2,626 research outputs found
A domain wall between single-mode and bimodal states and its transition to dynamical behavior in inhomogeneous systems
We consider domain walls (DW's) between single-mode and bimodal states that
occur in coupled nonlinear diffusion (NLD), real Ginzburg-Landau (RGL), and
complex Ginzburg-Landau (CGL) equations with a spatially dependent coupling
coefficient. Group-velocity terms are added to the NLD and RGL equations, which
breaks the variational structure of these models. In the simplest case of two
coupled NLD equations, we reduce the description of stationary configurations
to a single second-order ordinary differential equation. We demonstrate
analytically that a necessary condition for existence of a stationary DW is
that the group-velocity must be below a certain threshold value. Above this
threshold, dynamical behavior sets in, which we consider in detail. In the CGL
equations, the DW may generate spatio-temporal chaos, depending on the
nonlinear dispersion.Comment: 16 pages (latex) including 11 figures; accepted for publication in
Physica
Excess Floppy Modes and Multi-Branched Mechanisms in Metamaterials with Symmetries
Floppy modes --- deformations that cost zero energy --- are central to the
mechanics of a wide class of systems. For disordered systems, such as random
networks and particle packings, it is well-understood how the number of floppy
modes is controlled by the topology of the connections. Here we uncover that
symmetric geometries, present in e.g. mechanical metamaterials, can feature an
unlimited number of excess floppy modes that are absent in generic geometries,
and in addition can support floppy modes that are multi-branched. We study the
number of excess floppy modes by comparing generic and symmetric
geometries with identical topologies, and show that is extensive,
peaks at intermediate connection densities, and exhibits mean field scaling. We
then develop an approximate yet accurate cluster counting algorithm that
captures these findings. Finally, we leverage our insights to design
metamaterials with multiple folding mechanisms.Comment: Main text has 4 pages and 5 figures, and is further supported by
Supplementary Informatio
Rheology of Weakly Vibrated Granular Media
We probe the rheology of weakly vibrated granular flows as function of flow
rate, vibration strength and pressure by performing experiments in a vertically
vibrated split-bottom shear cell. For slow flows, we establish the existence of
a novel vibration dominated granular flow regime, where the driving stresses
smoothly vanish as the driving rate is diminished. We distinguish three
qualitatively different vibration dominated rheologies, most strikingly a
regime where the shear stresses no longer are proportional to the pressure.Comment: 14 pages, 19 figures, submitted to PR
Particle Diffusion in Slow Granular Bulk Flows
We probe the diffusive motion of particles in slowly sheared three
dimensional granular suspensions. For sufficiently large strains, the particle
dynamics exhibits diffusive Gaussian statistics, with the diffusivity
proportional to the local strain rate - consistent with a local, quasi static
picture. Surprisingly, the diffusivity is also inversely proportional to the
depth of the particles within the flow - at the free surface, diffusivity is
thus ill defined. We find that the crossover to Gaussian displacement
statistics is governed by the same depth dependence, evidencing a non-trivial
strain scale in three dimensional granular flows.Comment: 6 page
Silver(I) triflate-catalyzed protocol for the post-ugi synthesis of spiroindolines
A silver(I) triflate-catalyzed protocol for the post-Ugi synthesis of tetracyclic spiroindolines has been developed. The protocol worked best for indole-3-carbaldehyde-derived Ugi adducts obtained using anilines and 3-aryl propiolic acids. Thus, it is complementary to the previous cationic gold-catalyzed procedure that was developed for analogues Ugi substrates derived from aliphatic amines and 3-alkyl propiolic acids. Furthermore, we have demonstrated that under our new settings this domino Friedel-Crafts ipso cyclization / imine trapping process could be efficiently combined with the preceding four-component Ugi reaction into a two-step one-pot transformation
Non-affine response: jammed packings versus spring networks
We compare the elastic response of spring networks whose contact geometry is
derived from real packings of frictionless discs, to networks obtained by
randomly cutting bonds in a highly connected network derived from a
well-compressed packing. We find that the shear response of packing-derived
networks, and both the shear and compression response of randomly cut networks,
are all similar: the elastic moduli vanish linearly near jamming, and
distributions characterizing the local geometry of the response scale with
distance to jamming. Compression of packing-derived networks is exceptional:
the elastic modulus remains constant and the geometrical distributions do not
exhibit simple scaling. We conclude that the compression response of jammed
packings is anomalous, rather than the shear response.Comment: 6 pages, 6 figures, submitted to ep
Rheology of sedimenting particle pastes
We study the local and global rheology of non-Brownian suspensions in a
solvent that is not density-matched, leading to either creaming or
sedimentation of the particles. Both local and global measurements show that
the incomplete density matching leads to the appearance of a critical shear
rate above which the suspension is homogenized by the flow, and below which
sedimentation or creaming happens. We show that the value of the critical shear
rate and its dependence on the experimental parameters are governed by a simple
competition between the viscous and gravitational forces, and present a simple
scaling model that agrees with the experimental results from different types of
experiments (local and global) in different setups and systems
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
Continuum approach to wide shear zones in quasi-static granular matter
Slow and dense granular flows often exhibit narrow shear bands, making them
ill-suited for a continuum description. However, smooth granular flows have
been shown to occur in specific geometries such as linear shear in the absence
of gravity, slow inclined plane flows and, recently, flows in split-bottom
Couette geometries. The wide shear regions in these systems should be amenable
to a continuum description, and the theoretical challenge lies in finding
constitutive relations between the internal stresses and the flow field. We
propose a set of testable constitutive assumptions, including
rate-independence, and investigate the additional restrictions on the
constitutive relations imposed by the flow geometries. The wide shear layers in
the highly symmetric linear shear and inclined plane flows are consistent with
the simple constitutive assumption that, in analogy with solid friction, the
effective-friction coefficient (ratio between shear and normal stresses) is a
constant. However, this standard picture of granular flows is shown to be
inconsistent with flows in the less symmetric split-bottom geometry - here the
effective friction coefficient must vary throughout the shear zone, or else the
shear zone localizes. We suggest that a subtle dependence of the
effective-friction coefficient on the orientation of the sliding layers with
respect to the bulk force is crucial for the understanding of slow granular
flows.Comment: 11 pages, 7 figure
Sources and sinks separating domains of left- and right-traveling waves: Experiment versus amplitude equations
In many pattern forming systems that exhibit traveling waves, sources and
sinks occur which separate patches of oppositely traveling waves. We show that
simple qualitative features of their dynamics can be compared to predictions
from coupled amplitude equations. In heated wire convection experiments, we
find a discrepancy between the observed multiplicity of sources and theoretical
predictions. The expression for the observed motion of sinks is incompatible
with any amplitude equation description.Comment: 4 pages, RevTeX, 3 figur
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