189 research outputs found
Role of vibrations in the jamming and unjamming of grains discharging from a silo
We present experimental results of the jamming of non-cohesive particles
discharged from a flat bottomed silo subjected to vertical vibration. When the
exit orifice is only a few grain diameter wide, the flow can be arrested due to
the formation of blocking arches. Hence, an external excitation is needed to
resume the flow. The use of a continuous gentle vibration is a usual technique
to ease the flow in such situations. Even though jamming is less frequent, it
is still an issue in vibrated silos. There are, in principle, two possible
mechanisms through which vibrations may facilitate the flow: (i) a decrease in
the probability of the formation of blocking arches, and (ii) the breakage of
blocking arches once they have been formed. By measuring the time intervals
inside an avalanche during which no particles flow through the outlet, we are
able to estimate the probability of breaking a blocking arch by vibrations. The
result agrees with the prediction of a bivariate probabilistic model in which
the formation of blocking arches is equally probable in vibrated and
non-vibrated silos. This indicates that the second aforementioned mechanism is
the main responsible for improving the flowability in gently vibrated silos
Identification of arches in 2D granular packings
We identify arches in a bed of granular disks generated by a molecular
dynamic-type simulation. We use the history of the deposition of the particles
to identify the supporting contacts of each particle. Then, arches are defined
as sets of mutually stable disks. Different packings generated through tapping
are analyzed. The possibility of identifying arches from the static structure
of a deposited bed, without any information on the history of the deposition,
is discussed.Comment: 12 pages, 7 figure
Pattern formation without heating in an evaporative convection experiment
We present an evaporation experiment in a single fluid layer. When latent
heat associated to the evaporation is large enough, the heat flow through the
free surface of the layer generates temperature gradients that can destabilize
the conductive motionless state giving rise to convective cellular structures
without any external heating. The sequence of convective patterns obtained here
without heating, is similar to that obtained in B\'enard-Marangoni convection.
This work present the sequence of spatial bifurcations as a function of the
layer depth. The transition between square to hexagonal pattern, known from
non-evaporative experiments, is obtained here with a similar change in
wavelength.Comment: Submitted to Europhysics Letter
Unjamming a granular hopper by vibration
We present an experimental study of the outflow of a hopper continuously
vibrated by a piezoelectric device. Outpouring of grains can be achieved for
apertures much below the usual jamming limit observed for non vibrated hoppers.
Granular flow persists down to the physical limit of one grain diameter, a
limit reached for a finite vibration amplitude. For the smaller orifices, we
observe an intermittent regime characterized by alternated periods of flow and
blockage. Vibrations do not significantly modify the flow rates both in the
continuous and the intermittent regime. The analysis of the statistical
features of the flowing regime shows that the flow time significantly increases
with the vibration amplitude. However, at low vibration amplitude and small
orifice sizes, the jamming time distribution displays an anomalous statistics
Convective instabilities in two superposed horizontal liquid layers heated laterally
This work is devoted to the theoretical study of the stability of two
superposed horizontal liquid layers bounded by two solid planes and subjected
to a horizontal temperature gradient.
The liquids are supposed to be immiscible with a nondeformable interface.
The forces acting on the system are buoyancy and interfacial tension. Four
different flow patterns and temperature profiles are found for the basic state.
A linear perturbative analysis with respect to two and three dimensional
perturbations reveals the existence of three kind of patterns. Depending on the
relative height of both liquids several situations are predicted: either wave
propagation from cold to the hot regions, or waves propagating in the opposite
direction or still stationary longitudinal rolls. The behavior of three
different pairs of liquids which have been used in experiments on bilayers
under vertical gradient by other authors have been examined. The instability
mechanisms are discussed and a qualitative interpretation of the different
behaviors exhibited by the system is provided. In some configurations it is
possible to find a codimension-two point created by the interaction of two Hopf
modes with different frequencies and wavenumbers. These results suggest to
consider two liquid layers as an interesting prototype for the study of
propagation and interaction of waves in the context of the B\'enard-Marangoni
problem.Comment: 21 pages, 9 figures, 2 tables;accepted to be published in PR
An experimental route to spatiotemporal chaos in an extended 1D oscillators array
We report experimental evidence of the route to spatiotemporal chaos in a
large 1D-array of hotspots in a thermoconvective system. Increasing the driving
force, a stationary cellular pattern becomes unstable towards a mixed pattern
of irregular clusters which consist of time-dependent localized patterns of
variable spatiotemporal coherence. These irregular clusters coexist with the
basic cellular pattern. The Fourier spectra corresponding to this
synchronization transition reveals the weak coupling of a resonant triad. This
pattern saturates with the formation of a unique domain of great spatiotemporal
coherence. As we further increase the driving force, a supercritical
bifurcation to a spatiotemporal beating regime takes place. The new pattern is
characterized by the presence of two stationary clusters with a characteristic
zig-zag geometry. The Fourier analysis reveals a stronger coupling and enables
to find out that this beating phenomena is produced by the splitting of the
fundamental spatiotemporal frequencies in a narrow band. Both secondary
instabilities are phase-like synchronization transitions with global and
absolute character. Far beyond this threshold, a new instability takes place
when the system is not able to sustain the spatial frequency splitting,
although the temporal beating remains inside these domains. These experimental
results may support the understanding of other systems in nature undergoing
similar clustering processes.Comment: 12 pages, 13 figure
1-D Cluster Array at the Three Phase Contact Line in Diluted Colloids Subjected to A.C. Electric Fields
Colloidal particles provide an efficient mean of building multiple scale
structured materials from colloidal dispersions. In this Brief Report, we
account for experimental evidence on the formation of a colloidal cluster array
at a three-phase contact line. We study the influence of low frequency external
alternating electric fields on a diluted colloidal dispersion opened to the
air. We focus on the cluster formation and their evolution in the meniscus by
measuring characteristic times and lengths. We observe that the clusters are
separated by a well-defined length and that, in our experimental conditions,
they survive between five a fifteen minutes. These new results could be of
technological relevance in building tailored colloidal structures in
non-patterned substrates.Comment: There is supplementary information (see refs. 24, 27), which is
available upon request ([email protected]
Phase instabilities in hexagonal patterns
The general form of the amplitude equations for a hexagonal pattern including
spatial terms is discussed. At the lowest order we obtain the phase equation
for such patterns. The general expression of the diffusion coefficients is
given and the contributions of the new spatial terms are analysed in this
paper. From these coefficients the phase stability regions in a hexagonal
pattern are determined. In the case of Benard-Marangoni instability our results
agree qualitatively with numerical simulations performed recently.Comment: 6 pages, 6 figures, to appear in Europhys. Let
RUNX/AML and C/EBP factors regulate CD11a integrin expression in myeloid cells through overlapping regulatory elements
10 Figures. The publication costs of this article were defrayed in part by page charge
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. section 1734.The CD11a/CD18 (leukocyte function-associated antigen 1 [LFA-1]) integrin mediates critical leukocyte adhesive interactions during immune and inflammatory responses. The CD11a promoter directs CD11a/CD18 integrin expression, and its activity in lymphoid cells depends on a functional RUNX1/AML-1–binding site (AML-110) within the MS7 sequence. We now report that MS7 contains a C/EBP-binding site (C/EBP-100), which overlaps with AML-110 and is bound by C/EBP factors in myeloid cells. C/EBP and RUNX/AML factors compete for binding to their respective cognate elements and bind to the CD11a promoter MS7 sequence in a cell lineage- and differentiation-dependent manner. In myeloid cells MS7 is primarily recognized by C/EBP factors in proliferating cells whereas RUNX/AML factors (especially RUNX3/AML-2) bind to MS7 in differentiated cells. RUNX3/AML-2 binding to the CD11a promoter correlates with increased RUNX3/AML-2 protein levels and enhanced CD11a/CD18 cell surface expression. The relevance of the AML-110 element is underscored by the ability of AML-1/ETO to inhibit CD11a promoter activity, thus explaining the low CD11a/CD18 expression in t(8;21)–containing myeloid leukemia cells. Therefore, the expression of the CD11a/CD18 integrin in myeloid cells is determined through the differential occupancy of the CD11a proximal promoter by transcription factors implicated in the pathogenesis of myeloid leukemia.From the Centro de Investigaciones Biológicas, Consejo Superior de
Investigaciones CientÃficas, Madrid, Spain; ClÃnica Universitaria, Universidad de Navarra, Spain; Institute of Human Genetics, Aarhus, Denmark; Hospital Universitario Gregorio Maranón, Madrid, Spain; University of Colorado Health Sciences Center, Denver; and Department of Molecular Genetics, The
Weizmann Institute of Science, Rehovot Israel. Supported by grants 08.3/0026/2000.1 from Comunidad Auto´noma de Madrid,
01/0063-01 from Fondo de Investigaciones Sanitarias, and SAF2002-04615-
C02-01 from Ministerio de Ciencia y TecnologÃa (A.L.C.). We gratefully acknowledge Drs Ana Aranda and Aurora Sánchez-Pacheco for their very generous help with ChIP assays.Peer reviewe
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