Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B16: Mechanical Singularities in Soft Matter I |
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Sponsoring Units: GSOFT GSNP Chair: Joshua Dijksman, Wageningen University and Research Room: 275 |
Monday, March 13, 2017 11:15AM - 11:27AM |
B16.00001: Flexible Magnetic Membranes Pablo Vazquez-Montejo, Joshua Dempster, Mykola Tasinkevych, Monica Olvera de la Cruz Flexible magnetic filaments can be synthesized by joining superparamagnetic beads with elastic linkers, giving rise to interesting phenomena due to the combinations of their elastic and magnetic properties, which have found diverse applications, such as micro-mechanical sensors and self-propelled swimmers. We present a theoretical study of their two-dimensional counterparts, i.e. membranes composed of linked paramagnetic beads. In our model, the conformations of these magnetic membranes are governed by the competition between the bending energy and the dipolar interactions of nearest neighbors induced by a precessing magnetic field. We exploit the symmetries of these energies to determine equilibrium configurations of open and closed magnetic membranes. We characterize these shapes in terms of the area and material parameters of the membrane, as well as of the strength and precession angle of the magnetic field. In particular, we show how depending on the precession angle open membranes may form either rippled or helicoidal surfaces, whereas closed membranes may elongate or flatten. These kind of membranes might be suitable for many potential applications due to their controllable conformational changes. [Preview Abstract] |
Monday, March 13, 2017 11:27AM - 11:39AM |
B16.00002: The Smectic Order of Wrinkles Hillel Aharoni, Desislava V. Todorova, Octavio Albarr\'{a}n, Lucas Goehring, Randall D. Kamien, Eleni Katifori A thin elastic sheet lying on a soft substrate develops wrinkled patterns when subject to an external forcing or as a result of geometric incompatibility. Thin sheet elasticity and substrate response equip such wrinkles with a global preferred wrinkle spacing length and with resistance to wrinkle curvature. We show how the behavior of these systems can be described compactly by the theory of liquid crystalline smectics at intermediate length scales. This insight allows better understanding of the wrinkling patterns seen in such systems, with which we explain pattern breaking into domains, the properties of domain walls and wrinkle undulation. We compare our predictions with numerical simulations and experimental observations. [Preview Abstract] |
Monday, March 13, 2017 11:39AM - 11:51AM |
B16.00003: The free energy of singular sticky-sphere clusters Yoav Kallus, Miranda Holmes-Cerfon Many model systems for self-assembly use colloidal particles with an interaction range much smaller than their diameter. The cluster chemistry of such particles can be studied in a universal approach, independent of the particular interaction potential shape, by use of the ``sticky sphere'' limit. Rigid clusters---contact arrangements such that no nonrigid motion is available without breaking at least one contact---take the place of energy minima, with energy determined by the number of contacts. The relative stability of rigid clusters with the same number of contacts is determined by entropic contributions, such as the vibration entropy. The harmonic approximation gives the leading asymptotic term in the vibration entropy if the cluster has no zero-frequency modes, but diverges otherwise. We derive the leading asymptotic term for singular clusters. We use our result to characterize the free-energy landscape of a system of sticky spheres by calculating the free energy of rigid clusters of up to $N = 19$ particles, which have been previously enumerated. [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B16.00004: Effective medium model for a granular monolayer on an elastic substrate Alexei Maznev Effective medium models have been shown to work well in describing experimental observations of the interaction of surface Rayleigh waves with contact vibrations of a monolayer of microspheres (see e.g. Boechler et al., Phys. Rev. Lett. 111, 036103 (2013)) . However, these models contain intrinsic conceptual problems: for example, the local displacement of the substrate at the contact point is equated to the effective medium average value of the surface displacement. I will present a rigorous derivation of the effective medium model for a random arrangement of mass-spring oscillators on an elastic half-space using elastodynamic surface Green's function formalism. We will see that the model equating the local surface displacement to the effective medium displacement is indeed valid if the spring constant of the oscillators is modified to include the stiffness of the contact calculated in the quasistatic approximation. In the case of contact vibrations of microspheres, this means using the spring constant calculated using the Hertzian contact model. Thus the results obtained in the prior work were correct despite the apparent inconsistencies in the model. The presented analysis will provide a solid foundation for effective medium models used to describe dynamics of microparticle arrays adhered to a solid substrate. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B16.00005: Dynamic Phases, Clustering, and Lane Formation for Driven Disk Systems in the Presence of Quenched Disorder Yang Yang, Danielle McDermott, Cynthia J. Olson Reichhardt, Charles Reichhardt We numerically examine the dynamic phases and pattern formation of 2D monodisperse repulsive disks driven over random quenched disorder. We show that there is a series of distinct dynamic regimes as a function of increasing drive, including a clogged or pile-up phase near depinning, a homogeneous disordered flow state, and a dynamically phase separated regime consisting of high density crystalline regions surrounded by a low density of disordered disks. At the highest drives the disks arrange into 1D moving lanes. The phase separated regime has parallels with phase separation observed in active matter systems, and arises in the disk system due to the combination of nonequilibrium fluctuations and density dependent mobility. We discuss how this system exhibits pronounced differences from previous studies of driven particles moving over random substrates where the particles, such as superconducting vortices or electron crystals, have longer range repulsive interactions, and where dynamical phase separation and strong one-dimensional moving chain effects are not observed. The system we consider could be realized experimentally using sterically interacting colloids driven over random pinning arrays or quasi-two-dimensional granular matter flowing over rough landscapes. [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B16.00006: Redundancy: a Bridge Between Rigidity and Connectivity Percolation Models Varda F. Hagh, M. F. Thorpe We employ the concept of redundancy in networks - stress in rigidity and loops in connectivity- to perform a one on one comparison between the two models. In the case of rigidity percolation on a generic spring network, redundant bonds are those that cause an internal stress in the system and introduce finite forces that characterize over-constrained regions. In connectivity percolation, bonds that cause a loop are redundant and all the bonds that are part of a loop are equivalent to over-constrained bonds in rigidity percolation. To illustrate this we start with a network in 2D and use numerical tools such as Pebble Game algorithm to study the behavior of over-constrained regions near rigidity transition in hierarchical networks and lattices. We then connect all the sites to a ghost site which makes every bond inside a loop become rigidly over-constrained. This allows us to use our numerical tools to look into the behavior of loops in the same networks. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 12:39PM |
B16.00007: Rigid clusters in frictional particle packings Kuang Liu, Jonathan Kollmer, James Puckett, Karen Daniels, Silke Henkes, J.M. Schwarz We recently developed an algorithm to identify rigid clusters in frictional particle packings. The algorithm was applied to numerically generated frictional particle packings and the rigid cluster identification revealed the existence of a broad-tailed rigid cluster size distribution near the onset of frictional jamming, suggesting a continuous transition in the formation of rigid clusters.\footnote{Silke Henkes, David A. Quint, Yaouen Fily, J.M. Schwarz, \textbf{Phy. Rev. Lett.} 116, 028301 (2016)} We, therefore, look for other signatures of criticality in frictional particle packings in two ways. First, we numerically study rigidity percolation with friction on a honeycomb lattice with randomly added next nearest neighbor bonds. We find a second order transition, suggesting a fractal spanning rigid cluster, and numerically determine related exponents and universal scaling functions. Second, we implement our rigid cluster decomposition on experimentally obtained frictional jammed packings to test for signatures of criticality under realistic conditions. [Preview Abstract] |
Monday, March 13, 2017 12:39PM - 12:51PM |
B16.00008: Percolation transition in the packing of bidispersed particles on curved surfaces Andrew Mascioli, Christopher Burke, Timothy Atherton We study packings of bidispersed spherical particles on a spherical surface. The presence of curvature necessitates defects even for monodispersed particles; bidispersity either leads to a more disordered packing for nearly equal radii, or a higher fill fraction when the smaller particles are accomodated in the interstices of the larger spheres. Variation in the packing fraction is explained by a percolation transition, as chains of defects or scars previously discovered in the monodispersed case grow and eventually disconnect the neighbor graph. [Preview Abstract] |
Monday, March 13, 2017 12:51PM - 1:03PM |
B16.00009: Punctuating Instability of~a 2D Dusty Plasma Colloidal Crystal Guram Gogia, Justin Burton When placed in a weakly-ionized RF plasma,~colloidal microparticles~can be trapped in the~narrow Debye~sheath region above a~capacitively-coupled electrode. Known as a ''dusty plasma", the particles become negatively charged, leading to the formation of large, 2D~crystalline monolayers.~At low pressures the particles can~experience vertical oscillations due to plasma density fluctuations in the sheath. As a result of these fluctuations, we have~found that at low pressures and low bias~voltage, the colloidal crystal~experiences temporally reoccurring instabilities. Such "punctuating"~instabilities are caused by the~redistribution of kinetic energy from~vertical vibrations~to horizontal motion, essentially melting the crystal into a gas-like state. After the incipient instability, without changing any external parameters, the system loses kinetic energy to damping with the surrounding gas, then~eventually~recrystallizes and~remains stable until next punctuating instability. The period of the instability~ranges from seconds to minutes depending on the system parameters, and can vary significantly within a given system. Using simple simulations of 2D crystals driven by a~vertical Langevian forcing, we are able to capture the salient features of the punctuating instability. [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B16.00010: Bifurcation at the origin of shear band formation in a granular material Axelle Amon, Thai Binh Nguyen, Jerome Crassous, Sean McNamara The spontaneous localization of the deformation in a granular material is a long-standing problem. The incidences of this phenomenon are numerous from soil stability in civil engineering to fault formation in geophysics. Numerous works have been devoted to this problem, which is still nevertheless largely open. We present an experimental study of the shear band formation in a dry granular sample submitted to a biaxial test. We measure the spatial repartition of the deformation in a plane-strain configuration using an interferometric method based on multiple scattering. We quantify objectively the degree of localization in the experimental strain maps and the anisotropy of this field. We show that a bifurcation takes place but without the sudden formation of a slip plane. On the contrary, after the bifurcation, plasticity is still widely distributed in the sample. Still a breaking of symmetry has occurred: the spatial repartition of the deformation is not isotropic anymore but displays a large-scale orientation. After the bifurcation, we observe a progressive concentration of the strain field which evolves as the loading proceed from a wide diffuse band to a narrow stationary one. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B16.00011: Investigating strain softening and hardening in soft amorphous solids Mehdi Bouzid, Emanuela Del Gado Disordered elastic solids of soft condensed matter like proteins, colloids or polymers are ubiquitous in nature and important for modern technologies. They belong to the class of amorphous systems and can form even at very low solid volume fraction via aggregation into a variety of complex and often poorly connected networks.The ability to explain and tune their mechanical properties in terms of their microscopic structure remains a challenge. We use molecular dynamics simulations of a model system to investigate the emergence of the non-linear behavior. Under shear deformations the system exhibits strong localization of tensile stresses that may be released through the breaking of bonds. We show how the interplay between structural connectivity and local internal stresses controls the mechanical response, leading to a strain softening and/or hardening. Our findings help rationalize various experimental findings. [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B16.00012: Particle Rearrangements in Fluctuating Disordered Solids Qikai Wu, Thibault Bertrand, Corey O'Hern, Mark Shattuck We numerically study the evolution of interparticle contact networks in packings of frictionless bidisperse disks that interact via purely repulsive contact forces as a function of increasing temperature. We start with mechanically stable packings at zero temperature generated using an isotropic compression protocol. After each small increase in temperature, we run constant energy simulations for a given amount of time. At each temperature, we measure the average and variance in the particle positions as a function of time to identify particle rearrangements. In addition, we rapidly re-quench configurations from the constant energy simulations to zero temperature to determine when the system switches from one mechanically stable packing to another. From these simulations, we will better understand the organization and structure of mechanically stable disk packings in configuration space. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 1:51PM |
B16.00013: Universal signatures of plasticity in disordered solids Robert Ivancic, Ekin Cubuk, Samuel Schoenholz, Daniel Strickland, Daniel Gianola, Andrea Liu We present aggregated data from the UPenn MRSEC IRG on the ``Mechanics of Disordered Packings," obtained from experiments and simulations of disordered solids ranging from metallic glasses to granular packings. This data exhibits a remarkable commonality in the size of rearrangements at strains near or below the yield strain in systems spanning over 7 decades in particle size. Additionally, we find commonality in the magnitude of the macroscopic yield strain of disordered materials in systems spanning over 13 orders of magnitude in the Young’s modulus. To understand these commonalities, we use a machine learning approach to calculate a microscopic structural quantity, ``softness," which correlates strongly with rearrangements. We find that there is emergent commonality in the spatial extent of softness correlations and in the response of softness to strain, rationalizing the commonality observed in the rearrangement size and yield strain. [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B16.00014: Reversibility and rearrangements in sheared 2D systems: probing saddle points in the energy landscape Peter Morse, Lisa Manning, Martin van Hecke, Sven Wijtmans, Merlijn van Deen Under shear, a jammed packing of particles transitions between mechanically stable states. An open question is whether these transitions are one-to-one with changes to the particle contact network; the answer is important for characterizing energy barriers to rearrangements and understanding the role that small force distributions play in scaling theories for marginal stability. To answer the question, we analyze all contact change events in simulations of a sheared 2D disks and find they can be grouped into two types: One type, which we call a “network event”, is associated with a smooth change in stress but a discontinuous change in the shear modulus, while the other, called a “rearrangement event”, is accompanied by a drop in the stress and significant particle displacements. This suggests that not all contact changes are associated with saddle points, although all saddle points are accompanied by contact changes. We also examine the eigenvalues of the dynamical matrix, and find that different particle interaction potentials exhibit different signatures at these events. Finally, at high pressures, we find that network events are reversible under cyclic strain, while rearrangements are not, although the situation becomes messier at low pressures close to jamming. [Preview Abstract] |
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