50 research outputs found
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
Different scenarios of dynamic coupling in glassy colloidal mixtures
Colloidal mixtures represent a versatile model system to study transport in
complex environments. They allow for a systematic variation of the control
parameters, namely size ratio, total volume fraction and composition. We study
the effects of these parameters on the dynamics of dense suspensions using
molecular dynamics simulations and differential dynamic microscopy experiments.
We investigate the motion of the small particles through the matrix of large
particles as well as the motion of the large particles. A particular focus is
on the coupling of the collective dynamics of the small and large particles and
on the different mechanisms leading to this coupling. For large size ratios,
about 1:5, and an increasing fraction of small particles, the dynamics of the
two species become increasingly coupled and reflect the structure of the large
particles. This is attributed to the dominant effect of the large particles on
the motion of the small particles which is mediated by the increasing crowding
of the small particles. Furthermore, for moderate size ratios, about 1:3, and
sufficiently high fractions of small particles, mixed cages are formed and
hence the dynamics are also strongly coupled. Again, the coupling becomes
weaker as the fraction of small particles is decreased. In this case, however,
the collective intermediate scattering function of the small particles shows a
logarithmic decay corresponding to a broad range of relaxation times
Swelling and shrinking kinetics of a lamellar gel phase
We investigate the swelling and shrinking of L_beta lamellar gel phases
composed of surfactant and fatty alcohol after contact with aqueous
poly(ethylene-glycol) solutions. The height change is
diffusion-like with a swelling coefficient, S: . On
increasing polymer concentration we observe sequentially slower swelling,
absence of swelling, and finally shrinking of the lamellar phase. This behavior
is summarized in a non-equilibrium diagram and the composition dependence of S
quantitatively described by a generic model. We find a diffusion coefficient,
the only free parameter, consistent with previous measurements.Comment: 3 pages, 4 figures to appear in Applied Physics Letter