Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A57: Mechanics of Networks I: Allostery and Designed ResponseFocus
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Sponsoring Units: GSOFT GSNP DPOLY Chair: Pedro Reis, Massachusetts Institute of Technology-MIT Room: LACC 518 |
Monday, March 5, 2018 8:00AM - 8:12AM |
A57.00001: Exploring the limits of multifunctionality in tunable networks Jason Rocks, Henrik Ronellenfitsch, Andrea Liu, Sidney Nagel, Eleni Katifori Nature is rife with networks that are functionally optimized to propagate localized inputs to perform specific tasks. For example, allosteric proteins globally change conformation upon the binding of a ligand, controlling the activity of a distant active site. As another example, the vascular network in the brain can reroute blood flow to enhance oxygen levels to locally support active neurons. Whether via genetic evolution or dynamic adaptation, many networks create functionality by locally tuning edge properties. To explore this behavior, we optimize both mechanical and flow networks to perform specific functions by adding and removing edges. We define a single function as a tuned response (strain or pressure drop, respectively) of a single target edge when another specified part of the network is activated (similarly via strain or pressure drop, respectively). Using structures generated via such optimization, we answer the question of how many simultaneous functions a given network can be programmed to fulfill. We find that both types of networks display similar phase transitions in the number of targets that can be tuned, implying that both tuning problems can be understood in the context of a broader class of constraint-satisfaction problems. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A57.00002: Role of Connectivity in the Conformational Control of Maxwell Frames Jason Kim, Danielle Bassett Understanding conformational changes is crucial for programming and controlling the function of many mechanical systems such as allosteric enzymes, auxetics, and tunable metamaterials. Of particular interest is the relationship between the network setup (connection topology and geometry) and specific observed motions under controlling perturbations. Here we study this relationship in mechanical networks of 2-D and 3-D Maxwell frames composed of point masses connected by rigid rods with zero bending modulus about the masses. We first develop some basic principles that yield a simplified and intuitive understanding of how complex network connectivity gives rise to specific zero modes. Using these principles, we then construct networks with desirable properties such as modularity, tunable Poisson ratio, and multiple modes of conformation. |
Monday, March 5, 2018 8:24AM - 8:36AM |
A57.00003: Isotropic Auxetic Metamaterials from Disordered Networks Daniel Reid, Nidhi Pashine, Alec Bowen, Justin Wozniak, Sidney Nagel, Juan De Pablo Recent work indicates that selective pruning of disordered networks consisting of nodes connected by bonds can generate materials with non-trivial mechanical properties, including auxetic networks having a negative Poisson's ratio. Until now, auxetic networks created based on this strategy have not been successfully realized in experiment. Here a new model that includes angle-bending forces and the experimental boundary conditions is introduced for pruning-based design of auxetic materials. By pruning the appropriate bonds, the Poisson's ratio can be tuned to values approaching the lower mechanical limit of -1, and the corresponding laboratory networks exhibit good agreement with model predictions. While pruning algorithms allow for the creation of highly auxetic materials with an anisotropic Poisson's ratio, the auxetic effect is limited in the isotropic case. To address this, we employ optimization algorithms which selectively modify the moduli of individual bonds. Using such procedures, we are able to create isotropic highly auxetic networks in both simulation and experiment. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A57.00004: Designing allostery-inspired response in mechanical networks Invited Speaker: Sidney Nagel Recent advances in designing metamaterials have demonstrated that global mechanical properties of disordered spring networks can be tuned by selectively modifying only a small subset of bonds [1-3]. We have extended this idea to tune more general properties of networks [4]. Inspired by the long-range coupled conformational changes that constitute allosteric function in proteins, we can produce a strain between any two target nodes in a network in response to an applied source strain on a distant pair of nodes by removing only ∼1% of the bonds. Moreover, we can also control multiple pairs of target nodes, each with a different individual response, from a single source, and tune multiple independent source/target responses simultaneously. We have fabricated physical networks in macroscopic 2D and 3D systems that exhibit these responses. The fact that allostery is a common means for regulation in biological molecules suggests that it is a relatively easy property to develop through evolution. In analogy, our results show that long-range coupled mechanical responses are similarly easy to achieve in disordered networks. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A57.00005: Architecture of Allosteric Materials and Principles for Optimal Cooperativity Le Yan, Riccardo Ravasio, Carolina Brito, Matthieu Wyart Allostery, a long-range mediated interaction, is a key feature in the functionality of several proteins, resulting to be crucial for life. Understanding the nature of the information transmitted and the architectures optimising such transmission remains a challenge. We recently introduced a numerical scheme to evolve functional materials that can accomplish a specific task. Architectures optimised to be cooperative, which propagate efficiently energy, strongly differ from previously investigated materials optimised to propagate strain. Although we observe a large diversity of functioning cooperative architectures —including shear, hinge and twist designs, they all obey the same principle of displaying a mechanism, i.e. an extended and nearly zero energy mode. We compute its optimal frequency, and show that —for such designs— cooperativity decays logarithmically with the system size L for d=2 and does not decay for d=3 where d is the spatial dimension, in great contrast with undesigned materials for which cooperativity decays as L-d. Overall, our approach leads to a natural explanation for several observations in allosteric proteins, and suggests a path to discover new ones, also viewing allostery as a consequence of design in disordered media. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A57.00006: Geometry and mechanics of knitted fabric Michael Dimitriyev, Elisabetta Matsumoto Knitted fabric consists of yarn that is stitched together in a repeated pattern. The fabric is held together by internal stresses due to the large contortions of its threads, which are linked together in slipknots, placing physical constraints on thread displacement. The result is a material with extremely soft bending elasticity that is also able to stretch. Due to the complex structure of knitted fabrics, much is unknown about the collective mechanics of these materials and there is not currently an accurate constitutive model. As such, modeling is limited to computationally-intensive yarn-level simulations. Additionally, global properties of the fabric are sensitive to the local geometry of the stitches. The intrinsic asymmetry between the front and the back of stitches yields a fabric that has the propensity to curl, rather than lay flat. We seek a set of constitutive relations for a variety of stitch patterns by examining the equilibrium stress distribution in a model of linked elastica, with the ultimate goal of developing a robust continuum theory. Our results provide a guide to developing fabrics with intrinsic geometry and mechanics that are controlled by stitch pattern, as well as allowing for fast and accurate simulations for CGI films and video games. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A57.00007: Limits on the Programmability of Spring Networks Salem Al Mosleh, Christian Santangelo Mechanical meta-materials offer great potential for making materials with desired properties and functionalities that are not possible with ordinary materials. In this work we explore the possibility of programming in the precise lowest energy motions of a spring network by tuning the rigidities and configurations of bonds in the network. After demonstrating that the "obvious" ways of accomplishing this don't work we explore different approaches that can allow us to have precise control over the network motions. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A57.00008: The effect of varying crosslinking motifs on the microscale mechanics of co-entangled actin and microtubules Shea Ricketts, Jennifer Ross, Rae Anderson The cytoskeleton is a complex composite network of proteins, including filamentous proteins such as actin and microtubules, as well as numerous binding proteins that crosslink these filaments. The physical interactions between semiflexible actin filaments and rigid microtubules, as well as the wide range of crosslinking motifs that their corresponding binding proteins offer, allow cells to precisely tune their strength and structure to support mechanical processes such as apoptosis and meiosis. To determine the role that crosslinking versus steric interactions has on cytoskeleton composites we: (1) design composites of co-entangled actin and microtubules with varying crosslinking motifs, and (2) use optical tweezers microrheology and dual-color confocal microscopy to characterize the mechanics and structure of composites. Specifically, we create equimolar actin-microtubule composites in which only actin is crosslinked, only microtubules are crosslinked, and both filaments are co-crosslinked. We then measure the force response resulting from optically driving microspheres through the composites; and image spectrally distinct actin and microtubules to quantify composite mobility and morphology. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A57.00009: Associative memory in mechanical systems Menachem Stern, Arvind Murugan Associative memory is the ability of recurrent neural networks to retrieve one of several stored configurations (`memories') using only partial or corrupted information about the desired memory. We find the conditions under which associative memory is seen in a general family of mechanical networks that include spring networks and folding sheets (origami). We show that the capacity for associative memory requires strong mechanical non-linearities and grows with the range of interactions and the dimensionality of the system. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A57.00010: Plastic fluctuations in a knitted fabric Samuel Poincloux, Mokhtar Adda-Bedia, Frederic Lechenault A knitted fabric is a topologically constrained elastic yarn following a periodic path. The mechanical behavior of the fabric appears to be drastically different from the yarn it is made of. To explain this discrepancy, we introduced a network model which features three ingredients, a dominant bending energy, an unaltered topology and yarn length conservation. This model provides a quantitative comparison with experiments done on a model knitted fabric, both in the force-elongation relation and in the deformation field. However, yarn-yarn friction at the crossing points induces fluctuations around elasticity in the force response and deformation field. The fluctuations are identified as sudden drops of the force correlated with localized lines of deformation in the network, and are caused by the propagation of stick-slip events at contact points. The plastic events are characterized and their properties predicted owing to the network constraints that still apply. A good understanding of the friction induced plastic events allows quantitative comparison with similar plastic events arising in other a-thermal soft amorphous solids. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A57.00011: Manipulating the chain network in glassy polymers to alter its mechanical responses Jianning Liu, Xiaoxiao Li, Weiyu Wang, Yue Lu, Zhichen Zhao, Jimmy Mays, Shiqing Wang We study a simplest mechanical network, a glassy polymer, in terms of its nonlinear mechanical responses. Like all other networks, the key mechanics questions are how deformable is the network, what determines the mechanical strength, how the system undergoes structural failure. Some glassy polymers are ductile well below their glass transition temperatures, some are incapable of yielding and suffer brittle fracture. Polystyrene is brittle in tensile extension but ductile in compression at room temperature. Is polystyrene always ductile in compression? Our presentation discusses the latest experiments inspired by a recent molecular model1 for mechanics of glassy polymers. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A57.00012: Tuning the nature of mechano-memory in actin networks Danielle Scheff, Sayantan Majumdar, Margaret Gardel To adapt to external stimuli, cells can dynamically alter their material properties through rearrangement of the actin cytoskeleton, a cross-linked network of actin filaments. In addition to its importance in cell biology, understanding this mechanical response provides strategies for creation of novel materials. A recent study demonstrated that applied shear stress can encode mechanical memory in these networks, characterized by an anisotropic response under subsequent shear deformations. The network becomes stiffer in the direction of the previously applied stress. Here, we vary both the cross-linker concentration and stiffness to examine the effect of network architecture on this mechano-memory. While networks cross-linked with intermediate concentrations of cross-linkers both stiffen in the direction of the previously applied stress and weaken in the opposite direction, networks with high cross-linker concentrations only weaken. Remarkably, this effect is independent of cross-linker type and rigidity. Overall, our study is crucial for understanding the process by which networks rearrange under external stress and the resulting effect on response to shear. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A57.00013: Nonlinear Mechanics of Origami’s Critical Point: Understanding and Designing around Flat State Transitions Andrew Gillman, Kazuko Fuchi, Alexander Cook, Alexander Pankonien, Phil Buskhol Origami, the art of paper folding, has proven to be a transformative technology in science and engineering with applications including structural composites, electromagnetic devices, mechanical metamaterials, and space deployable structures. The large localized rotations and stiffness mismatch between folding and facet stretching/bending creates a highly nonlinear structure that exhibits unique properties, such as multi-stability and/or auxetic behavior. However, navigating the origami design space is challenging due to branching and limited energetic discrimination between possible fold paths. To investigate the challenge, we combine a nonlinear truss model and topology optimization techniques to identify methods of robust fold path selection. We focus on design of multistable origami structures with multiple internal vertices, and introduce techniques for exploring critical points and bifurcations (which often occur during a flat state transition). Objective functions are explored where these bifurcations are avoided or encouraged in addition to design of the overall energy landscape (location and number of equilibrium points). This analysis and design framework allows for generic tuning of the equilibrium states of the structure to achieve application specific performance. |
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