1,809 research outputs found
Influence of an adsorbing polymer in the aging dynamics of Laponite clay suspensions
Clay-polymer dispersions in aqueous solutions have attracted a great interest
in recent years due to their industrial applications and intriguing physical
properties. Aqueous solutions of bare Laponite particles are known to age
spontaneously from an ergodic state to a non ergodic state in a time varying
from hours to months depending on Laponite concentration. When a polymer
species like Polyethylene Oxide (PEO) is added to the solution, it weakly
adsorbs on clay particle surfaces modifying the effective interaction potential
between Laponite particles. A dynamic light scattering study, varying polymer
concentration at fixed polymer molecular weight (Mw=200.000 g/mol), has been
performed in order to understand the effect of polymer on the aging dynamics of
the system. The results obtained show that arresting phenomena between clay
particles are hindered if PEO is added and consequently the aging dynamics
slows down with increasing PEO concentration. This process is possibly due to
the progressive coverage of the clay surface by polymers that grow with
increasing PEO concentration and may lead to steric stabilization.Comment: 13 pages, 6 figures, manuscript accepted for publication on
Philosophical Magazin
More on phase diagram of Laponite
The phase diagram of a charged colloidal system (Laponite) has been
investigated by dynamic light scattering in a previously unexplored range of
salt and clay concentrations. Specifically the clay weight and salt molar
concentrations have been varied in the ranges Cw=0.004- 0.025, Cs=(1x 10^-3- 5x
10^-3) M respectively. As in the case of free salt water samples (Cs= 1x 10^-4
M) an aging dynamics towards two different arrested phases is found in the
whole examined Cw and Cs range. Moreover a transition between these two
different regimes is found for each investigated salt concentration. It is
clear from these measurements that a revision of the phase diagram is necessary
and a new "transition" line between two different arrested states is drawn.Comment: 16 pages, 5 figures, submitted to Langmui
Routes to gelation in a clay suspension
The gelation of water suspension of a synthetic clay (Laponite) has been
studied by dynamic light scattering in a wide range of clay weight
concentration (Cw = 0.003-0.031). At variance with previous determination,
indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the
gelation takes actually place in the whole examined Cw range. More importantly,
we find that Cw* marks the transition between two different routes to gelation.
We hypothesize that at low concentration Laponite suspension behaves as an
attractive colloid and that the slowing down of the dynamics is attained by the
formation of larger and larger clusters while at high concentration the basic
units of the gel could be the Debye Huckel spheres associated to single
Laponite plates.Comment: 5 pages, 4 figure
Polaron Crystallization and Melting: Effects of the Long-Range Coulomb Forces
On examining the stability of a Wigner crystal in an ionic dielectric, two
competitive effects due to the polaron formation are found to be important: (i)
the screening of the Coulomb force, which destabilizes the crystal, compensated
by (ii) the increase of the carrier mass (polaron mass). The competition
between the two effects is carefully studied, and the quantum melting of the
polaronic Wigner crystal is examined by varying the density at zero
temperature. By calculating the quantum fluctuations of both the electron and
the polarization, we show that there is a competition between the dissociation
of the polarons at the insulator-to-metal transition (IMT), and a melting
towards a polaron liquid. We find that at strong coupling, a liquid state of
dielectric polarons cannot exist, and the IMT is driven by the polaron
dissociation. Next, taking into account the dipolar interactions between
localized carriers, we show that these are responsible for an instability of
the transverse vibrational modes of the polaronic Wigner crystal as the density
increases. This provides a new mechanism for the IMT in doped dielectrics,
which yields interesting dielectric properties below and beyond the transition.
An optical signature of such a mechanism for the IMT is provided.Comment: 10 pages, 3 figures, to be published in Int.J.Mod.Phys.
Aging of the Nonlinear Optical Susceptibility of colloidal solutions
Using Z-scan and dynamic light scattering measurements we investigate the
nonlinear optics response of a colloidal solution undergoing dynamics slowing
down with age. We study the high optical nonlinearity of an organic dye
(Rhodamine B) dispersed in a water-clay (Laponite) solution, at different clay
concentrations (2.0 wt% - 2.6 wt%), experiencing the gelation process. We
determine the clay platelets self diffusion coefficient and, by its comparison
with the structural relaxation time, we conclude that the gelation process
proceeds through the structuring of interconnecting clay platelets network
rather than through clusters growth and aggregation.Comment: 4 figures, 4 page
Photodynamic Therapy of Necrobiosis Lipoidica - A Multicenter Study of 18 Patients
Background: Necrobiosis lipoidica (NL) is a granulomatous skin disease of unknown origin, and no reliably effective treatment option exists to handle this often disfiguring disease. Recently, a patient with long-lasting NL was reported to be cured by topical photodynamic therapy (PDT). Objective: To evaluate the overall potential of PDT in the treatment of NL on the lower legs. Methods: Retrospective study of 18 patients (aged 16 - 62 years) from 3 European university departments of dermatology treated with PDT for NL. Methyl aminolevulinate or 5-aminolevulinic acid were used as topically applied photosensitizers. Illumination followed with red light-emitting diode light. Results: Complete response was seen in 1/18 patients after 9 PDT cycles, and partial response in 6/18 patients (2 - 14 PDT cycles) giving an overall response rate of 39% (7/18). Conclusion: Although almost 40% of the cases showed some degree of response, PDT cannot currently be recommended as first-line therapy of NL. Subpopulations of therapy-resistant NL patients may, however, benefit from PDT. Copyright (C) 2008 S. Karger AG, Base
Charge-density wave formation in Sr_{14}Cu_{24}O_{41}
The electrodynamic response of the spin-ladder compound
SrCaCuO () has been studied from
radiofrequencies up to the infrared. At temperatures below 250 K a pronounced
absorption peak appears around 12 cm in SrCuO for
the radiation polarized along the chains/ladders ().
In addition a strongly temperature dependent dielectric relaxation is observed
in the kHz - MHz range. We explain this behavior by a charge density wave which
develops in the ladders sub-system and produces a mode pinned at 12 cm.
With increasing Ca doping the mode shifts up in frequency and eventually
disappears for because the dimensionality of the system crosses over from
one to two dimensions, giving way to the superconducting ground state under
pressure.Comment: One name added to author list 4 pages, 2 figures, email:
[email protected]
Competing interactions in arrested states of colloidal clays
Using experiments, theory and simulations, we show that the arrested state
observed in a colloidal clay at intermediate concentrations is stabilized by
the screened Coulomb repulsion (Wigner glass). Dilution experiments allow us to
distinguish this high-concentration disconnected state, which melts upon
addition of water, from a low-concentration gel state, which does not melt.
Theoretical modelling and simulations reproduce the measured Small Angle X-Ray
Scattering static structure factors and confirm the long-range electrostatic
nature of the arrested structure. These findings are attributed to the
different timescales controlling the competing attractive and repulsive
interactions.Comment: Accepted for publication in Physical Review Letter
Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41
The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized
by dc resistivity, low frequency dielectric and optical spectroscopy in all
three crystallographic directions. The collective charge-density wave screened
mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to
the mode along the legs. For x=8 and 9, the charge-density-wave response along
the rungs fully vanishes, while the one along the legs persists. The transport
perpendicular to the planes is always dominated by hopping.Comment: 4 pages, 3 figures, submitted to PRB R
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