766 research outputs found
Force balance in canonical ensembles of static granular packings
We investigate the role of local force balance in the transition from a
microcanonical ensemble of static granular packings, characterized by an
invariant stress, to a canonical ensemble. Packings in two dimensions admit a
reciprocal tiling, and a collective effect of force balance is that the area of
this tiling is also invariant in a microcanonical ensemble. We present
analytical relations between stress, tiling area and tiling area fluctuations,
and show that a canonical ensemble can be characterized by an intensive
thermodynamic parameter conjugate to one or the other. We test the equivalence
of different ensembles through the first canonical simulations of the force
network ensemble, a model system.Comment: 9 pages, 9 figures, submitted to JSTA
Geometry and Topology of Escape I: Epistrophes
We consider a dynamical system given by an area-preserving map on a
two-dimensional phase plane and consider a one-dimensional line of initial
conditions within this plane. We record the number of iterates it takes a
trajectory to escape from a bounded region of the plane as a function along the
line of initial conditions, forming an ``escape-time plot''. For a chaotic
system, this plot is in general not a smooth function, but rather has many
singularities at which the escape time is infinite; these singularities form a
complicated fractal set. In this article we prove the existence of regular
repeated sequences, called ``epistrophes'', which occur at all levels of
resolution within the escape-time plot. (The word ``epistrophe'' comes from
rhetoric and means ``a repeated ending following a variable beginning''.) The
epistrophes give the escape-time plot a certain self-similarity, called
``epistrophic'' self-similarity, which need not imply either strict or
asymptotic self-similarity.Comment: 15 pages, 9 figures, to appear in Chaos, first of two paper
Sticky Matter: Jamming and rigid cluster statistics with attractive particle interactions
While the large majority of theoretical and numerical studies of the jamming
transition consider athermal packings of purely repulsive spheres, real complex
fluids and soft solids generically display attraction between particles. By
studying the statistics of rigid clusters in simulations of soft particles with
an attractive shell, we present evidence for two distinct jamming scenarios.
Strongly attractive systems undergo a continuous transition in which rigid
clusters grow and ultimately diverge in size at a critical packing fraction.
Purely repulsive and weakly attractive systems jam via a first order
transition, with no growing cluster size. We further show that the weakly
attractive scenario is a finite size effect, so that for any nonzero attraction
strength, a sufficiently large system will fall in the strongly attractive
universality class. We therefore expect attractive jamming to be generic in the
laboratory and in nature.Comment: 4 pages, 5 figure
Model for the Scaling of Stresses and Fluctuations in Flows near Jamming
We probe flows of soft, viscous spheres near the jamming point, which acts as
a critical point for static soft spheres. Starting from energy considerations,
we find nontrivial scaling of velocity fluctuations with strain rate. Combining
this scaling with insights from jamming, we arrive at an analytical model that
predicts four distinct regimes of flow, each characterized by rational-valued
scaling exponents. Both the number of regimes and values of the exponents
depart from prior results. We validate predictions of the model with
simulations.Comment: 4 pages, 5 figures (revised text and one new figure). To appear in
Phys. Rev. Let
Jamming in finite systems: stability, anisotropy, fluctuations and scaling
Athermal packings of soft repulsive spheres exhibit a sharp jamming
transition in the thermodynamic limit. Upon further compression, various
structural and mechanical properties display clean power-law behavior over many
decades in pressure. As with any phase transition, the rounding of such
behavior in finite systems close to the transition plays an important role in
understanding the nature of the transition itself. The situation for jamming is
surprisingly rich: the assumption that jammed packings are isotropic is only
strictly true in the large-size limit, and finite-size has a profound effect on
the very meaning of jamming. Here, we provide a comprehensive numerical study
of finite-size effects in sphere packings above the jamming transition,
focusing on stability as well as the scaling of the contact number and the
elastic response.Comment: 20 pages, 12 figure
Relaxations and rheology near jamming
We determine the form of the complex shear modulus in soft sphere
packings near jamming. Viscoelastic response at finite frequency is closely
tied to a packing's intrinsic relaxational modes, which are distinct from the
vibrational modes of undamped packings. We demonstrate and explain the
appearance of an anomalous excess of slowly relaxing modes near jamming,
reflected in a diverging relaxational density of states. From the density of
states, we derive the dependence of on frequency and distance to the
jamming transition, which is confirmed by numerics.Comment: 4 pages, 3 figure
High-resolution imaging at the SOAR telescope
Bright single and binary stars were observed at the 4.1-m telescope with a
fast electron-multiplication camera in the regime of partial turbulence
correction by the visible-light adaptive optics system. We compare the angular
resolution achieved by simple averaging of AO-corrected images (long-exposure),
selection and re-centering (shift-and-add or "lucky" imaging) and speckle
interferometry. The effect of partial AO correction, vibrations, and image
post-processing on the attained resolution is shown. Potential usefulness of
these techniques is evaluated for reaching the diffraction limit in
ground-based optical imaging. Measurements of 75 binary stars obtained during
these tests are given and objects of special interest are discussed. We report
tentative resolution of the astrometric companion to Zeta Aqr B. A concept of
advanced high-resolution camera is outlined.Comment: Accepted for publication in PASP. 14 pages, 9 figures, 2 tabl
Analysis of Chaos-Induced Pulse Trains in the Ionization of Hydrogen
We examine excitation (by a short laser pulse) of a hydrogen atom in parallel electric and magnetic fields, from an initial tightly bound state to a state above the classical ionization threshold. We predict that the atom ionizes by emitting a train of electron pulses. This prediction is based on the classical dynamics of electron escape. In particular, the pulse train is due to classical chaos, which occurs for nonvanishing magnetic field. We connect the structure of the pulse train to fractal structure in the escape dynamics, and discuss several issues of experimental interest, with a particular emphasis on understanding the resolution of individual pulses. A brief account of this work appeared previously as a Letter [Phys. Rev. Lett. 92, 073001 (2004)]
Chaos-Induced Pulse Trains in the Ionization of Hydrogen
We predict that a hydrogen atom in parallel electric and magnetic fields, excited by a short laser pulse to an energy above the classical saddle, ionizes via a train of electron pulses. These pulses are a consequence of classical chaos induced by the magnetic field. We connect the structure of this pulse train (e.g., pulse size and spacing) to fractal structure in the classical dynamics. This structure displays a weak self-similarity, which we call “epistrophic self-similarity.” We demonstrate how this self-similarity is reflected in the pulse train
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