98 research outputs found
From Equilibrium to Steady State: The Transient Dynamics of Colloidal Liquids under Shear
We investigate stresses and particle motion during the start up of flow in a
colloidal dispersion close to arrest into a glassy state. A combination of
molecular dynamics simulation, mode coupling theory and confocal microscopy
experiment is used to investigate the origins of the widely observed stress
overshoot and (previously not reported) super-diffusive motion in the transient
dynamics. A link between the macro-rheological stress versus strain curves and
the microscopic particle motion is established. Negative correlations in the
transient auto-correlation function of the potential stresses are found
responsible for both phenomena, and arise even for homogeneous flows and almost
Gaussian particle displacements.Comment: 24 pages, 14 figures, J. Phys.: Condens. Matter, in pres
Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear
We investigate mixing effects on the glass state of binary colloidal
hard-sphere-like mixtures with large size asymmetry, at a constant volume
fraction phi = 0.61. The structure, dynamics and viscoelastic response as a
function of mixing ratio reflect a transition between caging by one or the
other component. The strongest effect of mixing is observed in systems
dominated by caging of the large component. The possibility to pack a large
number of small spheres in the free volume left by the large ones induces a
pronounced deformation of the cage of the large spheres, which become
increasingly delocalised. This results in faster dynamics and a strong
reduction of the elastic modulus. When the relative volume fraction of small
spheres exceeds that of large spheres, the small particles start to form their
own cages, slowing down the dynamics and increasing the elastic modulus of the
system. The large spheres become the minority and act as an impurity in the
ordering beyond the first neighbour shell, i.e. the cage, and do not directly
affect the particle organisation on the cage level. In such a system, when
shear at constant rate is applied, melting of the glass is observed due to
facilitated out-of-cage diffusion which is associated with structural
anisotropy induced by shear.Comment: 8 pages, 7 figures, Proceedings of the 4th International Symposium on
Slow Dynamics in Complex Systems, Sendai, 2-7 December 201
Yielding of Hard-Sphere Glasses during Start-Up Shear
Concentrated hard-sphere suspensions and glasses are investigated with rheometry, confocal microscopy, and Brownian dynamics simulations during start-up shear, providing a link between microstructure, dynamics, and rheology. The microstructural anisotropy is manifested in the extension axis where the maximum of the pair-distribution function exhibits a minimum at the stress overshoot. The interplay between Brownian relaxation and shear advection as well as the available free volume determine the structural anisotropy and the magnitude of the stress overshoot. Shear-induced cage deformation induces local constriction, reducing in-cage diffusion. Finally, a superdiffusive response at the steady state, with a minimum of the time-dependent effective diffusivity, reflects a continuous cage breakup and reformation
Small-angle neutron scattering of percolative perfluoropolyether water-in-oil microemulsions
A water in oil microemulsion system composed of water, surfactant, and oil, the latter two components of perfluoropolyether (PFPE) type, has been studied by small-angle neutron scattering (SANS) with the aim of knowing the microstructure of the system and to have an insight on the connection between microstructure characterization and percolation behavior. In fact, along the dilution line W/S = 11 of the phase diagram, dielectric spectroscopy and conductivity studies revealed a dynamic percolation process taking place approaching and above the dynamic percolation threshold, leading to a system composed of droplet clusters with percolation thresholds varying with temperature from a 0.501 volume fraction of the dispersed phase at 9.3 °C to 0.205 at 32.5 °C. The SANS experimental spectra of this work have been studied by modeling the microemulsion droplets as adhesive hard spheres. For all of the samples, the surfactant area per polar head has been also measured in the Porod region of the SANS spectra. Geometric and potential parameters as well as the osmotic pressure, the second virial coefficient, and the distance between droplets have been extracted from data as a function of droplets concentration. At low concentration, that is, below percolation thresholds, the droplets behave as hard spheres, whereas at threshold and above, adhesion changes significantly the samples. In fact, for each temperature, the measured size increases versus concentration from 30 to 50 Å, and the area per polar head decreases correspondingly, suggesting that a process of dynamic fusion of droplets occurs in the system above threshold, that is, couples of droplets stick and unstick continuously with interdigitation of the surfactant tails.The EC for support via the ”Human Capital and Mobility - Access to Large Scale Facilities” Program, Contract ERB CHGECT920001, and the Italian MIUR, PRIN 2005, INFM, and CSGI for financial support.Peer reviewe
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