Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J44: Invited Session: Self-assembly in the Macro-World |
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Sponsoring Units: GSNP Chair: Corey O'Hern, Yale University Room: 214D |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J44.00001: Self-assembly and the Formation of Structure in Granular Materials Invited Speaker: Robert Behringer Particle systems self-assemble in ways that are sensitive to their environments. Proteins fold, polymers crosslink, and molecular systems form crystals. Granular materials, unlike proteins, polymers or molecules, are not sensitive to temperature, and will only form new structures when they are driven. This raises the question of how a granular state depends on the preparation protocol, and an even more basic question of what is needed to specify a granular state. I will focus on granular systems near jamming, where key state variables include the density and stresses. Systems of frictionless grains follow the Liu-Nagel$^1$ scenario of jamming, with a lowest packing fraction, $\phi_J$, such that any system with $\phi < \phi_J$ is unjammed, and all isotopic states (shear stress $\tau = 0$) are jammed for $\phi > \phi_J$. For frictional grains the picture changes. For a given $\phi$ in the range $\phi_S < \phi < \phi_J$, it is possible to have stress-free (unjammed) states, highly anisotropic fragile states, and robustly jammed states. The fragile and strongly jammed states form spontaneously in response to shear. By inference, $\phi$ is not a state variable, but recent experiments$^2$ indicate that the non-rattler fraction, $f_{NR}$ is. In $\phi_S < \phi < \phi_J$, the system response is inherently non-linear; under cyclic shear, the system self-organizes to new steady states via a process that resembles thermal activation, with shear stress replacing energy$^3$. The activation is provided by shear strain. We observe similar relaxation under cyclic compression. An important question is, what is (are) the organizing principle(s) which govern jamming by shear, and systematic reorganization under cyclic driving. $1$Liu, A. \& S. Nagel, Nature 396, 21 (1998). $2$D. Bi et al., Nature 480, 355 (2011). $3$ J. Ren et al. Phys. Rev. Lett. {\bf 110}, 018302 (2013) [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J44.00002: Emergent Behavior in the Macro World: Rigidity of Granular Solids Invited Speaker: Bulbul Chakraborty Diversity in the natural world emerges from the collective behavior of large numbers of interacting objects. The origin of collectively organized structures over the vast range of length scales from the subatomic to colloidal is the competition between energy and entropy. Thermal motion provides the mechanism for organization by allowing particles to explore the space of configurations. This well-established paradigm of emergent behavior breaks down for collections of macroscopic objects ranging from grains of sand to asteroids. In this macro-world of particulate systems, thermal motion is absent, and mechanical forces are all important. We lack understanding of the basic, unifying principles that underlie the emergence of order in this world. In this talk, I will explore the origin of rigidity of granular solids, and present a new paradigm for emergence of order in these athermal systems. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 4:18PM |
J44.00003: Designing Jammed Materials from the Particle Up Invited Speaker: Marc Miskin Identifying which microscopic features produce a desired macroscopic behavior is a problem at the forefront of materials science. This task is materials design, and within it, new challenges have emerged from tailoring packings of particles jammed into a rigid state. For these materials, particle shape is a key parameter by which the response of a packing can be tuned. Yet designing via shape faces two unique complications: first there is no general theory that calculates the response of an aggregate given a particle shape, and second, there is no straightforward way to explore the space of all particle geometries. This talk summarizes recent results that address these challenges to design jammed materials from the particle up. It shows how simulations, experiments, and state-of-the-art optimization engines come together to form a complete system that identifies extreme materials. As examples, it will show how this system can create particle shapes that form the stiffest, softest, densest, loosest, most dissipative and strain-stiffening aggregates. Finally, it will discuss the how these results relate to the general task of materials design and the exciting possibilities associated with optimizing, tuning and rationally constructing new breeds of jammed materials. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:54PM |
J44.00004: Self-assembly of granular crystals Invited Speaker: Mark Shattuck Acoustic meta-materials are engineered materials with the ability to control, direct, and manipulate sound waves. Since the 1990s, several groups have developed acoustic meta-materials with novel capabilities including negative index materials for acoustic super-lenses, phononic crystals with acoustic band gaps for wave guides and mirrors, and acoustic cloaking device. Most previous work on acoustic meta-materials has focused on continuum solids and fluids. In contrast, we report on coordinated computational and experimental studies to use macro-self-assembly of granular materials to produce acoustic meta-materials. The advantages of {\it granular} acoustic materials are three-fold: 1) {\it Microscopic control:} The discrete nature of granular media allows us to optimize acoustic properties on both the grain and network scales. 2) {\it Tunability:} The speed of sound in granular media depends strongly on pressure due to non-linear contact interactions and contact breaking. 3) {\it Direct visualization:} The macro-scale size of the grains enables visualization of the structure and stress propagation within granular assemblies. We report simulations and experiments of vibrated particles that form a variety of self-assembled ordered structures in two- and three-dimensions. In the simplest case of mono-disperse spheres, using a combination of pressure and vibration we produce crystals with long-range order on the scale of 100's of particles. Using special particle shapes that form ``lock and key'' structures we are able to make binary crystals with prescribed stoichiometries. We discuss the mechanical properties of these structures and methods to create more complicated structures. [Preview Abstract] |
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