Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F49: Focus Session: Wet Granular Matter |
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Sponsoring Units: GSOFT GSNP DFD Chair: Arshad Kudrolli, Clark University Room: 217D |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F49.00001: Wet sand flows better than dry sand Invited Speaker: Christian Wagner Wet sand that does not contain too much water is known to be stiff enough to build sand castles or in physical words has a significant yield stress. However, we could recently show that there are quite a few conditions under which such wet sand opposes less resistant to flow than its dry counterpart. This effect might have been already known to the old Egyptians: The Ancient painting of El Bersheh at the tomb of Tehutihetep shows that there was liquid poured in front of the sledge that was used to transport heavy weight stones and statues. While archeologist have attributed this to a sacral ceremony, our data clearly show that wetting the sand ground drastically decreases the effective sliding friction coefficient. We first study the stress-strain behavior of sand with and without small amounts of liquid under steady and oscillatory shear. Using a technique to quasistatically push the sand through a tube with an enforced parabolic (Poiseuille-like) profile, we minimize the effect of avalanches and shear localization. We observe that the resistance against deformation of the wet (partially saturated) sand is much smaller than that of the dry sand, and that the latter dissipates more energy under flow. Second we show experimentally that the sliding friction on sand is greatly reduced by the addition of some---but not too much---water. The formation of capillary water bridges increases the shear modulus of the sand, which facilitates the sliding. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F49.00002: Wet granular materials submitted to thermal cycling Geoffroy Lumay, Francois Ludewig, Jorge Fiscina, Maryam Pakpour, Nicolas Vandewalle, Stephane Dorbolo Many phenomenons observed in nature are related to the particular behavior of wet granular materials submitted to temperature cycling: ice-lens formation in soil leading to frost heaving, landslides, structures formation in permafrost, stone heave and possibly some geological formations observed on Mars. We present experimental results concerning the effect of thermal cycling on the packing fraction of equal spheres with the presence of water. First, the case corresponding to completely immersed granular piles is considered. Afterward, the effect of thermal cycling on unsaturated granular piles is discussed. The pile is submitted to temperature cycling ranging from T1 to T2. If the temperature is always higher than 4$^{\circ}$C, the temperature increase (or decrease) induces a dilatation (or contraction) of the grains and of the water. We show that the packing fraction variation is mainly related to water dilatation and contraction. If the temperature decreases under 0$^{\circ}$C during a cycle, the water situated between the grains experiences a strong dilatation during the freezing step and a contraction during the ice melting step. In this case, we show how the freeze-thaw transition affects the packing fraction of the pile. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F49.00003: Mechanical Properties of Sheared Wet Granular Piles Ralf Seemann, Marc Schaber, Somnath Karmakar, Anna-Lena Hippler, Mario Scheel, Marco Di Michiel, Martin Brinkmann The mechanical properties of dry and wet granulates are explored when being sheared with a parabolic profile at constant shear volume. The dissipated energy increase linearly with external pressure both for a wet and a dry granulate. However, the dissipated energy for wet a granulate has a finite value for the limiting case of vanishing external pressure and increases slower with external pressure compared to the dry granulate. Using a down sized version of the shear cell the reorganization of a granulate and liquid is additionally imaged in real time using x-ray micro-tomography. With the insight from x-ray tomography the contribution of the breaking capillary bridges to the dissipated energy can be analyzed. We could also shed light on the influence of dilatation effects on the dissipated energy upon inverting the shear direction. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F49.00004: The Effect Liquid Loading on the Rheology of Granular Flows Sankaran Sundaresan, Ali Ozel, Yile Gu, Stefan Radl Discrete element simulations of simple shear flows of dense and homogeneous assemblies of uniform, spherical, soft and dry particles reveal three regimes: (i) a quasi-static regime, where the stress is independent of shear rate, (ii) an inertial regime where the stress varies quadratically with shear rate and (iii) an intermediate regime where the stress manifests power-law dependence with n~1/2. Inclusion of inter-particle cohesion due to van der Waals force has been shown to lead to bifurcation of the inertial regime into two regimes: (a) a cohesive rate-independent regime and (b) an inertial regime. In the present study, we perform analogous simulations for wet particles. We account for capillary and viscous interaction forces between particles, which result from the liquid bridges, and allow for liquid transfer between the particles and the liquid bridge. It is found that the bifurcation of the inertial regime observed with van der Waals interaction persists for capillary cohesion and that the span of the cohesive rate-independent regime increases with liquid loading in the pendular regime. A simple model for steady shear rheology is obtained by blending the results in various regimes. The presentation will also discuss the effect liquid viscosity on the flow behavior. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F49.00005: Shear bands at the Jamming Transition: The role of Weak Attractive Interactions Ehsan Irani, Pinaki Chaudhuri, Claus Heussinger We study the rheology of a particulate sytem close to jamming in the presence of weakly attractive interactions. Lees-Edwards boundary conditions are used to simulate a shear-controlled flow. In addition to Bagnold scaling at large shear rates, the attraction results in a finite yield stress in the limit of small shear rates. In the yield regime a fragile solid is formed and the rheology can be explained by a scaling argument that exploits the vicinity to the isostatic state. In the transition region the shear stress develops a minimum, which (in large enough systems) leads to the formation of persistent shear bands. These features are rationalized by a scenario that involves the competition between attraction-induced structure formation and its break-down because of shearing. Properties of shear bands are studied in order to reveal the physical mechanisms that underly the non-monotonic flow curve and the flow heterogenities in the transition region. This work may help to elucidate the origin of shear bands in different materials with finite and short-ranged attractive forces. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F49.00006: Flow and clogging of submerged hoppers Juha Koivisto, Douglas Durian The discharge rate for granular hoppers was recently found to depend on the filling height when the hopper is submerged in water\footnote{T.J. Wilson et al., Pap. Phys. \textbf{6}, 060009 (2014).}. This effect is further studied with an automated experimental setup consisting of cylindrical flat bottomed hoppers with various diameters and orifices. The grains are spherical glass beads of diameter $1.1 \pm 0.1$~mm. The flow rate is measured with an electric scale connected to a computer. With this, we confirm the counterintuitive surge in the flow rate as the filling height decreases toward zero. We also find a similar surge for dry gains, but the size of the effect is much smaller and to our knowledge is previously unseen. In both cases we notice that the flow of grains near the wall changes from creep like behavior to mass flow as the hopper diameter decreases. The hypothesis for the surge effect is changes in compatible stresses and force chains. To alter such behavior, on-going work includes changing the fluid pressure and flow rate near the orifice as well as changing the roughness of the walls. Work has also begun on clogging for small orifices in submerged hoppers, where preliminary observations show an exponential distribution of flow durations. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F49.00007: Armoring, stability, and transport driven by fluid flow over a granular bed Benjamin Allen, Arshad Kudrolli We discuss experiments investigating the evolution of a granular bed by a fluid flow as a function of shear rate at the fluid-bed interface. This is a model system to investigate a variety of physical examples including wind blowing over sand, sediment transport in rivers, tidal flows interacting with beaches, flows in slurry pipelines, and sand proppants in hydraulic fracturing. In order to examine the onset and entrainment of the granular bed under steady state conditions, we have constructed a novel conical rheometer system which allows a variable amount of shear to be applied to the granular bed. The grain-fluid system is index matched so that we can visualize the grains away from the sides as well as visualize the fluid flow above and below the interface by using fluorescent tracer particles. We demonstrate that the onset of erosion arises as particles rotate out of their stable position highlighting the importance of torque balance to onset. We find significant armoring of the bed, as the bed is sheared by the fluid flow. Above onset, at least three distinct regions of bed mobility can be found. We will discuss the measured integrated granular flux as a function of shear rate and compare them with empirical laws found in the geophysical literature. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F49.00008: Creep and Dynamical Heterogeneities of Fluid-Driven Granular Flows Carlos Ortiz, Morgane Houssais, Douglas Durian, Douglas Jerolmack Earth's surface is a fluid-sediment interface evolving through fluid-driven granular flow. To probe long-time dynamics, we construct an annular chamber that mimics an infinitely-long river channel. We use non-Brownian, spherical plastic grains, fully submerged in a less dense index-matching fluid. We drive the packs with a laminar flow and record dynamics by laser scanned particle tracking. ``Bed load'' grains near the surface exhibit relatively fast shear. By long-time averaging grain trajectories, we find that grains deep in the pack, which appear frozen by eye, exhibit a slow creep dynamics. The transition between bed load and creep occurs at a critical value of the local relaxation time, characterized by a critical dimensionless shear rate, the viscous number. We also characterize the important length and time scales for dynamical heterogeneities as a function of depth and find that grain dynamics are spatiotemporally heterogeneous at all depths. The dynamics slow down monotonically as a function of depth, but the domain size is largest at the transition to creeping. We propose a new phase diagram for fluid-sheared granular transport, where ``bed load'' sediment transport is defined as a dense granular flow driven by fluid shear from above and granular creep from below. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F49.00009: Onset and cessation of grain motion in riverbed erosion experiments Julia Salevan, Abram Clark, Mark Shattuck, Corey O'Hern, Nicholas Ouellette Erosion due to fluid flow plays a principal role in shaping landscapes. However, the complexity of the coupling between hydrodynamic shear, sediment transport, and internal granular bed rearrangements limits our understanding of the particle-scale physics that governs erosion. In particular, it is unclear whether particle rearrangements in an immersed bed are controlled largely by fluid forcing or by mechanical instabilities in the network of interparticle forces, and how the onset and cessation of particle motion is linked to the prior shear history of the bed. To address these questions, we perform experimental studies in a recirculating water flume in which we drive turbulent flow across beds of glass beads. We use optical imaging to characterize both the turbulence and dynamics of the bed, and we study the differences in flow properties required to initiate and maintain particle motion. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F49.00010: Jamming and unjamming in model riverbeds Abe Clark, Julia Salevan, Mark Shattuck, Nicholas Ouellette, Corey O'Hern When fluid flows laterally over a granular bed, it exerts shear stress on the particles. The ratio of this stress to the gravitational stress is known as the Shields number, and bulk sediment transport is thought to occur once the Shields number has passed a critical threshold. However, the particle-scale mechanisms that control this transition are not well understood. Here, we perform molecular dynamics simulations of a model riverbed to understand the particle-scale origins of jamming and unjamming in these systems. The particles interact via purely repulsive harmonic forces and are coupled to the flow using a Stokes-like drag model. The interstitial fluid velocity is determined from the local packing density using a relation similar to Darcy's law. Near the transition to sediment transport, we observe hysteresis and avalanches, and connect their statistical properties to the packing geometry at the particle scale. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F49.00011: Erosion and flow of hydrophobic granular materials Brian Utter, Thomas Benns, Benjamin Foltz, Joseph Mahler We experimentally investigate submerged granular flows of hydrophobic and hydrophilic grains both in a rotating drum geometry and under erosion by a surface water flow. While slurry and suspension flows are common in nature and industry, effects of surface chemistry on flow behavior have received relatively little attention. In the rotating drum, we use varying concentrations of hydrophobic and hydrophilic grains of sand submerged in water rotated at a constant angular velocity. Sequential images of the resulting avalanches are taken and analyzed. High concentrations of hydrophobic grains result in an effectively cohesive interaction between the grains forming aggregates, with aggregate size and repose angle increasing with hydrophobic concentration. However, the formation and nature of the aggregates depends significantly on the presence of air in the system. We present results from a related experiment on erosion by a surface water flow designed to characterize the effects of heterogeneous granular surfaces on channelization and erosion. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F49.00012: Fluid and particulate suspension flows at fracture junctions Tak S. Lo, Joel Koplik Suspended particles can be a serious problem in geological contexts such as fluid recovery from reservoirs because they alter the rheology of the flowing liquids and may obstruct transport by narrowing flow channels due to deposition or gravitational sedimentation. In particular, the irregular geometry of the fracture walls can trap particles, induce jamming and cause unwanted channeling effects. We have investigated particle suspension flows in tight geological fractures using lattice Boltzmann method in the past. In this work we extend these studies to flows at a junction where two fractures intersect, an essential step towards a complete understanding of flows in fracture networks. The fracture walls are modeled as realistic self-affine fractal surfaces, and we focus on the case of tight fractures, where the wall roughness, the aperture and the particle size are all comparable. The simulations provide complete detail on the particle configurations and the fluid flow field, from which the stresses in the fluid and the forces acting on the bounding walls can be computed. With these information, phenomena such as particle mixing and dispersion, mechanical responses of the solid walls, possible jamming and release at junctions, and other situations of interest can be investigated. [Preview Abstract] |
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