Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B53: Mechanics of Networks II |
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Sponsoring Units: GSOFT GSNP DPOLY Room: LACC 513 |
Monday, March 5, 2018 11:15AM - 11:27AM |
B53.00001: Quasi-Continuum Analysis of Networked Materials Ahmed Ghareeb, Ahmed Elbanna The skeleton of many natural and artificial structures may be abstracted as networks of elements interacting in a non-linear fashion. Examples include rubber, gels, soft tissues, and lattice materials. Understanding the multiscale nature of deformation and failure of networked structure holds key for uncovering origins of fragility in many complex systems including biological tissues and enables designing novel materials. However, these processes are intrinsically multiscale and for large scale structures it is computationally prohibitive to adopt a full discrete approach. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B53.00002: Hierarchical Elastic Network Mechanics Jonathan Michel, Peter Yunker Structural hierarchy is ubiquitous in biological tissues, a long-used architectural strategy, and an increasingly common feature of mechanical metamaterials. All of these cases feature networks comprised of bonds and nodes. While the mechanics of frames with features on a single length scale has been studied for over a century, there exists a lingering need for general rules explaining the interplay of structures on disparate length scales. We present the results of normal mode analysis and uniaxial tensile simulations of ball-and-spring networks with a diluted hexatic lattice structure on two separate scales. Broadly, small-scale structure is found to govern the vibrational density of states, while large-scale structure is found to be important in frustrating low-energy bending modes. We additionally comment upon tolerance in hierarchical structures of random errors in assembly, a crucial attribute in biological systems. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B53.00003: Linear Elasticity of DNA Hydrogels as a Function of DNA Nanostar Valence Nate Conrad, Tynan Kennedy, Omar Saleh, Deborah Fygenson Crosslink valence, or the number of network strands that connect at a junction, is a key structural feature of hydrogel networks. To better understand the effect of crosslink valence on network mechanics, we measure the linear, frequency-dependent elastic response of hydrogels made of DNA nanostars of valence f = {4, 5, 6} using bulk rheology. A DNA nanostar (DNAns) is a cross-linking structure of defined valence that self-assembles when a solution of partially-complementary DNA strands is slowly cooled from near boiling to room temperature. When designed with short, self-complementary sequences at one end of each strand, DNAns experience temperature-dependent binding interactions which support network formation. When the time scale of deformation is shorter than the lifetime of the binding interactions, the storage modulus (G') of the DNAns hydrogel exceeds the loss modulus (G'') and plateaus to a constant value G'p. For all f, G'p scales with DNAns concentration to a power 1 < x < 2, suggesting that the hydrogel is a flocculated fractal network rather than a cross-linked polymer network. We report the fractal dimension of hydrogels made from DNAns of different valence, and compare the magnitude of G'p to theoretical predictions. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B53.00004: Testing Rigidity Percolation Models of Articular Cartilage via Enzymatic Degradation Thomas Wyse Jackson, Lena Bartell, Moumita Das, Lawrence Bonassar, Itai Cohen The shear modulus of articular cartilage has been shown to change by several orders of magnitude with depth, and these changes correlate well with very slight changes in concentration of the constituents of the tissue, namely collagen fiberils and aggrecans. A Rigidity Percolation Model has been proposed to explain this jump in shear modulus. Here, I will describe results of studies where we use readily accessible enzymes to selectively degrade the constituents of articular cartilage and measure the mechanical properties with depth using confocal elastography. Comparing the mechanical measurements with the constitutive concentrations of cartilage, measured using FTIR imaging, we test and further develop our rigidity percolation models to describe the mechanical behavior of articular cartilage. Ultimately, development of these models will enable creating new artificial constructs with properties based on the model predictions, for use as tissue replacements, or in soft robotics. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B53.00005: Stretch-Induced Intramolecular Phase Separation in Polyrotaxane Glasses for the Mechanical Toughness Kazuaki Kato, Kaito Nemoto, Koichi Mayumi, Hideaki Yokoyama, Kohzo Ito We present here a unique mechanism for ductile behavior observed in a new series of polymer glasses made only of polyrotaxanes, which are necklace-like mechanically interlocked polymers consisting of cyclic molecules and threading linear polymers. The polymer glasses exhibit a Young’s modulus of ca. 1 GPa and are ductile with crazing, necking and a total elongation of >300%. Synchrotron X-ray scattering measurements under uniaxial tensile test showed that an amorphous hallo corresponding to the correlation distance between the cyclic components was shifted to a lower q value parallel to the tensile direction, and then began to shift in the opposite direction as the transmittance was suddenly increased. It indicates that the cyclic components located at the necking part were approaching against the tensile direction. In addition, the threading polymers sparsely covered with the rings were orientated to the tensile direction and crystallized slowly. It is attributed to the continuously exposed polymers threading, because an aging with such crystallization was interfered by the distributed threaded rings. These results indicate that an intramolecular phase separation between the aggregated rings and the exposed polymers were induced at the stress concentrated to be toughened. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B53.00006: Fracturing of topological Maxwell lattices Leyou Zhang, Xiaoming Mao In this talk, we will present our research on how topologically polarized Maxwell lattices fracture when they are stretched by applied stress. Maxwell lattices are mechanical structures containing equal numbers of degrees of freedom and constraints and are thus on the verge of mechanical instability. Recent progress in topological mechanics leads to the discovery of topologically protected floppy modes and states of self-stress at edges and domain walls of Maxwell lattices. We find that applied stress on topological Maxwell lattices is attracted by state-of-self-stress domain walls, and the lattice will gradually fracture at these domain walls first. We will explain the results using topological mechanics and further discuss how these domain walls protect the rest of the structures against cracks. Our results lead to possible designs of new mechanical metamaterials that exhibit high strength against fracturing and well-controlled fracturing process. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B53.00007: Network formation and molecular dynamics in hydrogen-bonding telechelic polymers: a competition between association lifetime and structural relaxation Martin Tress, Kunyue Xing, Peng-Fei Cao, Shiwang Cheng, Tomonori Saito, Vladimir Novikov, Alexei Sokolov Reversible bonds between polymer chains can form supra-molecular networks which exhibit extraordinary mechanical properties. In fact, reversible bonds represent a promising route to functional materials with e.g. self-healing properties. We study short telechelic polymers with H-bonding end-groups of different interaction strength and backbone flexibility. The glass transition temperature of flexible polydimethyl siloxanes (PDMS) does not vary with H-bond strength of the end-groups, but differs strongly from Tg in methyl-terminated PDMS. At the same time, Tg of the much stiffer telechelic polypropylene glycol (PPG) depends significantly on the H-bond interaction strength. In contrast, viscosity strongly depends on the H-bond strength in the PDMS series while it remains almost the same in PPG with different end groups. Complementary measurements of the shear modulus and dielectric relaxation indicate that these observations can be explained by competing lifetimes of the supra-molecular associations and the structural relaxations. Only if the association lifetime exceeds the characteristic segmental or chain relaxation time, the glass transition or viscosity will be affected by chain associations. Otherwise the chain end associations do not influence significantly Tg or viscosity. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B53.00008: Strain Adaptive Stiffening in Self-Assembled Bottlebrush Networks Andrey Dobrynin, Heyi Liang, Andrew Keith, Yidan Cong, Sergei Sheiko We study the strain adaptive behavior of the self-assembled linear-brush-linear (ABA) triblock copolymer networks using analytical calculations, molecular dynamics simulations and experiments. During microphase separation of the architecturally distinct blocks, stiff bottlebrush strands are physically crosslinked by aggregates of linear chains and form a network with extreme softness and intense strain-stiffening. The mechanical response of such networks under uniaxial deformations is a two-stage process, which starts with extension of the bottlebrush network strands (elastic regime) followed by pulling out of the linear chains from A-domains (yielding regime). In the elastic regime, the stress-strain curves can be described by a network model, which considers bottlebrush strands as worm-like chains. In the yielding regime, force generated in bottlebrush strands is sufficient to pull linear chains from the aggregates. This pull-out occurs at a constant force which results in a linear dependence of the true stress on the network elongation ratio, σtrue∝λ. Such two-stage process is confirmed by molecular dynamics simulations of the self-assembled ABA copolymer networks and experimental results on PMMA-bbPDMS-PMMA plastomers. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B53.00009: Hardening and Softening Materials: the role of network topology in soft gels Minaspi Bantawa, Mehdi Bouzid, Emanuela Del Gado The structural complexity of soft gels is at the origin of a versatile mechanical response that allows for large deformation, controlled elastic recovery, and toughness in the same material. A limit to exploiting the potential of such materials is the insufficient fundamental understanding of the microstructural origin of the bulk mechanical properties. Here we investigate the role of the network topology and of frozen-in stresses in a model gel through 3D numerical simulations. Our study links the topology of the network organization in space to the stress redistribution under shear and to its nonlinear rheological response preceding yielding and damage: our analysis elucidates how the network connectivity alone could be used to modify the gel mechanics at large strains, from strain-softening to hardening and even to a brittle response. These findings provide new insight for smart material design and for understanding the nontrivial mechanical response of a potentially wide range of technologically relevant materials. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B53.00010: Correlated rigidity percolation and colloidal gels Shang Zhang, Mehdi Bouzid, Leyou Zhang, D. Zeb Rocklin, Emanuela Del Gado, Xiaoming Mao Rigidity percolation occurs when mechanical stability emerges in disordered networks as more components or constraints are introduced. Classical theories of rigidity percolation elucidated critical phenomena at the rigidity percolation transition in systems where components are uncorrelated. Many experimental systems, such as colloidal gels, involve components that exhibit interactions which induce positional correlation. In this talk, we discuss the effect of correlation at the rigidity percolation transition and discuss its implications for colloidal gels. We find, through numerical simulations of site-diluted triangular lattices, that short-range positional correlation shifts the rigidity percolation transition to lower volume fraction, while keeping the same critical exponents, consistent with the scenario that correlation acts as an irrelevant perturbation at rigidity percolation. We further explore the emergence of rigidity in colloidal gels through molecular dynamics simulations and structural rigidity analysis. In particular, we examine the relation between the emergence of rigidity and the gelation transition at different temperatures, aiming at understanding the origin of structural rigidity in colloidal gels, as solid materials at extra-low volume fractions. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B53.00011: Mechanical Failure of Disordered Networks Estelle Berthier, Jonathan Kollmer, Karen Daniels Disordered networks are widely used to study heterogeneous material failure. These structures are inherent to many systems such as rigid foams or granular materials. Granular materials in particular exhibit highly heterogeneous force chains networks that control the response of such a medium to external perturbations. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B53.00012: Analyzing the Rigidity of Force Networks in Jammed Granular Packings Deshpreet Bedi, Bulbul Chakraborty An understanding of the rigidity of granular systems necessitates the extension of the concept of a mechanical network to include an abstract force-space network. Contact forces between dry grains in mechanical equilibrium can be represented in a dual force space as a network of edges that form a tiling. This force tiling paradigm has been found to be particularly useful in the characterization of stress-induced transitions in granular materials. As a generic network itself, the force tiling network can be studied using techniques used in the investigation of more conventional networks. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B53.00013: The influence of adhesion on crumpled films Andrew Croll, Timothy Twohig, Theresa Elder Understanding the mechanics of random networks is currently one of the most important problems facing the soft-matter community due, in part, to the broad role random networks play in Nature. Recently, researchers have focused on the crumpling of a thin sheet as a useful model random network with unique features due to the increased non-crossing constraints faced by a sheet (compared to a chain). While several models have been proposed, all have ignored the role of inter-sheet adhesion in the process. In this work, we will show how adhesion modifies several features of the crumpling process. We perform compression experiments with several polymeric materials in which adhesion can be controlled, examining the static and dynamic features of the crumpled state. Most notable of our results is that fitting force-displacement data to a simple empirical power law reveals an order of magnitude increase in the effective Young’s modulus of the crumpled network. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B53.00014: Origins of the Poynting effect in elastic networks Brian Tighe, Karsten Baumgarten Poynting observed that some elastic networks expand when sheared, while others contract. We study the origins of this nonlinear effect in random spring networks, a minimal model for biopolymer gels, foams, and other mechanical networks. We show theoretically and verify numerically that the sign of the Poynting effect is predicted by the microscopic Grüneisen parameter, which measures how stretching the system shifts its eigenfrequencies. Spring networks contract under shear (a negative Poynting effect) because their eigenfrequencies shift upwards. We find that the amplitude of the Poynting effect is sensitive to the network's preparation protocol, and it diverges at the isostatic point. Finally, we illustrate correlations between the Poynting effect and the manner in which a network stiffens under strain. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B53.00015: Stochastic buckling of a colloidal chain assembled by critical Casimir forces Simon Stuij, Jan Maarten van Doorn, Thomas Kodger, Joris Sprakel, Corentin Coulais, Peter Schall Mechanical instabilities occur when structures spontaneously deform or change conformation under stress, and are relevant for all materials, at scales from ~10meters to nanometers. So far, they have been mostly considered in a pure mechanical context, where thermal fluctuations are negligible. However, these latter become important when the mechanical structures become small as is the case e.g. for filaments or membranes in soft and biological materials. Here, we study the buckling of a colloidal chain in the presence of thermal noise and unveil a new mechanical critical transition at the chain buckling points. We assemble short colloidal chains by critical Casimir forces. These solvent-mediated forces allow us to vary the attractive particle interactions and thus tune the chain stiffness with temperature. Two laser tweezers are used to hold the chain at its ends, and to push on it. When compressing the chain to a critical state we observe buckling reminiscent of macroscopic structures, yet the thermal fluctuations introduce a crucial new element: multiple buckling modes occur in sequences, accompanied by a divergence of bending fluctuations when a new buckling point is reached. |
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