réf Journal: Physical Review E, 92, 032307, 2015

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DOI: 10.1088/0953-8984/27/19/194103
WoS: WOS:000353778800005
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8 Citations
Résumé:

We explore the influence of particle softness and internal structure on both the bulk andinterfacial rheological properties of colloidal suspensions. We probe bulk stresses byconventional rheology, by measuring the flow curves, shear stress versus strain rate, forsuspensions of soft, deformable microgel particles and suspensions of near hard-sphere-likesilica particles. A similar behaviour is seen for both kinds of particles in suspensions atconcentrations up to the random close packing volume fraction, in agreement with recenttheoretical predictions for sub-micron colloids. Transient interfacial stresses are measured byanalyzing the patterns formed by the interface between the suspensions and their solvent, dueto a generalized Saffman–Taylor hydrodynamic instability. At odds with the bulk behaviour,we find that microgels and hard particle suspensions exhibit vastly different interfacial stressproperties. We propose that this surprising behaviour results mainly from the difference inparticle internal structure (polymeric network for microgels versus compact solid for the silicaparticles), rather than softness alone.

Résumé:

The dynamics of gels formed by strongly attractive colloidal particles exhibit very peculiar behavior: correlation functions measured by scattering methods decay steeper than exponentially (as opposed to the stretched exponential typically observed in glassy systems), and particle motion is ballistic (as opposed to the more usual diffusive behavior). Quite remarkably, over the last years the same behavior has been observed in a large variety of systems, including concentrated emulsions, foams, surfactant phases, actin networks and even hard condensed matter systems such as molecular or metallic glass formers. In this seminar, I'll review these experiments and discuss current models, based on the relaxation of internal stresses.

Résumé:

Surface tension is the tendency of a liquid to minimize its interface with another substance. Surface tension literally shapes the world around us: soap bubbles and liquid drops, insects striding on a water pond, and water transport in plants are but a few phenomena where surface tension plays a key role. Does a surface tension exist between two miscible fluids? While the interface between two such fluids inevitably fades out as the liquids mix due to diffusion, physicist and mathematician D. Korteweg proposed more than a century ago that an effective, off-equilibrium surface tension rules the behavior of the interface on time scales shorter than mixing. A direct proof of Korteweg’s ideas, however, was still lacking. Here, 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. We show that the interface shape is ruled by an off-equilibrium surface tension and validate quantitatively Korteweg’s predictions. 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:

Soft matter comprises micron or sub-micron-sized objects (solid colloidal particles, liquid emulsion drops, air bubbles, polymers...) suspended in a background solvent. In dilute systems, these objects undergo a characteristic erratic motion, due to the collisions with the solvent molecules, as first reported by British botanist Robert Brown for pollen grains. In more concentrated systems, the interactions between the suspended particles lead to slower, more complex dynamics, a crowding effect qualitatively similar to the slowing down of the microscopic dynamics in dense granular systems or molecular amorphous materials, such as glass formers. In this seminar, I'll review the slow dynamics of a variety of soft materials, from model systems for the glass transition (dense suspensions of colloidal hard spheres), to more complex systems such as colloidal gels made of attractive particles or gels made of actin filaments, as found in the cytoskeleton of cells. The emphasis will be on the striking similarities found in the behavior of many systems, in spite of their vastly different structure and composition.

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