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ItemSignatures of glass formation in a fluidized bed of hard spheres(Georgia Institute of Technology, 2006-04-14) Goldman, Daniel I. ; Swinney, Harry L.We demonstrate that a fluidized bed of hard spheres during defluidization displays properties associated with formation of a glass. The final state is rate dependent, and as this state is approached, the bed exhibits heterogeneity with increasing time and length scales. The formation of a glass results in the arrest of macroscopic particle motion and thus the loss of fluidization. Microscopic motion persists in this state, but the bed can be jammed by application of a small increase in flow rate. Thus a fluidized bed can serve as a test system for studies of glass formation and jamming.
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ItemStationary state volume fluctuations in a granular medium(Georgia Institute of Technology, 2005-03-30) Schroeter, Matthias ; Goldman, Daniel I. ; Swinney, Harry L.A statistical description of static granular material requires ergodic sampling of the phase space spanned by the different configurations of the particles. We periodically fluidize a column of glass beads and find that the sequence of volume fractions ϕ of postfluidized states is history independent and Gaussian distributed about a stationary state. The standard deviation of ϕ exhibits, as a function of ϕ, a minimum corresponding to a maximum in the number of statistically independent regions. Measurements of the fluctuations enable us to determine the compactivity X, a temperaturelike state variable introduced in the statistical theory of Edwards and Oakeshott [Physica A 157, 1080 (1989)].
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ItemMach cone in a shallow granular fluid(Georgia Institute of Technology, 2004-12-23) Heil, Patrick ; Rericha, Erin C. ; Goldman, Daniel I. ; Swinney, Harry L.We study the V-shaped wake (Mach cone) formed by a cylindrical rod moving through a thin, vertically vibrated granular layer. The wake, analogous to a shock (hydraulic jump) in shallow water, appears for rod velocities vR greater than a critical velocity c. We measure the half angle u of the wake as a function of vR and layer depth h. The angle satisfies the Mach relation, sin u =c/vR, where c=Îgh, even for h as small as one-particle diameter.
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ItemCrucial role of sidewalls in velocity distributions in quasi-two-dimensional granular gases(Georgia Institute of Technology, 2004-10-19) van Zon, J. S. ; Kreft, J. ; Goldman, Daniel I. ; Miracle, D. ; Swift, J. B. ; Swinney, Harry L.Our experiments and three-dimensional molecular dynamics simulations of particles confined to a vertical monolayer by closely spaced frictional walls (sidewalls) yield velocity distributions with non-Gaussian tails and a peak near zero velocity. Simulations with frictionless sidewalls are not peaked. Thus interactions between particles and their containers are an important determinant of the shape of the distribution and should be considered when evaluating experiments on a constrained monolayer of particles.
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ItemDynamics of drag and force distributions for projectile impact in a granular medium(Georgia Institute of Technology, 2004-05-14) Ciamarra, Massimo Pica ; Lara, Antonio H. ; Lee, Andrew T. ; Goldman, Daniel I. ; Vishik, Inna ; Swinney, Harry L.Our experiments and molecular dynamics simulations on a projectile penetrating a two-dimensional granular medium reveal that the mean deceleration of the projectile is constant and proportional to the impact velocity. Thus, the time taken for a projectile to decelerate to a stop is independent of its impact velocity. The simulations show that the probability distribution function of forces on grains is time independent during a projectile’s deceleration in the medium. At all times the force distribution function decreases exponentially for large forces.
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ItemPersistent holes in a fluid(Georgia Institute of Technology, 2004-05-07) Merkt, Florian S. ; Deegan, Robert D. ; Goldman, Daniel I. ; Rericha, Erin C. ; Swinney, Harry L.We observe stable holes in a vertically oscillated 0.5 cm deep aqueous suspension of cornstarch. Holes appear only if a finite perturbation is applied to the layer for accelerations α above 10g. Holes are circular and approximately 0.5 cm wide, and can persist for more than 10⁶ cycles. Above α ≃ 17g the rim of the hole becomes unstable, producing fingerlike protrusions or hole division. At higher acceleration, the hole delocalizes, growing to cover the entire surface with erratic undulations. We find similar behavior in an aqueous suspension of glass microspheres.
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ItemNoise, coherent fluctuations, and the onset of order in an oscillated granular fluid(Georgia Institute of Technology, 2004-04-30) Goldman, Daniel I. ; Swift, J. B. ; Swinney, Harry L.We study fluctuations in a vertically oscillated layer of grains below the critical acceleration for the onset of ordered standing waves. As onset is approached, transient disordered waves with a characteristic length scale emerge and increase in power and coherence. The scaling behavior and the shift in the onset of order agrees with the Swift-Hohenberg theory for convection in fluids. However, the noise in the granular system is an order of magnitude larger than the thermal noise in the most sensitive convecting fluid experiments to date; the effect of the granular noise is observable 20% below the onset of order.
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ItemKink-induced transport and segregation in oscillated granular layers(Georgia Institute of Technology, 2003-09-26) Moon, Sung Joon ; Goldman, Daniel I. ; Swift, J. B. ; Swinney, Harry L.We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counterrotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles.
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ItemLattice dynamics and melting of a nonequilibrium pattern(Georgia Institute of Technology, 2003-03-14) Goldman, Daniel I. ; Shattuck, M. D. ; Moon, Sung Joon ; Swift, J. B. ; Swinney, Harry L.We present a new description of nonequilibrium square patterns as a harmonically coupled crystal lattice. In a vertically oscillating granular layer, different transverse normal modes of the granular square-lattice pattern are observed for different driving frequencies (fd) and accelerations. The amplitude of a mode can be further excited by either frequency modulation of (fd) or reduction of friction between the grains and the plate. When the mode amplitude becomes large, the lattice melts (disorders), in accord with the Lindemann criterion for melting in two dimensions.
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ItemPhase bubbles and spatiotemporal chaos in granular patterns(Georgia Institute of Technology, 2001-12-04) Moon, Sung Joon ; Shattuck, M. D. ; Bizon, C. ; Goldman, Daniel I. ; Swift, J. B. ; Swinney, Harry L.We use inelastic hard sphere molecular dynamics simulations and laboratory experiments to study patterns in vertically oscillated granular layers. The simulations and experiments reveal that phase bubbles spontaneously nucleate in the patterns when the container acceleration amplitude exceeds a critical value, about 7g, where the pattern is approximately hexagonal, oscillating at one-fourth the driving frequency (f/4). A phase bubble is a localized region that oscillates with a phase opposite (differing by π) to that of the surrounding pattern; a localized phase shift is often called an arching in studies of two-dimensional systems. The simulations show that the formation of phase bubbles is triggered by undulation at the bottom of the layer on a large length scale compared to the wavelength of the pattern. Once formed, a phase bubble shrinks as if it had a surface tension, and disappears in tens to hundreds of cycles. We find that there is an oscillatory momentum transfer across a kink, and the shrinking is caused by a net collisional momentum inward across the boundary enclosing the bubble. At increasing acceleration amplitudes, the patterns evolve into randomly moving labyrinthian kinks (spatiotemporal chaos). We observe in the simulations that f/3 and f/6 subharmonic patterns emerge as primary instabilities, but that they are unstable to the undulation of the layer. Our experiments confirm the existence of transient f/3 and f/6 patterns.