232 research outputs found
Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry
Two-dimensional colloidal suspensions subject to laser interference patterns
with decagonal symmetry can form an Archimedean-like tiling phase where rows of
squares and triangles order aperiodically along one direction [J. Mikhael et
al., Nature 454, 501 (2008)]. In experiments as well as in Monte-Carlo and
Brownian dynamics simulations, we identify a similar phase when the laser field
possesses tetradecagonal symmetry. We characterize the structure of both
Archimedean-like tilings in detail and point out how the tilings differ from
each other. Furthermore, we also estimate specific particle densities where the
Archimedean-like tiling phases occur. Finally, using Brownian dynamics
simulations we demonstrate how phasonic distortions of the decagonal laser
field influence the Archimedean-like tiling. In particular, the domain size of
the tiling can be enlarged by phasonic drifts and constant gradients in the
phasonic displacement. We demonstrate that the latter occurs when the
interfering laser beams are not adjusted properly
Directed percolation identified as equilibrium pre-transition towards non-equilibrium arrested gel states
The macroscopic properties of gels arise from their slow dynamics and load-bearing network structure, which are exploited by nature and in numerous industrial products. However, a link between these structural and dynamical properties has remained elusive. Here we present confocal microscopy experiments and simulations of gel-forming colloid–polymer mixtures. They reveal that gel formation is preceded by continuous and directed percolation. Both transitions lead to system-spanning networks, but only directed percolation results in extremely slow dynamics, ageing and a shrinking of the gel that resembles synaeresis. Therefore, dynamical arrest in gels is found to be linked to a structural transition, namely directed percolation, which is quantitatively associated with the mean number of bonded neighbours. Directed percolation denotes a universality class of transitions. Our study hence connects gel formation to a well-developed theoretical framework, which now can be exploited to achieve a detailed understanding of arrested gels
Photon Channelling in Foams
Experiments by Gittings, Bandyopadhyay, and Durian [Europhys. Lett.\
\textbf{65}, 414 (2004)] demonstrate that light possesses a higher probability
to propagate in the liquid phase of a foam due to total reflection. The authors
term this observation photon channelling which we investigate in this article
theoretically. We first derive a central relation in the work of Gitting {\em
et al.} without any free parameters. It links the photon's path-length fraction
in the liquid phase to the liquid fraction . We then construct
two-dimensional Voronoi foams, replace the cell edges by channels to represent
the liquid films and simulate photon paths according to the laws of ray optics
using transmission and reflection coefficients from Fresnel's formulas. In an
exact honeycomb foam, the photons show superdiffusive behavior. It becomes
diffusive as soon as disorder is introduced into the foams. The dependence of
the diffusion constant on channel width and refractive index is explained by a
one-dimensional random-walk model. It contains a photon channelling state that
is crucial for the understanding of the numerical results. At the end, we
shortly comment on the observation that photon channelling only occurs in a
finite range of .Comment: 9 pages, minor change
One-Bead Microrheology with Rotating Particles
We lay the theoretical basis for one-bead microrheology with rotating
particles, i.e, a method where colloids are used to probe the mechanical
properties of viscoelastic media. Based on a two-fluid model, we calculate the
compliance and discuss it for two cases. We first assume that the elastic and
fluid component exhibit both stick boundary conditions at the particle surface.
Then, the compliance fulfills a generalized Stokes law with a complex shear
modulus whose validity is only limited by inertial effects, in contrast to
translational motion. Secondly, we find that the validity of the Stokes regime
is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let
Proliferation of anomalous symmetries in colloidal monolayers subjected to quasiperiodic light fields
Quasicrystals provide a fascinating class of materials with intriguing
properties. Despite a strong potential for numerous technical applications, the
conditions under which quasicrystals form are still poorly understood.
Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry
but no single example with 7 or 9-fold symmetry has ever been observed. Here we
report on geometrical constraints which impede the formation of quasicrystals
with certain symmetries in a colloidal model system. Experimentally, colloidal
quasicrystals are created by subjecting micron-sized particles to
two-dimensional quasiperiodic potential landscapes created by n=5 or seven
laser beams. Our results clearly demonstrate that quasicrystalline order is
much easier established for n = 5 compared to n = 7. With increasing laser
intensity we observe that the colloids first adopt quasiperiodic order at local
areas which then laterally grow until an extended quasicrystalline layer forms.
As nucleation sites where quasiperiodicity originates, we identify highly
symmetric motifs in the laser pattern. We find that their density strongly
varies with n and surprisingly is smallest exactly for those quasicrystalline
symmetries which have never been observed in atomic systems. Since such high
symmetry motifs also exist in atomic quasicrystals where they act as
preferential adsorption sites, this suggests that it is indeed the deficiency
of such motifs which accounts for the absence of materials with e.g. 7-fold
symmetry
Diffusive transport of light in a two-dimensional disordered packing of disks: Analytical approach to transport-mean-free path
We study photon diffusion in a two-dimensional random packing of monodisperse
disks as a simple model of granular media and wet foams. We assume that the
intensity reflectance of disks is a constant. We present an analytic expression
for the transport-mean-free path in terms of the velocity of light in the disks
and host medium, radius and packing fraction of the disks, and the intensity
reflectance. For the glass beads immersed in the air or water, we estimate
transport-mean-free paths about half the experimental ones. For the air bubbles
immersed in the water, transport-mean-free paths is an inverse function of
liquid volume fraction of the model wet foam. This throws new light on the
empirical law of Vera et. al, and promotes more realistic models.Comment: 9 pages, 6 figure
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