4,708 research outputs found
Coupling between aging and convective motion in a colloidal glass of Laponite
We study thermal convection in a colloidal glass of Laponite in formation.
Low concentration preparation are submitted to destabilizing vertical
temperature gradient, and present a gradual transition from a turbulent
convective state to a steady conductive state as their viscosity increases. The
time spent under convection is found to depend strongly on sample
concentration, decreasing exponentially with mass fraction of colloidal
particles. Moreover, at fixed concentration, it also depends slightly on the
pattern selected by the Rayleigh B\'{e}nard instability: more rolls maintain
the convection state longer. This behavior can be interpreted with recent
theoretical approaches of soft glassy material rheology.Comment: Eur. Phys. J. B 55, 101-107 (2007) The original publication is
available at http://www.springerlink.co
New N=1 Extended Superconformal Algebras with Two and Three Generators
In this paper we consider extensions of the super Virasoro algebra by one and
two super primary fields. Using a non-explicitly covariant approach we compute
all SW-algebras with one generator of dimension up to 7 in addition to the
super Virasoro field. In complete analogy to W-algebras with two generators
most results can be classified using the representation theory of the super
Virasoro algebra. Furthermore, we find that the SW(3/2, 11/2)-algebra can be
realized as a subalgebra of SW(3/2, 5/2) at c = 10/7. We also construct some
new SW-algebras with three generators, namely SW(3/2, 3/2, 5/2), SW(3/2, 2, 2)
and SW(3/2, 2, 5/2).Comment: 30 pages (Plain TeX), BONN-HE-92-0
Dynamical heterogeneity in aging colloidal glasses of Laponite
Glasses behave as solids due to their long relaxation time; however the
origin of this slow response remains a puzzle. Growing dynamic length scales
due to cooperative motion of particles are believed to be central to the
understanding of both the slow dynamics and the emergence of rigidity. Here, we
provide experimental evidence of a growing dynamical heterogeneity length scale
that increases with increasing waiting time in an aging colloidal glass of
Laponite. The signature of heterogeneity in the dynamics follows from dynamic
light scattering measurements in which we study both the rotational and
translational diffusion of the disk-shaped particles of Laponite in suspension.
These measurements are accompanied by simultaneous microrheology and
macroscopic rheology experiments. We find that rotational diffusion of
particles slows down at a faster rate than their translational motion. Such
decoupling of translational and orientational degrees of freedom finds its
origin in the dynamic heterogeneity since rotation and translation probe
different length scales in the sample. The macroscopic rheology experiments
show that the low frequency shear viscosity increases at a much faster rate
than both rotational and translational diffusive relaxation times.Comment: 12 pages, 5 figures, Accepted in Soft Matter 201
Microwave Conductivity due to Impurity Scattering in a d-wave Superconductor
The self-consistent t-matrix approximation for impurity scattering in
unconventional superconductors is used to interpret recent measurements of the
temperature and frequency dependence of the microwave conductivity of YBCO
crystals below 20K. In this theory, the conductivity is expressed in terms of a
fequency dependent single particle self-energy, determined by the impurity
scattering phase shift which is small for weak (Born) scattering and approaches
for unitary scattering. Inverting this process, microwave
conductivity data are used to extract an effective single-particle self-energy
and obtain insight into the nature of the operative scattering processes. It is
found that the effective self-energy is well approximated by a constant plus a
linear term in frequency with a small positive slope for thermal quasiparticle
energies below 20K. Possible physical origins of this form of self-energy are
discussed.Comment: 5 pages, 4 figure
Shear thickening of cornstarch suspensions as a re-entrant jamming transition
We study the rheology of cornstarch suspensions, a dense system of
non-Brownian particles that exhibits shear thickening, i.e. a viscosity that
increases with increasing shear rate. Using MRI velocimetry we show that the
suspension has a yield stress. From classical rheology it follows that as a
function of the applied stress the suspension is first solid (yield stress),
then liquid and then solid again when it shear thickens. The onset shear rate
for thickening is found to depend on the measurement geometry: the smaller the
gap of the shear cell, the lower the shear rate at which thickening occurs.
Shear thickening can then be interpreted as the consequence of the Reynolds
dilatancy: the system under flow wants to dilate but instead undergoes a
jamming transition because it is confined, as confirmed by measurement of the
dilation of the suspension as a function of the shear rate
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
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