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ABSTRACT: We explore the initial moments of impact between two dense granular clusters
in a two-dimensional geometry. The particles are composed of solid CO$_{2}$ and
are levitated on a hot surface. Upon collision, the propagation of a dynamic
"jamming front" produces a distinct regime for energy dissipation in a granular
gas in which the translational kinetic energy decreases by over 90%.
Experiments and associated simulations show that the initial loss of kinetic
energy obeys a power law in time, $\Delta E=-Kt^{3/2}$, a form that can be
predicted from kinetic arguments.
06/2013;
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ABSTRACT: When a dense granular jet hits a target, it forms a large dead zone and
ejects a highly collimated conical sheet with a well-defined opening angle.
Using experiments, simulations, and continuum modeling, we find that this
opening angle is insensitive to the precise target shape and the dissipation
mechanisms in the flow. We show that this surprising insensitivity arises
because dense granular jet impact, though highly dissipative, is nonetheless
controlled by the limit of perfect fluid flow.
04/2013;
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ABSTRACT: At zero temperature and applied stress, an amorphous packing of spheres
exhibits, as a function of packing fraction, a jamming transition where the
system is sensitive to boundary conditions even in the thermodynamic limit.
Upon further compression, the system should become insensitive to boundary
conditions but only if it is sufficiently large. Here we explore the linear
response to a large class of boundary perturbations in 2 and 3 dimensions. We
consider each finite packing with periodic-boundary conditions as the basis of
an infinite square or cubic lattice and study properties of vibrational modes
at arbitrary wave vector. Our results can be understood in terms of competition
between plane waves and the anomalous vibrational modes associated with the
jamming transition; boundary perturbations become irrelevant for systems that
are larger than a previously identified transverse length that diverges at the
jamming transition.
01/2013;
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ABSTRACT: We study the dependence on the packing fraction of the pair-correlation function g(r) and particle mobility in a dense three-dimensional packing of soft colloids made of poly N-isopropyl acrylamide (pNIPAM), a thermosensitive gel. We find that g(r) for our samples is qualitatively like that of a liquid at all packing fractions. There is a peak in g_{1}, the height of the first peak of g(r), as a function of the packing fraction. This peak is identified as a vestige, which remains at finite temperature, of the divergence found at the jamming transition in simulations of soft frictionless spheres at zero temperature. As the density is increased, the particle dynamics slow down and near the packing fraction where there is a peak in g_{1} the particles become arrested on the time scale of the experiment.
Physical Review E 01/2013; 87(1-1):012303. · 2.26 Impact Factor
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ABSTRACT: We present an analysis of finite-size effects in jammed packings of N soft,
frictionless spheres at zero temperature. There is a 1/N correction to the
discrete jump in the contact number at the transition so that jammed packings
exist only above isostaticity. As a result, the canonical power-law scalings of
the contact number and elastic moduli break down at low pressure. These
quantities exhibit scaling collapse with a non-trivial scaling function,
demonstrating that the jamming transition can be considered a phase transition.
Scaling is achieved as a function of N in both 2 and 3 dimensions, indicating
an upper critical dimension of 2.
04/2012;
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ABSTRACT: Drop coalescence is central to diverse processes involving dispersions of drops in industrial, engineering, and scientific realms. During coalescence, two drops first touch and then merge as the liquid neck connecting them grows from initially microscopic scales to a size comparable to the drop diameters. The curvature of the interface is infinite at the point where the drops first make contact, and the flows that ensue as the two drops coalesce are intimately coupled to this singularity in the dynamics. Conventionally, this process has been thought to have just two dynamical regimes: a viscous and an inertial regime with a cross-over region between them. We use experiments and simulations to reveal that a third regime, one that describes the initial dynamics of coalescence for all drop viscosities, has been missed. An argument based on force balance allows the construction of a new coalescence phase diagram.
Proceedings of the National Academy of Sciences 04/2012; 109(18):6857-61. · 9.68 Impact Factor
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ABSTRACT: A liquid drop impacting a solid surface may splash by emitting a thin liquid
sheet that subsequently breaks apart or by promptly ejecting droplets from the
advancing liquid-solid contact line. Using high-speed imaging, we show that air
pressure and surface roughness influence both splash mechanisms. Roughness
increases prompt splashing at the advancing contact line but inhibits the
formation of the thin sheet. If the air pressure is lowered, droplet ejection
is suppressed not only during thin-sheet formation but for prompt splashing as
well. The threshold pressure depends on impact velocity, liquid viscosity and
surface roughness.
03/2012;
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ABSTRACT: A drop impacting a solid surface with sufficient velocity will emit many small droplets creating a splash. However, splashing is completely suppressed if the surrounding gas pressure is lowered. The mechanism by which the gas affects splashing remains unknown. We use high-speed interference imaging to measure the air beneath all regions of a spreading viscous drop as well as optical absorption to measure the drop thickness. Although an initial air bubble is created on impact, no significant air layer persists until the time a splash is created. This suggests that splashing in our experimentally accessible range of viscosities is initiated at the edge of the drop as it encroaches into the surrounding gas.
Physical Review Letters 10/2011; 107(15):154502. · 7.37 Impact Factor
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ABSTRACT: Out-of-equilibrium disordered systems may form memories of external driving in a remarkable fashion. The system "remembers" multiple values from a series of training inputs yet "forgets" nearly all of them at long times despite the inputs being continually repeated. Here, learning and forgetting are inseparable aspects of a single process. The memory loss may be prevented by the addition of noise. We identify a class of systems with this behavior, giving as an example a model of non-Brownian suspensions under cyclic shear.
Physical Review Letters 07/2011; 107(1):010603. · 7.37 Impact Factor
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ABSTRACT: Using an electrical method and high-speed imaging, we probe drop coalescence down to 10 ns after the drops touch. By varying the liquid viscosity over two decades, we conclude that, at a sufficiently low approach velocity where deformation is not present, the drops coalesce with an unexpectedly late crossover time between a regime dominated by viscous and one dominated by inertial effects. We argue that the late crossover, not accounted for in the theory, can be explained by an appropriate choice of length scales present in the flow geometry.
Physical Review Letters 03/2011; 106(11):114501. · 7.37 Impact Factor
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ABSTRACT: After impact onto a smooth dry surface, a drop of viscous liquid initially spreads in the form of a thick lamella. If the drop splashes, it first emits a thin fluid sheet that can ultimately break up into droplets causing the splash. Ambient gas is crucial for creating this thin sheet. The time for sheet ejection, t{ejt}, depends on impact velocity, liquid viscosity, gas pressure, and molecular weight. A central air bubble is trapped below the drop at pressures even below that necessary for this sheet formation. In addition, air bubbles are entrained underneath the spreading lamella when the ejected sheet is present. Air entrainment ceases at a lamella velocity that is independent of drop impact velocity as well as ambient gas pressure.
Physical Review E 09/2010; 82(3 Pt 2):036302. · 2.26 Impact Factor
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ABSTRACT: When a system jams, it undergoes a transition from a flowing to a rigid state. Despite this important change in the dynamics, the internal structure of the system remains disordered in the solid as well as the fluid phase. In this way jamming is quite different from crystallization, the other common way in which a fluid solidifies. Jamming is a paradigm for thinking about how many different types of fluids—from molecular liquids to macroscopic granular matter—develop rigidity. Here we review recent work on the jamming transition. We start with perhaps the simplest model: frictionless spheres interacting via repulsive finite-range forces at zero temperature. In this highly idealized case, the transition has aspects of both first- and second-order transitions. From studies of the normal modes of vibration for the marginally jammed solid, new physics has emerged for how a material can be rigid without having the elastic properties of a normal solid. We first survey the simulation data and theoretical argumen...
07/2010; 1:347-369.
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ABSTRACT: Sorting the integers 1 through N into an ordered list is a simple task that can be done rapidly. However, using an algorithm based on the thermally activated pairwise exchanges of neighboring list elements, we find sorting can display many features of a glass, even for lists as small as N=5. This includes memory and rejuvenation effects during aging-two hallmarks of glassy dynamics that have been difficult to reproduce in standard glass simulations.
Physical Review Letters 06/2010; 104(25):257201. · 7.37 Impact Factor
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ABSTRACT: The jamming scenario of disordered media, formulated about 10 years ago, has
in recent years been advanced by analyzing model systems of granular media.
This has led to various new concepts that are increasingly being explored in in
a variety of systems. This chapter contains an introductory review of these
recent developments and provides an outlook on their applicability to different
physical systems and on future directions. The first part of the paper is
devoted to an overview of the findings for model systems of frictionless
spheres, focussing on the excess of low-frequency modes as the jamming point is
approached. Particular attention is paid to a discussion of the cross-over
frequency and length scales that govern this approach. We then discuss the
effects of particle asphericity and static friction, the applicability to
bubble models for wet foams in which the friction is dynamic, the dynamical
arrest in colloids, and the implications for molecular glasses.
06/2010;
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ABSTRACT: We calculate numerically the normal modes of vibrations in three-dimensional jammed packings of soft spheres as a function of the packing fraction and obtain the energy diffusivity, a spectral measure of transport that controls sound propagation and thermal conductivity. The crossover frequency between weak and strong phonon scattering is controlled by the coordination and shifts to zero as the system is decompressed toward the critical packing fraction at which rigidity is lost. We present a scaling analysis that relates the packing fraction dependence of the crossover frequency to the anomalous scaling of the shear modulus with compression. Below the crossover, the diffusivity displays a power-law divergence with inverse frequency consistent with Rayleigh law, which suggests that the vibrational modes are primarily transverse waves, weakly scattered by disorder. Above it, a large number of modes appear whose diffusivity plateaus at a nearly constant value before dropping to zero above the localization frequency. The thermal conductivity of a marginally jammed solid just above the rigidity threshold is calculated and related to the one measured experimentally at room temperature for most glasses.
Physical Review E 02/2010; 81(2 Pt 1):021301. · 2.26 Impact Factor
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ABSTRACT: We show that the slowing of the dynamics in simulations of several model glass-forming liquids is equivalent to the hard-sphere glass transition in the low-pressure limit. In this limit, we find universal behavior of the relaxation time by collapsing molecular-dynamics data for all systems studied onto a single curve as a function of T/p, the ratio of the temperature to the pressure. At higher pressures, there are deviations from this universal behavior that depend on the interparticle potential, implying that additional physical processes must enter into the dynamics of glass formation.
Physical Review Letters 12/2009; 103(24):245701. · 7.37 Impact Factor
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ABSTRACT: Rigid particles pack into structures, such as sand dunes on the beach, whose overall stability is determined by the average number of contacts between particles. However, when packing spatially extended objects with flexible shapes, additional concepts must be invoked to understand the stability of the resulting structure. Here, we examine the disordered packing of chains constructed out of flexibly connected hard spheres. Using x-ray tomography, we find that long chains pack into a low-density structure whose mechanical rigidity is mainly provided by the backbone. On compaction, randomly oriented, semi-rigid loops form along the chain, and the packing of chains can be understood as the jamming of these elements. Finally, we uncover close similarities between the packing of chains and the glass transition in polymers.
Science 10/2009; 326(5951):408-10. · 31.20 Impact Factor
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ABSTRACT: We compare the harmonic and anharmonic properties of the vibrational modes in 3-dimensional jammed packings of frictionless spheres interacting via repulsive, finite range potentials. A crossover frequency is apparent in the density of states, the diffusivity and the participation ratio of the modes. At this frequency, which shifts to zero at the jamming threshold, the vibrational modes have a very small participation ratio implying that the modes are quasi-localized. The most anharmonic modes occur at low frequency which is opposite to what is normally found in crystals. The lowest frequency modes have the strongest response to the pressure and the lowest energy barriers to mechanical failure. Comment: 6 pages, 5 figures
09/2009;
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ABSTRACT: When the packing fraction is increased sufficiently, loose particulates jam to form a rigid solid in which the constituents are no longer free to move. In typical granular materials and foams, the thermal energy is too small to produce structural rearrangements. In this zero-temperature (T = 0) limit, multiple diverging and vanishing length scales characterize the approach to a sharp jamming transition. However, because thermal motion becomes relevant when the particles are small enough, it is imperative to understand how these length scales evolve as the temperature is increased. Here we used both colloidal experiments and computer simulations to progress beyond the zero-temperature limit to track one of the key parameters-the overlap distance between neighbouring particles-which vanishes at the T = 0 jamming transition. We find that this structural feature retains a vestige of its T = 0 behaviour and evolves in an unusual manner, which has masked its appearance until now. It is evident as a function of packing fraction at fixed temperature, but not as a function of temperature at fixed packing fraction or pressure. Our results conclusively demonstrate that length scales associated with the T = 0 jamming transition persist in thermal systems, not only in simulations but also in laboratory experiments.
Nature 06/2009; 459(7244):230-3. · 36.28 Impact Factor
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ABSTRACT: We calculate the normal modes of vibration in jammed sphere packings to obtain the energy diffusivity, a spectral measure of transport. At the boson peak frequency, we find an Ioffe-Regel crossover from a diffusivity that drops rapidly with frequency to one that is nearly frequency independent. This crossover frequency shifts to zero as the system is decompressed towards the jamming transition, providing unambiguous evidence of a regime in frequency of nearly constant diffusivity. Such a regime, postulated to exist in glasses to explain the temperature dependence of the thermal conductivity, therefore appears to arise from properties of the jamming transition.
Physical Review Letters 02/2009; 102(3):038001. · 7.37 Impact Factor
