Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y07: Soft Matter Physics in a Geophysical ContextFocus Live
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Sponsoring Units: DSOFT DFD Chair: Karen Daniels, North Carolina State University |
Friday, March 19, 2021 11:30AM - 12:06PM Live |
Y07.00001: What can the deformation of colloids tell us about earthquake hazards? Invited Speaker: Michele Cooke Faults within the crust of the Earth (and other planetary bodies) evolve through the accumulation of earthquakes over thousands to millions of years. One way to understand fault evolution is by simulating crustal deformation within the laboratory using scaled materials, such as clay colloids. The scalings of length and time from the Earth’s crust to the laboratory table-top are governed by the strength and viscosity of the analog materials. With suitable rheology of analog materials, we can simulate millions of years of crustal deformation that occurs over hundreds of km within a table-top device over the course of a few hours. The evolution of laboratory fault systems captures the processes of fault initiation, propagation and linkage of faults within the Earth’s crust. One advantage of colloids over elastic materials is that stress relaxation off of faults simulates similar processes within crustal materials. The competition between stress accumulation on faults and stress relaxation off of faults modulates fault growth and earthquake recurrence. Furthermore, laboratory observations of fault slip rate variations inform estimates of seismic hazard from recent crustal fault activity |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y07.00002: Tuning sedimentation of clay particles through surface charge interactions Ali Seiphoori, Andrew L Gunn, Sebastien Kosgodagan Acharige, Paulo Arratia, Douglas J. Jerolmack Clays are abundant in natural environments and have various applications in industrial processes. Featuring a platelike morphology, clay particles typically have negatively charged faces and positively charged edges that encourage their aggregation when suspended in water. The screening of repulsive surface charges by dissolved cations facilitates particle aggregation, while intercalation of negatively charged ions can impose repulsion that suppresses aggregation. Here we tune the interaction of kaolinite clay particles from attractive to repulsive to examine the effects of surface charge on the rate and style of sedimentation. Classic hindered settling theory reasonably describes sedimentation rate for repulsive particles, which form a concentrated jamming front. The hindered settling description breaks down for attractive particles, which aggregate to form a clay gel that consolidates under its own weight. As the gel compacts, expulsion of interstitial water drives upward flow, which opens fracture-like channels in the bulk of the gel; these channels disappear as the gel enters a creep regime. Results help towards understanding the transport and erodibility of cohesive sediments. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y07.00003: Glassy hillslopes and quiescent sandpiles: rendering sub-yield granular creep with Diffusing Wave Spectroscopy Nakul Deshpande, David Furbish, Paulo Arratia, Douglas J. Jerolmack Hills and mountainsides are wrapped in a blanket of soil: an amorphous granular packing, constrained by geologic boundaries and pulled by gravity. At first glance, soil-mantled hillslopes beneath the angle of repose appear jammed and remain as such until a yield threshold is reached. Here we render creep dynamics in a sub-threshold granular heap with Diffusing Wave Spectroscopy in the absence of excitations or driving. The system is isolated from vibrations and is for all practical purposes quiescent. Nonetheless, we observe persistent creeping strains (10-6). The exponential decay of the relaxation dynamics is governed by a single time scale, which is sufficient to collapse the data into a single master curve. We also observe the growth of dynamical heterogeneities and a quadrapolar shape of the spatial correlations, signatures of glassy behavior which are consistent with the phenomenology of supercooled liquids and amorphous solids. Further, depth-averaged strain profiles are consistent with decades-long records of soil deformation, which hint that these dynamics are at play in natural hillslopes. This opens the possibility for the re-interpretation of field data through the lens of soft matter and for future work examining the role of weak mechanical disturbances. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y07.00004: Pressure response on granular impacts: how far the Boussinesq model can go? Francisco Martinez Carreaux, Maria P. Urrea, Claudia M. Gonzalez, German Varas The Boussinesq law is one of the classical models to predict the overpressure inside a soil induced by applying a single static load. In the present report, we question how this model can be extended when dealing with granular impacts. For this, we have conducted experimental measures of the pressure distribution at the bottom of a quasi-uniform dry sandy deposit impacted by a steel sphere of 8cm diameter, falling from dropping heights variable up to 100 cm. Our measures shown that this distribution follows a symmetrical Boussinesq-like form, characterized by a center peak of pressure that can be modeled by assuming a simple point-force model acting at the granular pile's surface. This point-force considers the contribution of the weight of the intruder and its momentum at the impact time. To the best of our knowledge, this is the first time where these kinds of measures are reported for granular beds, proposing at the same time many questions and challenges deserving to be explored with more detail. |
Friday, March 19, 2021 12:42PM - 1:18PM Live |
Y07.00005: Mud as a soft matter system: Stability of aggregates under time-periodic flow Invited Speaker: Cacey Bester Mud, a mixture of fine-grained sediments (such as clays and sand), organic material, and water, exhibits complex rheological behavior while being important to many environmental processes. The cohesive nature of clay minerals dominates the rheology of mud, since these constituents tend to aggregate through collisions and bonding of clay particles. Aggregation of cohesive sediment is highly influenced by hydrodynamic forces, such as that experienced in rivers, in ways that are far from understood. This motivates our experimental study, in which we model cohesive sediment by investigating a granular medium of spherical polystyrene grains that can aggregate due to interfacial deformation as they float at an air-liquid interface. The grains are introduced to well-characterized, two-dimensional time-periodic flows which are controlled by Lorentz forcing. We thereby describe the formation, breakup, and transport of granular aggregates due to the relative importance of shear stresses and interparticle forces. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y07.00006: Stick-slip dynamics generated by granular materials with varying grain angularity Ryan Kozlowski, Hu Zheng, Karen Daniels, Joshua Socolar Experiments on two-dimensional granular materials and corresponding simulations and theoretical treatments typically feature circular or elliptical grains. However, grains found in natural systems often have flat faces that introduce local rotational constraints; these have been shown to affect, for example, the jamming transition, discontinuous shear thickening, and ordered states in colloids and thermalized grains. In this work, we experimentally investigate the effects of grain angularity on stick-slip dynamics in a slowly driven granular medium consisting of regular polygonal grains; the gravity-packed granular bed is sheared via a weighted slider pulled by a spring. We find that packings of triangular or square grains have slightly higher shear strengths than packings of pentagons, hexagons, heptagons, or disks. Moreover, as the number of sides increases, sticking periods become shorter on average with the material yielding at smaller applied stresses. We report on measurements of the pulling force on the slider, particle dynamics during slip events, and properties of force-bearing contact networks identified via photoelasticity. The data are consistent with a continuous change in dynamics as the circular grain limit is approached. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y07.00007: Probing Granular Materials in Extraterrestrial Gravities Jack Featherstone, Robert E Bullard, Tristan Emm, Anna Jackson, Riley Reid, Sean Shefferman, Adrienne Dove, Joshua Colwell, Jonathan E Kollmer, Karen Daniels The Hayabusa2 and OSIRIS-REx missions have identified the surfaces of asteroids Ryugu and Bennu as being composed of weakly attracted grains primarily bound by cohesive forces, raising questions about granular dynamics in these exotic conditions. Operating instruments on such surfaces will require efficient and intentional design principles; we identify the insertion of a flexible probe as a function of speed and gravity as a prototypical example to explore these dynamics. We employ a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible intruder during its insertion into a laboratory system of bi-disperse, cm-sized model grains at various speeds and under four different levels of gravity: terrestrial, martian, lunar, and microgravity. This technique is used to identify the grain-scale forces throughout the system, and shows good agreement with direct load cell measurements. In most cases, the interparticle forces are characterized by discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. For microgravity experiments, stick-slip behaviors are negligible. We additionally find that varying the probe speed can suppress stick-slip behavior. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y07.00008: Densest Packings of Binary Frictional Particles Leo Silbert, Ishan Srivastava, Scott Roberts, Gary Grest, Jeremy Lechman Using a pressure-controlled discrete element method, implementing a highly efficient neighbor-detection algorithmic scheme, we have generated a large number of binary sphere-packings spanning a wide range in particle size ratio and filling fraction of small particles, over several orders of magnitude in particle friction coefficient. Our results are consistent with a generalized Furnas model [1] that takes into account friction dependence of the global packing fraction φJ. Our systems approach the theoretical result in the limit of large size ratios (20:1), with packings containing 1-10 million spheres. We provide evidence that the densest mechanically-stable jammed packings of binary frictional particles (circa φJ ~ 0.82 - 0.85) are produced at a friction-dependent optimal composition of large and small particles. |
Friday, March 19, 2021 1:54PM - 2:06PM Live |
Y07.00009: Slow dynamic nonlinear elasticity in novel systems John Yoritomo, Richard L. Weaver Slow dynamic nonlinear elastic recovery is found to be universal amongst geological materials and other materials with complex microstructures, such as concrete and cracked glass blocks. The slow dynamic behavior is characterized by a log(time) recovery after an initial drop of stiffness, induced by strain events as small as a microstrain. Slow dynamics has also been observed on the seismological scale with the recovery occurring over years. Insomuch as materials that exhibit slow dynamics form fault gouge, it is thought that slow dynamics could play a role in dynamic earthquake triggering. Here we present studies of slow dynamics in simplified systems—unconsolidated bead packs and a single bead system—to inform theory. A consensus theoretical understanding of the universality of slow dynamics and, in particular, the log(time) recovery is still needed. Our results imply that some previously proposed mechanisms, i.e., force chains, glassy microstructures, and cracking, cannot play essential roles as they are presumably absent in one or more of the studied systems. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y07.00010: The lifetime of charge on a lofted particle Dana Harvey, Joshua Mendez, Tianshu Huang, Jake McGrath, Justin Burton The transport of small atmospheric particles across the world’s oceans is crucial for the global biome, for example, dust from the Sahara feeds the Amazon rain forest. This transport can be enhanced by electrostatic forces when individual particles are charged. Yet little is known about the lifetime of charge on such lofted particles. Laboratory measurements are challenging since particles must be levitated and manipulated without contact. Here we use an acoustic radiation trap to levitate and measure the net charge on isolated, millimetric particles for days. Particles are charged with an ionizer, and we adjust the phase of the acoustic field to move the particle through a Faraday cup. In dry, atmospheric environments, we find that the charge decays exponentially with a time constant between 1-10 days. Increasing the humidity leads to a much faster logarithmic decay of charge rather than exponential. This behavior is observed in polystyrene, amaranth, and aerogel particles. By varying the pressure and atmospheric composition we are currently testing two possible mechanisms to explain this behavior: water mobility and hydrophobicity of the surface, and ion recruitment by diffusion to the particle surface. |
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