10,775 research outputs found
Interface Motion in Random Media at Finite Temperature
We have studied numerically the dynamics of a driven elastic interface in a
random medium, focusing on the thermal rounding of the depinning transition and
on the behavior in the pinned phase. Thermal effects are quantitatively
more important than expected from simple dimensional estimates. For sufficient
low temperature the creep velocity at a driving force equal to the
depinning force exhibits a power-law dependence on , in agreement with
earlier theoretical and numerical predictions for CDW's. We have also examined
the dynamics in the pinned phase resulting from slowly increasing the
driving force towards threshold. The distribution of avalanche sizes
decays as , with , in agreement with
recent theoretical predictions.Comment: harvmac.tex, 30 pages, including 9 figures, available upon request.
SU-rm-94073
Nonlinear Waves on a String with Inhomogeneous Properties
Nonlinear waves on an infinite string with a rapid change in properties at one location are treated. The string is an idealized version of more complex configurations in both fluids and solids. This idealized version treats the property change as an interface with a discontinuity in properties. Packets of waves are then considered with a reduced model, here a set of nonlinear Schr¨odinger (NLS) equations. The stress and the displacement must both be matched at the interface, resulting in dynamic and kinematic interfacial conditions. The dynamic condition produces an inhomogeneous effect that cannot be treated successfully with separation-of-variables. This inhomogeneity is treated here with a time-evolution approach using Laplace transforms. The results show that this inhomogeneity creates a mean longitudinal displacement on both sides of the interface and a shift in the position of the interface as the waves transit the interface. This mean longitudinal displacement corresponds to a sustained strain in the string. The mean longitudinal displacement develops three distinct features. One feature has a length scale that is half the wave-length of the incident waves, while the lengths of the other two features have the same order as the length of the wave packet. The position of maximum strain as a result of this mean is often at the interface, depending on parameter values. These results apply to a variety of applications, such as waves in ocean ice, Rayleigh waves caused by earthquakes, internal waves in the oceans and atmosphere, as well as waves in stretched cables
On the compact wave dynamics of tensegrity beams in multiple dimensions
This work presents a numerical investigation on the nonlinear wave dynamics
of tensegrity beams in 1D, 2D and 3D arrangements. The simulation of impact
loading on a chain of tensegrity prisms and lumped masses allows us to apply on
a smaller scale recent results on the propagation of compression solitary waves
in 1D tensegrity metamaterials. Novel results on the wave dynamics of 2D and 3D
beams reveal - for the first time - the presence of compact compression waves
in two- and three-dimensional tensegrity lattices with slender aspect ratio.
The dynamics of such systems is characterized by the thermalization of the
lattice nearby the impacted regions of the boundary. The portion of the
absorbed energy moving along the longitudinal direction is transported by
compression waves with compact support. Such waves emerge with nearly constant
speed, and slight modifications of their spatial shape and amplitude, after
collisions with compression waves traveling in opposite direction. The analyzed
behaviors suggest the use of multidimensional tensegrity lattices for the
design and additive manufacturing of novel sound focusing devices
Growing Dynamical Facilitation on Approaching the Random Pinning Colloidal Glass Transition
Despite decades of research, it remains to be established whether the
transformation of a liquid into a glass is fundamentally thermodynamic or
dynamic in origin. While observations of growing length scales are consistent
with thermodynamic perspectives like the Random First-Order Transition theory
(RFOT), the purely dynamic approach of the Dynamical Facilitation (DF) theory
lacks experimental validation. Further, for glass transitions induced by
randomly freezing a subset of particles in the liquid phase, simulations
support the predictions of RFOT, whereas the DF theory remains unexplored.
Here, using video microscopy and holographic optical tweezers, we show that
dynamical facilitation in a colloidal glass-forming liquid unambiguously grows
with density as well as the fraction of pinned particles. In addition, we show
that heterogeneous dynamics in the form of string-like cooperative motion,
which is believed to be consistent with RFOT, emerges naturally within the
framework of facilitation. Most importantly, our findings demonstrate that a
purely dynamic origin of the glass transition cannot be ruled out.Comment: 13 pages, 3 figures. Submitted to Nature Communications on the 17th
of March, 201
Non-linear optomechanical measurement of mechanical motion
Precision measurement of non-linear observables is an important goal in all
facets of quantum optics. This allows measurement-based non-classical state
preparation, which has been applied to great success in various physical
systems, and provides a route for quantum information processing with otherwise
linear interactions. In cavity optomechanics much progress has been made using
linear interactions and measurement, but observation of non-linear mechanical
degrees-of-freedom remains outstanding. Here we report the observation of
displacement-squared thermal motion of a micro-mechanical resonator by
exploiting the intrinsic non-linearity of the radiation pressure interaction.
Using this measurement we generate bimodal mechanical states of motion with
separations and feature sizes well below 100~pm. Future improvements to this
approach will allow the preparation of quantum superposition states, which can
be used to experimentally explore collapse models of the wavefunction and the
potential for mechanical-resonator-based quantum information and metrology
applications.Comment: 8 pages, 4 figures, extensive supplementary material available with
published versio
Stokesian Dynamics simulation of Brownian suspensions
The non-equilibrium behaviour of concentrated colloidal dispersions is studied by Stokesian Dynamics, a general molecular-dynamics-like technique for simulating particles suspended in a viscous fluid. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the Péclet number, Pe, which measures the relative importance of shear and Brownian forces. Three clearly defined regions of behaviour are revealed. There is first a Brownian-motion-dominated regime (Pe ≤ 1) where departures from equilibrium in structure and diffusion are small, but the suspension viscosity shear thins dramatically. When the Brownian and hydrodynamic forces balance (Pe ≈ 10), the dispersion forms a new ‘phase’ with the particles aligned in ‘strings’ along the flow direction and the strings are arranged hexagonally. This flow-induced ordering persists over a range of Pe and, while the structure and diffusivity now vary considerably, the rheology remains unchanged. Finally, there is a hydrodynamically dominated regime (Pe > 200) with a dramatic change in the long-time self-diffusivity and the rheology. Here, as the Péclet number increases the suspension shear thickens owing to the formation of large clusters. The simulation results are shown to agree well with experiment
Interactions and Correlations of Particulate Inclusions in a Columnar Phase
We calculate the elastic field mediated interaction between macroscopic
particles in a columnar hexagonal phase. The interaction is found to be
long-ranged and non-central, with both attractive and repulsive parts. We show
how the interaction modifies the particle correlations and the column
fluctuations. We also calculate the interaction of particles with the
topological defects of the columnar phase. The particle-defect interaction
reduces the mobility of the defects.Comment: RevTeX4 8 pages, 4 eps figures, submitted to Euro. Phys. J.
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