Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z13: Granular Materials II |
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Sponsoring Units: GSNP DFD Chair: Arshad Kudrolli, Clark University Room: D225/226 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z13.00001: Perfect fluid flow from the impact of a dense granular jet Wendy W. Zhang, Jake Ellowitz, Nicholas Guttenberg, Herve Turlier, Sidney R. Nagel Axisymmetric collision of a cylindrical water jet with a circular target generates a thin conical sheet, also known as a water bell [Cheng et al. Phys. Rev. Lett. 99, 2007]. Intriguingly, recent experiments on granular jet impact in the regime of dense inertial flow reveal similar behavior: the angles by which the collimated sheets of particles are ejected from the target agree closely with the angles measured in the water-bell experiments [Clanet, C. J. Fluid Mech. 430, 2001] . This quantitative correspondence suggests that the collective granular motion during impact can be modeled as an incompressible, continuum fluid. Since viscous effects are weak in water-jet impact and the granular jet is comprised of non-cohesive particles (hence possessing zero surface tension), the simplest scenario is that the continuum motion corresponds to the flow of a perfect fluid. We show an exact solution of 2D perfect fluid impact agrees quantitatively with 2D discrete-particle simulation results. Therefore, the emergence of a highly collimated outgoing sheet does not necessarily signal the creation of a thermodynamic liquid phase. Such a coherent outcome results generically when the motion is nearly incompressible and dominated by inertia. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z13.00002: Jet-Induced Granular 2-D Crater Formation with Horizontal Symmetry Breaking Abe Clark, Robert Behringer We investigate the formation of a crater in a 2-D bed of granular material by a jet of impinging gas, motivated by the problem of a retrograde rocket landing on a planetary surface. As the strength and height of the jet are varied, the crater is characterized in terms of depth and shape as it evolves, as well as by the horizontal position of the bottom of the crater. The crater tends to grow logarithmically in time, a result which is common in related experiments. We also observe an unexpected horizontal symmetry breaking at certain well-defined conditions. We present data on the evolution of these asymmetric states and attempt to give insights into the mechanism behind the symmetry-breaking bifurcation. This horizontal symmetry breaking is highly suggestive of a pitchfork bifurcation, and we give evidence to classify it as forward or backward in different regimes of operation. As we will demonstrate, the formation of an asymmetric crater could be of considerable practical concern for lunar or planetary landers, particularly in the case of a backward pitchfork bifurcation, which is characterized by hysteresis and very rapid transitions. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z13.00003: Simulations of granular jet impact deadzone formation Nicholas Guttenberg, Jake Ellowitz, Wendy Zhang, Herve Turlier, Sidney Nagel Motivated by granular experiments showing the emergence of continuum-like dynamics when a dense jet hits a target, we simulate the impact of 2D and 3D granular jets of frictional, cohesion-less grains upon a fixed target. This is a inertial, dense jet regime where the motion is essentially incompressible. Impact deflects the material in the jet into a hollow conical sheet. The cone angles measured in simulation are consistent with previous experimental studies of the 3D granular jet impact. In addition, experiments have revealed the formation of a ``dead zone,'' a region where the grain motion is negligibly small. The simulation shows that this dead zone can only form when a no-slip boundary condition is enforced at the target. The presence or absence of the dead zone leads to a change in cone angle consistent with the experimentally observed differences in cone angle between the 3D granular flow and the corresponding water bell flow. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z13.00004: Endless penetration in impact cratering J. Carlos Ruiz-Suarez, Felipe Pacheco-Vazquez, J. Manuel Solano-Altamirano, Gabriel Caballero-Robledo The phenomena of impact cratering have been in the minds of physicists at least for two decades; the reason being the interest for elucidating the intriguing rheological response produced by granular systems when they are penetrated. With the great amount of work done in this regard, one could think that the problem is reasonably well understood. However, we study here a fascinating phenomenon never observed before in granular penetration experiments: depending on the mass of a projectile colliding onto a granular bed, it either stops at a given depth like normally expected, or keeps sinking with a terminal velocity as if the medium were a newtonian fluid. Understanding this intriguing behaviour could help us to know the subtleties of intrusion phenomena in granular media. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z13.00005: Avalanches of Singing Sand in the Laboratory Simon Dagois-Bohy, Sylvain Courrech Du Pont, St\'ephane Douady The song of dunes is a natural phenomenon that has arisen travellers' curiosity for a long time, from Marco Polo to R.A. Bagnold. Scientific observations in the XXth century have shown that the sound is emitted during a shear flow of these particular grains, the free surface of the flow having coherent vibrations like a loud speaker. The sound emission is also submitted to a threshold effect with many parameters like humidity, flow speed, surface of the grains. The sound has been reproduced in laboratory avalanche experiments close to the natural phenomenon on field, but set in a channel with a hard bottom and a few centimeters of sand flowing, which contradicts explanations of the sound that involve a sand dune under the avalanche flow. Flow rates measurements also show the presence of a plug region in the flow above the sheared band, with the same characteristic length as the coherence zones of the sound. Finally we show experimentally that the Froude number, once modified to take into account the height of this plug band, is the parameter that sets the amplitude of the sound, and produces a threshold that depends on the grain type. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z13.00006: Shear strength of vibrated granular/granular-fluid mixtures Brian Utter, Ralph Herman, Ben Foltz The behavior of dense granular materials can be characterized by the continuous forming and breaking of a strong force network resisting flow. This jamming/unjamming behavior is typical of a variety of systems and is influenced by factors such as grain packing fraction, applied shear stress, and the random kinetic energy of the particles. We present experiments on shear strength of granular and granular-water mixtures under the influence of external vibrations, one parameter that leads to unjamming. We use low vibration ($<$ 1g) and slow shear and measure avalanching statistics in a rotating drum and the torque required to move a stirrer through a sand/water mixture. We find that external vibration (i) increases granular strength at small vibrations in the dry system, (ii) removes history dependence (memory), and (iii) decreases shear strength at all accessible saturation levels in the sand-fluid system. Additionally, shear strength is found to be smallest for both dry and completely saturated mixtures. Additional ongoing experiments probe beyond a dimensionless acceleration of 1 and explore jamming and surface chemistry effects in the avalanching flow of granular/fluid mixtures. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z13.00007: Collisions between solitary waves in granular alignments Surajit Sen, Diankang Sun Solitary waves arise naturally when an unloaded alignment of elastic spheres, that is held between fixed end walls, is perturbed at one end. Unlike most known classes of solitary waves, those in granular materials are special and tend to break down and reform during any collision. Here we present what happens when two solitary waves of unequal magnitude suffer head-on and overtaking types of collisions. We will show that these collisions provide ways for solitary waves to not only become smaller but also become larger (within bounds) and that they are the underlying reason behind the emergence of the quasi-equilibrium phase. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z13.00008: Oil in Water Emulsion Flow in a 2D Hopper Xia Hong, Dandan Chen, Kenneth Desmond, Eric Weeks Granular flows are still somewhat poorly understood. One such case is the flow of 2D disks through a hopper. In a~prior experiment~by To~(K. To, et al. PRL 86(1) 2001), they found that as 2D disks flow through a hopper they may jam due to arch formations~at the hopper~exit, and that the jamming probability can be increased by enhancing the static friction between the disks.~In our study we remove the effects of static friction by using quasi-2D oil in water emulsion droplets flowing through a hopper to understand the role of friction in jamming. The droplets feel a viscous friction, but no static friction. Similar to the granular experiment, our oil droplets flow due to gravity. We have observed the transition between jammed and unjammed flows in our setup, and we are currently investigating its nature as the hopper size changes. In our experiments, jamming seems to occur only for very small hopper openings, and arches are always unstable. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z13.00009: Water Retention of Mixed Hydrogel Particles and Sandy Soil Yuli Wei, Douglas Durian We study the water-holding capacity of mixed hydrogel particles and a model sandy soil. To probe static behavior, we develop a custom pressure plate method that measures the expelled water per unit pressure increment per unit cross-sectional area; results are analyzed in terms of the water-accessible pore areas in the granular packing. To probe dynamic behavior, we build a raindrop impingement set-up that measures the retained water inside a dry granular packing during steady rain at a fixed rate. The percentage saturation of the granular packing is deduced. In both studies, we first determine the influence of the packing height and then of the gel concentration and size. Results from pressure plate method show that the swollen hydrogel particles partially clog the pores in the sandy soil, so that less water could be expelled for a given pressure increment. The total water-accessible area determined from the expelled water curve decreases exponentially as the gel concentration increases. Large hydrogel particles are less efficient in clogging the pores when no extra confinement is applied on the packing. Results from the raindrop impingement measurements also show that the water-holding capacity of sandy soil is improved by addition of hydrogel particles. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z13.00010: Granular ``electrophoresis'': in situ measurement of charge and size of freely-falling grains Scott Waitukaitis, Gustavo Castillo, Sebastian Gonzalez, Heinrich Jaeger We present measurements of tribocharged, chemically identical grains falling from a hopper. Tribocharging is the transfer of electrical charge between contacting surfaces. Granular interactions are governed by contacts, and not-surprisingly tribocharging can have important effects on bulk granular behavior. What is surprising is that this occurs even in grains of the same material. Typically same chemistry tribocharging (SCT) correlates with the particle size distribution: larger particles charge positively and smaller particles negatively. However, the detailed mechanism of SCT remains elusive. We have developed an experimental technique to make \emph{in situ} measurements of the particle size and charge on small ($\sim$100-500 $\mu$m) grains. With high speed videography of freely-falling grains we resolve particle sizes down to a few microns, charges as small as a few thousand electrons, and forces as small as a few picoNewtons. Our results confirm the qualitative charge segregation observed in previous SCT experiments and provide quantitative measurement for theoretical comparison. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z13.00011: Strain-stiffening in random packings of granular chains Heinrich Jaeger, Alice Nasto, Dylan Murphy, Eric Brown We report on triaxial compression experiments performed to characterize the mechanical response of random packings of granular particles. For a wide variety of particle shapes, the packings yield when the shear stress exceeds a value on the order of the confining pressure. In contrast, granular chains consisting of flexibly connected beads exhibit strain stiffening (i.e., the effective modulus increases with strain), sustain stresses far beyond the confining pressure, and do not yield until the chains break. The critical chain length required for significant strain-stiffening to occur corresponds to the minimum circumference of closed loops the chains are able to form during the formation of the packings. This strain-stiffening behavior is similar to that found in polymer materials, and chain packings therefore may serve as a model system to quantify the contribution of pure entanglement effects to the strength of polymer materials in the absence of Brownian motion. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z13.00012: Anisotropic diffusion of vibrated semi-flexible granular rods Vikrant Yadav, Arshad Kudrolli We discuss the diffusive dynamics of semi-flexible granular rods as a function of their concentration in a vertically vibrated container. These rods are composed of short beaded chains and are tracked with a camera, and their trajectories used to analyze the rotational and translational displacement as a function of area fraction $\phi$. We observe that the diffusion in the parallel and perpendicular direction in the body frame of reference deviate from those calculated for thermally excited elastic polymer rods. In particular we find that the diffusion perpendicular to the major axis in dilute regime is observed to be greater than that in the parallel direction due to rotation about the major axis of the rod. The motion is observed to become sub-linear above $\phi = 0.48$ and $0.54$ in the perpendicular and parallel directions, respectively, both lower than for spherical particles. Rotational diffusion is also investigated and found to deviate systematically from exponential decays with increase in $\phi$. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z13.00013: Vibrofluidized melting of geometrically cohesive granular media Nick Gravish, Geoffrey Russell, Scott V. Franklin, David Hu, Daniel I. Goldman Dry granular media composed of particles of special shapes (e.g. long rods or c-shaped particles) can display cohesive effects through particle geometry alone. We study the solid to gas transition in piles of c-shaped particles under vertical vibration as we vary acceleration and frequency. A cylindrical solid of particles is formed with wall angles near $90^\circ$ and is placed on a solid surface. For fixed frequency as acceleration increases, the pile undergoes two transitions. The first is from the solid-like state to a liquid-like state in which the wall angles relax but the mobile particles remain spatially localized. The second is from the liquid-like state to the gaseous state in which particles become separated (not entangled). Using video and accelerometer measurements, we record the temporal evolution of the spatial density and pile-plate collisional impulse. A critical energy scale, set by the particle geometry and gravitational potential energy, governs the liquid-gas transition. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z13.00014: Switchable capillary bridges in sphere packings Christoph G\"ogelein, Martin Brinkmann, Matthias Schr\"oter, Stephan Herminghaus If one adds a small amount of water to a heap of sand, it becomes paste-like since the grains get interconnected by capillary bridges. Due to this effect, we can easily sculpture wet sand (e.g., building a sand castle), whereas a heap of dry grains ripples away and cannot sustain any shape. In the present work, we use a non-Brownian suspension of glass spheres immersed in a binary liquid mixture. The suspending water-lutidine mixture exhibits a well studied lower critical point slightly above ambient temperature. Hence, the mixture starts to phase separate upon heating. Since the water-rich phase wets the hydrophilic glass spheres, capillary bridges are formed between adjacent particles. If the system is cooled below the demixing temperature, the bridges disappear within a few seconds by intermolecular diffusion. Thus, this systems offers the opportunity to switch the capillary bridges on and off by altering the temperature. In this presentation, we will show the temperature-induced formation of capillary bridges using confocal and bright light microscopy [1]. Furthermore, we will discuss the effect of capillary bridges on random sphere packings using a fluidized bed setup. \\[4pt] [1] C. G\"ogelein, M. Brinkmann, M. Schr\"oter, and S. Herminghaus, Langmuir 26 (2010) 22, 17184. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z13.00015: Stable Solitary Waves in Granular Alignments Yoichi Takato, Surajit Sen We study the propagation of an impulse in a loaded chain of elastic spheres where the spheres are held between fixed walls. We show that for a certain critical loading, propagating impulses develop into solitary waves and these solitary waves are not measurably affected by wall collisions, the latter being typically the case with granular solitary waves. The properties of these special solitary waves and of possible connections between this problem and the Fermi-Pasta-Ulam problem will be addressed. [Preview Abstract] |
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