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The glass transition of two-dimensional binary soft disk mixtures with large size ratios
We simulate binary soft disk systems in two dimensions, and investigate how
the dynamics slow as the area fraction is increased toward the glass
transition. The "fragility" quantifies how sensitively the relaxation time
scale depends on the area fraction, and the fragility strongly depends on the
composition of the mixture. We confirm prior results for mixtures of particles
with similar sizes, where the ability to form small crystalline regions
correlates with fragility. However, for mixtures with particle size ratios
above 1.4, we find that the fragility is not correlated with structural
ordering, but rather with the spatial distribution of large particles. The
large particles have slower motion than the small particles, and act as
confining "walls" which slow the motion of nearby small particles. The
rearrangement of these confining structures governs the lifetime of dynamical
heterogeneity, that is, how long local regions exhibit anomalously fast or slow
behavior. The strength of the confinement effect is correlated with the
fragility and also influences the aging behavior of glassy systems.Comment: 11 pages, 10 figure
Glassy dynamics and dynamical heterogeneity in colloids
Concentrated colloidal suspensions are a well-tested model system which has a
glass transition. Colloids are suspensions of small solid particles in a
liquid, and exhibit glassy behavior when the particle concentration is high;
the particles are roughly analogous to individual molecules in a traditional
glass. Because the particle size can be large (100 nm - 1000 nm), these samples
can be studied with a variety of optical techniques including microscopy and
dynamic light scattering. Here we review the phenomena associated with the
colloidal glass transition, and in particular discuss observations of spatial
and temporally heterogeneous dynamics within colloidal samples near the glass
transition. Although this Chapter focuses primarily on results from
hard-sphere-like colloidal particles, we also discuss other colloidal systems
with attractive or soft repulsive interactions.Comment: Chapter of "Dynamical heterogeneities in glasses, colloids, and
granular media", Eds.: L. Berthier, G. Biroli, J-P Bouchaud, L. Cipelletti
and W. van Saarloos (Oxford University Press, to appear), more info at
http://w3.lcvn.univ-montp2.fr/~lucacip/DH_book.ht
Soap films as two-dimensional fluids: Diffusion and flow fields
We observe tracer particles diffusing in soap films to measure the
two-dimensional (2D) viscous properties of the films. We make soap films with a
variety of water-glycerol mixtures and of differing thicknesses. The
single-particle diffusivity relates closely to parameters of the film (such as
thickness ) for thin films, but the relation breaks down for thicker films.
Notably, the diffusivity is faster than expected for thicker films, with the
transition at using the tracer particle diameter . This
indicates a transition from purely 2D diffusion to diffusion that is more
three-dimensional. Additionally, we measure larger length scale flow fields
from correlated particle motions and find good agreement with what is expected
from theory of 2D fluids for all our films, thin and thick. We measure the
effective 2D viscosity of a soap film using single-particle diffusivity
measurements in thin films, and using the two-particle correlation measurements
in all films
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