2,232 research outputs found
Convection in rotating annuli: Ginzburg-Landau equations with tunable coefficients
The coefficients of the complex Ginzburg-Landau equations that describe
weakly nonlinear convection in a large rotating annulus are calculated for a
range of Prandtl numbers . For fluids with , we
show that the rotation rate can tune the coefficients of the corresponding
amplitude equations from regimes where coherent patterns prevail to regimes of
spatio-temporal chaos.Comment: 4 pages (latex,multicol,epsf) including 3 figure
Contact Changes of Sheared Systems: Scaling, Correlations, and Mechanisms
We probe the onset and effect of contact changes in 2D soft harmonic particle
packings which are sheared quasistatically under controlled strain. First, we
show that in the majority of cases, the first contact changes correspond to the
creation or breaking of contacts on a single particle, with contact breaking
overwhelmingly likely for low pressures and/or small systems, and contact
making and breaking equally likely for large pressures and in the thermodynamic
limit. The statistics of the corresponding strains are near-Poissonian. The
mean characteristic strains exhibit scaling with the number of particles N and
pressure P, and reveal the existence of finite size effects akin to those seen
for linear response quantities. Second, we show that linear response accurately
predicts the strains of the first contact changes, which allows us to study the
scaling of the characteristic strains of making and breaking contacts
separately. Both of these show finite size scaling, and we formulate scaling
arguments that are consistent with the observed behavior. Third, we probe the
effect of the first contact change on the shear modulus G, and show in detail
how the variation of G remains smooth and bounded in the large system size
limit: even though contact changes occur then at vanishingly small strains,
their cumulative effect, even at a fixed value of the strain, are limited, so
that effectively, linear response remains well-defined. Fourth, we explore
multiple contact changes under shear, and find strong and surprising
correlations between alternating making and breaking events. Fifth, we show
that by making a link with extremal statistics, our data is consistent with a
very slow crossover to self averaging with system size, so that the
thermodynamic limit is reached much more slowly than expected based on finite
size scaling of elastic quantities or contact breaking strains
Stresses in Smooth Flows of Dense Granular Media
The form of the stress tensor is investigated in smooth, dense granular flows
which are generated in split-bottom shear geometries. We find that, within a
fluctuation fluidized spatial region, the form of the stress tensor is directly
dictated by the flow field: The stress and strain-rate tensors are co-linear.
The effective friction, defined as the ratio between shear and normal stresses
acting on a shearing plane, is found not to be constant but to vary throughout
the flowing zone. This variation can not be explained by inertial effects, but
appears to be set by the local geometry of the flow field. This is in agreement
with a recent prediction, but in contrast with most models for slow grain
flows, and points to there being a subtle mechanism that selects the flow
profiles.Comment: 5 pages, 4 figure
Contact Changes near Jamming
We probe the onset and effect of contact changes in soft harmonic particle
packings which are sheared quasistatically. We find that the first contact
changes are the creation or breaking of contacts on a single particle. We
characterize the critical strain, statistics of breaking versus making a
contact, and ratio of shear modulus before and after such events, and explain
their finite size scaling relations. For large systems at finite pressure, the
critical strain vanishes but the ratio of shear modulus before and after a
contact change approaches one: linear response remains relevant in large
systems. For finite systems close to jamming the critical strain also vanishes,
but here linear response already breaks down after a single contact change.Comment: 5 pages, 4 figure
Critical jamming of frictional grains in the generalized isostaticity picture
While frictionless spheres at jamming are isostatic, frictional spheres at
jamming are not. As a result, frictional spheres near jamming do not
necessarily exhibit an excess of soft modes. However, a generalized form of
isostaticity can be introduced if fully mobilized contacts at the Coulomb
friction threshold are considered as slipping contacts. We show here that, in
this framework, the vibrational density of states (DOS) of frictional discs
exhibits a plateau when the generalized isostaticity line is approached. The
crossover frequency to elastic behavior scales linearly with the distance from
this line. Moreover, we show that the frictionless limit, which appears
singular when fully mobilized contacts are treated elastically, becomes smooth
when fully mobilized contacts are allowed to slip.Comment: 4 pages, 4 figures, submitted to PR
Parent and Family Outcomes of PEERS: A Social Skills Intervention for Adolescents with Autism Spectrum Disorder
Raising a child with an Autism Spectrum Disorder (ASD) is associated with increased family chaos and parent distress. Successful long-term treatment outcomes are dependent on healthy systemic functioning, but the family impact of treatment is rarely evaluated. The Program for the Education and Enrichment of Relational Skills (PEERS) is a social skills intervention designed for adolescents with high-functioning ASD. This study assessed the impact of PEERS on family chaos, parenting stress, and parenting self-efficacy via a randomized, controlled trial. Results suggested beneficial effects for the experimental group in the domain of family chaos compared to the waitlist control, while parents in the PEERS experimental group also demonstrated increased parenting self-efficacy. These findings highlight adjunctive family system benefits of PEERS intervention and suggest the need for overall better understanding of parent and family outcomes of ASD interventions
Shocks near Jamming
Non-linear sound is an extreme phenomenon typically observed in solids after
violent explosions. But granular media are different. Right when they jam,
these fragile and disordered solids exhibit a vanishing rigidity and sound
speed, so that even tiny mechanical perturbations form supersonic shocks. Here,
we perform simulations in which two-dimensional jammed granular packings are
dynamically compressed, and demonstrate that the elementary excitations are
strongly non-linear shocks, rather than ordinary phonons. We capture the full
dependence of the shock speed on pressure and impact intensity by a
surprisingly simple analytical model.Comment: Revised version. Accepted for publication in Phys. Rev. Let
Anticipatory Smiling: Linking Early Affective Communication and Social Outcome
In anticipatory smiles, infants appear to communicate pre-existing positive affect by smiling at an object and then turning the smile toward an adult. We report two studies in which the precursors, development, and consequences of anticipatory smiling were investigated. Study 1 revealed a positive correlation between infant smiling at 6 months and the level of anticipatory smiling at 8 and 10 months during joint attention episodes, as well as a positive correlation between anticipatory smiling and parent-rated social expressivity scores at 30 months. Study 2 confirmed a developmental increase in the number of infants using anticipatory smiles between 9 and 12 months that had been initially documented in the Study 1 sample [Venezia, M., Messinger, D. S., Thorp, D., & Mundy, P. (2004). The development of anticipatory smiling. Infancy, 6(3), 397–406]. Additionally, anticipatory smiling at 9 months positively predicted parent-rated social competence scores at 30 months. Findings are discussed with regard to the importance of anticipatory smiling in early socioemotional development
Flow in linearly sheared two dimensional foams: from bubble to bulk scale
We probe the flow of two dimensional foams, consisting of a monolayer of
bubbles sandwiched between a liquid bath and glass plate, as a function of
driving rate, packing fraction and degree of disorder. First, we find that
bidisperse, disordered foams exhibit strongly rate dependent and inhomogeneous
(shear banded) velocity profiles, while monodisperse, ordered foams are also
shear banded, but essentially rate independent. Second, we introduce a simple
model based on balancing the averaged drag forces between the bubbles and the
top plate and the averaged bubble-bubble drag forces. This model captures the
observed rate dependent flows, and the rate independent flows. Third, we
perform independent rheological measurements, both for ordered and disordered
systems, and find these to be fully consistent with the scaling forms of the
drag forces assumed in the simple model, and we see that disorder modifies the
scaling. Fourth, we vary the packing fraction of the foam over a
substantial range, and find that the flow profiles become increasingly shear
banded when the foam is made wetter. Surprisingly, our model describes flow
profiles and rate dependence over the whole range of packing fractions with the
same power law exponents -- only a dimensionless number which measures the
ratio of the pre-factors of the viscous drag laws is seen to vary with packing
fraction. We find that , where , corresponding to the 2d jamming density, and suggest that this scaling
follows from the geometry of the deformed facets between bubbles in contact.
Overall, our work suggests a route to rationalize aspects of the ubiquitous
Herschel-Bulkley (power law) rheology observed in a wide range of disordered
materials.Comment: 16 pages, 14 figures, submitted to Phys. Rev. E. High quality version
available at: http://www.physics.leidenuniv.nl/sections/cm/gr
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