Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L16: Granular Flows Beyond Simple Mechanical ModelsFocus Live

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Sponsoring Units: GSNP DSOFT Chair: Stephen Teitel, University of Rochester 
Wednesday, March 17, 2021 8:00AM  8:12AM Live 
L16.00001: Controlling Rheology via Boundary Conditions in Dense Granular Flows Farnaz Fazelpour, Karen Daniels In the field of granular rheology, an important horizon is the influence of the boundary conditions on granular flows. Understanding how changing the roughness of a boundary changes the resulting flow can lead to developing predictive model for wall slip, roughness may also be changing the propagation of nonlocal effects through the creation of additional fluctuations. We perform experiments in a quasi2D annular shear cell under 5 different boundaries with controlled roughness. We characterize the properties of granular slip at the boundaries, and investigate which aspects of a dense granular flow can be controlled by the choice of boundary condition. We have observed that boundary roughness strongly controls both the flow profile v(r) and shear rate profile γ(r). We measure the shear and pressure stress fields by coarsegraining photoelastic measurements, and identify how the roughness of the boundary changes these fields. Finally, we investigate how the nonlocal rheology model, proposed by Kamrin and Koval, responds to various boundary conditions and how boundary roughness controls force chain fluctuations. 
Wednesday, March 17, 2021 8:12AM  8:24AM Live 
L16.00002: Quantifying the influence of rolling friction on force networks and rheology in sheared suspensions Abhinendra Singh, Rituparna Basak, Juan De Pablo, Lou Kondic, Heinrich Jaeger The mechanism of shear thickening in dense suspensions has been linked to a stresscontrolled transition from an unconstrained lubricated ``frictionless'' to a constrained unlubricated ``frictional'' rheology. Particle simulations that led to this concept have been successful in quantitatively reproducing the nonNewtonian behavior of thickening suspensions. We have recently shown the importance of rolling friction for the quantitative agreement of numerical simulations with the experimental data. Rolling friction can stabilize the force network, and hence lead to higher effective viscosity under shear. With an aim to make a connection between the frictional force network and the rheology, we perform persistent homology on systems with different combinations of sliding and rolling friction at several volume fractions. These measures allow for quantifying the force network properties that are responsible for the modified rheology of sheared suspensions. We will present the comparison between the statistics of the evolution of the force network in different systems and how different constraints affect the rheology of a dense suspension. 
Wednesday, March 17, 2021 8:24AM  9:00AM Live 
L16.00003: Generalized Constitutive Modeling of Granular Materials near the FlowArrest Transition Invited Speaker: Ishan Srivastava Flowing granular materials often abruptly arrest if not driven by sufficient applied stresses. Such abrupt cessation of motion can be economically expensive in industrial materials handling and processing, and is significantly consequential in intermittent geophysical phenomena such as landslides and earthquakes. However, the statistical and rheological properties of this nonequilibrium transition are not wellunderstood. We recently developed a fully stresscontrolled granular simulation framework to elucidate such phenomena at the macroscopic bulk scale. Through extensive simulations, we construct a flowarrest state diagram that clearly distinguishes between states of shear flow and arrest in granular materials in terms of their microstructural and rheological properties, and where interparticle friction is an important ingredient. Furthermore, granular flows in the vicinity of such a transition exhibit features that are not captured by the traditional rheological models, such as the presence of normal stress differences. I will describe our general tensorial constitutive model that suitably captures these features. The generality of the model will be demonstrated through its applicability in complex flow fields such as extensional and socalled triaxial flows, along with extensions to granular materials with enhanced compositional complexities such as large particle size dispersity. 
Wednesday, March 17, 2021 9:00AM  9:12AM Live 
L16.00004: PowerLaw Scaling in Granular Rheology Seongmin Kim, Kenneth N Kamrin We propose a new form of the constitutive equation for simple granular materials which is independent of packing fraction. By measuring coarsegrained continuum fields in discrete element method simulations of frictional particles, we have found that rescaling the stress ratio μ by a power of dimensionless granular temperature Θ makes the data from many different flow geometries collapse to a master curve which depends only on the inertial number I. This powerlaw structure appears robust to varying interparticle friction (roughness) in both 2D and 3D systems. Its functional form reflects the key physical idea that stronger velocity fluctuations (agitations) soften the granular material and assist flow. Our rheology fits and extends the frameworks such as kinetic theory and the nonlocal granular fluidity model. Finally, we show how this equation can be used to predict the velocity field in inclined chutes where flow depends on two spatial coordinates. 
Wednesday, March 17, 2021 9:12AM  9:24AM Live 
L16.00005: Pressuredependent shear modulus of jammed packings of nonspherical particles Jerry Zhang, Kyle R VanderWerf, Mark David Shattuck, Corey O'Hern There have been numerous studies of the structural and mechanical properties of jammed packings of spherical particles, yet there have been far fewer studies of jammed packings of nonspherical particles. Here, we generate jammed packings of circulolines in two spatial dimensions (2D) and study their mechanical response. Previously, we showed that the shear modulus of jammed disk packings decreases linearly with pressure, p, within geometric families, G_{f }= G_{0} – A p, where the contact network of the packing does not change. In contrast, the ensembleaveraged shear modulus increases as <G> » G_{0 }+ B p^{a}, where a» 0.5 for jammed packings of spherical particles, due to particle rearrangements. For jammed packings of circulolines, we find that the shear modulus of geometrical families can increase, as well as decrease with pressure: G_{f }= G_{0} ± A’ p. We show that the increase of G_{f} with pressure is related to local shearinduced dilation for particles with rotational degrees of freedom. We find that the ensembleaveraged shear modulus <G>  G_{0 }» p^{a} for packings of circulolines scales as a powerlaw with pressure, but a»0.8, which is larger than the value for packings of spherical particles. 
Wednesday, March 17, 2021 9:24AM  9:36AM Live 
L16.00006: Results that stick: how adhesive particle properties affect their collective flow behavior Zohreh Farmani, Joshua Dijksman, Jan Wieringa Interparticle forces are very relevant for granular flow dynamics. Here we experimentally probe whether adhesive characteristics of particles also affect the rheology of such particle suspensions. We used ceramic microparticles with a size distribution between 10100 micrometer; these particles are base materials for porcelain stoneware. We characterize particle adhesion forces by using colloid probe atomic force spectroscopy. We find that acidic and basic environments can induce an increase in the adhesion capacity of certain types of ceramic particles. Additionally, the adhesion force can be contact time and retraction speed dependent; results repeat over many cycles. We attribute the adhesion dynamics to the presence of clay inside the ceramic microparticles. The adhesive properties of the ceramic microparticles also may affect their collective behavior in suspension, as we confirm that the rheology of ceramic microparticle suspensions is strongly pH dependent. Our results suggest that adhesive particle properties are relevant to consider in the analysis of suspension dynamics. 
Wednesday, March 17, 2021 9:36AM  9:48AM Live 
L16.00007: Flow and clogging behavior of mixtures of lowfriction soft and frictional rigid spheres in a quasitwodimensional silo Bo Fan, Jing Wang, Tivadar Pongó, Kirsten Harth, Torsten Trittel, Ralf Stannarius, Maja Illig, Raúl Cruz Hidalgo, Tamás Börzsönyi Flow and clogging of a mixture of lowfriction deformable beads with frictional hard beads of similar size were studied in a quasitwodimensional (2D) silo. Previous work has demonstrated that silo discharge is significantly different for lowfriction deformable grains and conventional frictional hard particles. Here we show that the addition of a small amount of frictional hard beads to an ensemble of lowfriction deformable beads already has significant consequences. We compare the outflow velocities and the duration of nonpermanent clogs for different compositions of the mixtures. Experimental results are compared with numerical simulations. 
Wednesday, March 17, 2021 9:48AM  10:00AM Live 
L16.00008: Rheology of suspensions in the thermal crossover: an incline plane experiment under confocal observation. Alice Billon, Olivier Dauchot, Yoël Forterre, Olivier Pouliquen Hard sphere suspensions have been widely studied in the case of frictional athermal spheres as well as in the case of dilute colloidal suspensions. However, the crossover regime, where frictional and thermal effects are both significant [1], is still lacking of a complete description, despite its crucial importance in understanding realistic flows. Inspired by the macroscopic incline plane experiments which allowed significant progresses in the description of granular flows, we designed a microscopic incline plane experiment under confocal observation, using micronsize suspensions for which the thermal stress is of the same order of magnitude as the gravity stress. We compare the experimental velocity profiles with a theoretical model that extends to pressureimposed conditions the model of Ikeda et al [2] for the rheology of soft and brownian suspensions at the crossover of the glass and jamming transition. 
Wednesday, March 17, 2021 10:00AM  10:12AM Live 
L16.00009: Discharge Rate of a Silo with Rotating Bottom Tivadar Pongó, Dariel HernándezDelfin, Kiwing To, Tamás Börzsönyi, Raúl Cruz Hidalgo Discharge of spherical grains from a silo was investigated numerically with the bottom plate rotating around the vertical axis of the cylindrical silo. We observe that the flow rate exhibits an increasing trend for small, and a nonmonotonic trend for larger orifices as a function of rotation speed, which was also recently found in experiments. Our study sheds light on the cause of this peculiar behavior. Using a coarsegraining technique, we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure and the system dilatancy. This reduces the stability of the arches, and, consequently, the flow rate increases monotonically. For large orifices, however, we detected that the kinetic stress changes nonmonotonically, which might explain the nonmonotonic behavior of the flow rate when varying the rotational shear. 
Wednesday, March 17, 2021 10:12AM  10:24AM Live 
L16.00010: Localized Airflow Fluidization During Granular Intrusion Andras Karsai, Daniel I Goldman Resistive forces during intrusion into frictional granular media increase linearly with depth. These forces arise from the sum of external stresses needed to pass the local frictional threshold and successfully shear the media to intrude. This threshold depends heavily on the packing and lithostatic pressure of the media around the intruder. An increasing lithostatic pressure gradient can prevent unanchored intruders like autonomous robots from burrowing successfully, as the gradient pushes towards the free surface. We investigate in both experiment and CFDDEM simulations how rapid downward airflow from an intruder’s tip can create transient cavities in the media and help reduce resistive force for cylindrical intruders of 3 cm diameter in both 425850 micron granular sand and 3 mm glass beads. When the fluidizing intruder tip is above the granular surface, airflowinduced cavities are formed which exhibit different shapes and flow rates dependent on their distance from the free surface. Below the surface, the resistive force per unit depth on the intruder decreases as a function of airflow rate until a characteristic intrusion depth, where the force increases rapidly and approaches the resistive force for the same intruder with no airflow. 
Wednesday, March 17, 2021 10:24AM  10:36AM Live 
L16.00011: Impact of loading geometry on steadystate flow of frictional granular packings Jeremy Lechman, Joel Clemmer, Ishan Srivastava, Gary Grest Studies of granular rheology often focus on a single loading geometry or stress state such as simple shear. However, different loading geometries or stress states such as triaxial extension or compression have different yield conditions and result in distinct flows. We explored this dependence using discrete element model simulations. Systems are deformed to large strains to reach steadystate flow using generalized KraynikReinelt boundary conditions. Rheology is characterized for different constant bulk pressures, values of interparticle friction, strain rate, and loading geometries. Results are compared to common yield criteria such as the MohrCoulomb and Drucker Prager models. Additionally, we will discuss different methods of maintaining constant pressure in flow and compare results to constant volume simulations. 
Wednesday, March 17, 2021 10:36AM  10:48AM Live 
L16.00012: Dry granular rheology microstructure with rotational constraints Andrew Santos, Ishan Srivastava, Leo Silbert, Jeremy Lechman, Gary Grest Dense granular material requires a critical minimum ratio of applied shear stress and pressure to induce flow. Friction between particles causes this critical stress ratio to increase. Frictional constraints beyond sliding friction can lead to an increase in the critical stress ratio. Sliding, rolling and twisting friction are included in particlebased, discrete element simulations in bulklike stresscontrolled shear flows. These simulations show that by increasing the rolling and twisting friction coefficients the stress ratio can double. Furthermore, these constraints cause a larger decrease in the volume fraction and coordination number in the flowing state. However, the functional form of the μ(I) rheology is largely unchanged by these additional constraints. The normal stress differences and fabric tensor are used to further characterize the threedimensional microstructure and stress states of these flowing granular systems. 
Wednesday, March 17, 2021 10:48AM  11:00AM Live 
L16.00013: Unraveling the relationship between dense suspension flow and particle and solvent identity Michael Van der Naald, Grayson Jackson, Heinrich Jaeger

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