2,033 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
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
Power dependence of pure spin current injection by quantum interference
We investigate the power dependence of pure spin current injection in GaAs
bulk and quantum-well samples by a quantum interference and control technique.
Spin separation is measured as a function of the relative strength of the two
transition pathways driven by two laser pulses. By keeping the relaxation time
of the current unchanged, we are able to relate the spin separation to the
injected average velocity. We find that the average velocity is determined by
the relative strength of the two transitions in the same way as in classical
interference. Based on this, we conclude that the density of injected pure spin
current increases monotonically with the excitation laser intensities. The
experimental results are consistent with theoretical calculations based on
Fermi's golden rule.Comment: 6 pages, 4 figure
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
Arrested state of clay-water suspensions: gel or glass?
The aging of a charged colloidal system has been studied by Small Angle
X-rays Scattering, in the exchanged momentum range Q=0.03 - 5 nm-1, and by
Dynamic Light Scattering, at different clay concentrations (Cw =0.6 % - 2.8 %).
The static structure factor, S(Q), has been determined as a function of both
aging time and concentration. This is the first direct experimental evidence of
the existence and evolution with aging time of two different arrested states in
a single system simply obtained only by changing its volume fraction: an
inhomogeneous state is reached at low concentrations, while a homogenous one is
found at high concentrations.Comment: 5 pages, 2 figure
The Case against Mercury as the Angrite Parent Body (APB)
Angrites are not plausibly from Mercury based on their high FeO contents and ancient ages (e.g., [1]). Rather, the early crystallization ages of angrites argues for a small asteroidal-sized parent body for these meteorites (e.g., [2]). Despite this, recently it has been proposed that Mercury is the APB [3, 4, 5, 6]. Preserved corona and symplectite textures and the presence of 120 triple junctions in NWA 2999 have been cited as requiring a planetary origin [3, 4], with the symplectites in NWA 2999 resulting from rapid decompression during uplift via thrust faults on Mercury [4], and the coronas during subsequent cooling at low pressure. Glasses along grain boundaries and exsolution lamellae possibly indicative of rapid melting and cooling in NWA 4950 are cited as evidence of rapid decompression [6]. To explain the discrepancy between spectral observations of the Mercurian surface and the high FeO contents in angrites, an early (~4.5 Ga), collisionally-stripped FeO-rich basaltic surface has been suggested for Mercury [5, 6]
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