Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X54: Active Mechanics of Networks and Gels IFocus
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Sponsoring Units: GSOFT DBIO Chair: David Lubensky, Univ of Michigan - Ann Arbor Room: LACC 514 |
Friday, March 9, 2018 8:00AM - 8:36AM |
X54.00001: Topological modes in disordered fiber networks under active driving Invited Speaker: Xiaoming Mao Disordered fiber networks are ubiquitous in a broad range of natural (e.g., cytoskeleton) and manmade (e.g. aerogels) materials. The dilute nature of these fiber network structures permits floppy modes which only bend the fibers without changing their length, and these floppy modes govern mechanical response of the material, such as strain stiffening. In this talk, we show that the geometry of the fiber network dictates the nature of these floppy modes. In particular, an ideal network in which all fibers are straight hosts floppy modes that extend through the bulk, whereas perturbing the geometry from this ideal state and bending the fibers induce floppy modes exponentially localize on the edge of the network, leading to asymmetric edge stiffness. Various activities present in fiber networks, such as active driving of motors in the cytoskeleton, active pulling of cells in the extracellular matrix, and actuators in manmade fiber networks, could lead to such perturbed geometry and thus interesting consequences in the mechanical properties. We show that the localization of these edge floppy modes is protected by the topology of the phonon structure of the fiber networks, analogous to topological edge floppy modes in Maxwell lattices recently studied in topological mechanics. We also discuss our numerical results showing how such modes are generated by active driving, the resulting mechanical properties, and relevance for experimental systems in biology and engineering [1]. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X54.00002: Building active liquid crystals with tunable defect dynamics and variable elasticity Nitin Kumar, Rui Zhang, Jennifer Ross, Juan De Pablo, Margaret Gardel Constituents of active materials convert ambient free energy into directional motion giving rise to striking dynamical phenomena. Here we perform experiments on a two-dimensional sheet of F-actin filaments of length ranging from 1-2 μm, driven by myosin-II motors and show that it acts as an extensile active nematic liquid crystal. We demonstrate that the interaction between ±1/2 defect pairs can be switched from attraction to effective repulsion of strength linearly scaling with motor concentration, φ. To characterize the emerging complex flows, we measure velocity-velocity and orientational correlation lengths and confirm their scaling of φ-1/2. We show that the existence of hydrodynamic shear flows along the symmetry axis of the +1/2 defect leads to its shape change, signifying a lowered effective bend modulus in the active state determined through the defect morphology. We present a novel method of estimating active stress by quantifying the bending of microtubule bundles as probe particles. All our experimental observations are captured by continuum simulations based on the hydrodynamic model of active nematic liquid crystals. Our experiments demonstrate active liquid crystals with tunable defect dynamics and variable effective elasticities in the non-equilibrium state. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X54.00003: Localization of Semiflexible Filaments in a Random Potential. Valentin Slepukhin, Kei Mueller, Alexander Levine Semiflexible filaments, such as F-actin, can be cross linked into complex networks including those found in in vitro experiments and in the cytoskeleton of living cells. Recent work on transiently cross linked semiflexible gels [K.W. Mueller et al., PRL 112, 238102 (2014).] suggests that filaments are typically quenched during network formation into highly stressed states. In order |
Friday, March 9, 2018 9:00AM - 9:12AM |
X54.00004: Stress relaxation in 3-D semiflexible network with transient cross linkers: A finite-element kinetic Monte Carlo approach Lu Shen, Sanjay Dharmavaram, Alexander Levine Semiflexible biopolymer networks cross-linked by noncovalent bonds allow the network’s topology to evolve over time and in response to applied stress. The effects of transient cross linking include glassy power-law rheology at low frequencies1, energy dissipation through bond rupture2, mechanomemory effects3, and self-healing4. We developed a finite-element kinetic Monte Carlo simulation, which allows linkers to reorganize within a three-dimensional network, to explore these dynamics. We discuss this simulation strategy and present results for stress relaxation of sheared networks. We also discuss the statistical distribution of binding and rupture events to address whether stress relaxation occurs via isolated bond breaking events or correlated “ripping” events in which multiple linkers within regions of the networks fail in rapid succession. We conclude with proposals for new experiments to test these numerical results. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X54.00005: Active mechanics of fire-ant networks Franck Vernerey, Tong Shen, Shankar Lalitha Sridhar, Alberto Fernandez-Nieves Active networks are omnipresent in nature, from the molecular to the macroscale. Here, we will discuss the mechanics of fire-ant aggregations, whose swarming behavior has shown impressive dynamics that culminates with the aggregation’s capacity to self-heal and adapt to the environment. Although empirical models have enabled us to characterize these the combined elasticity and rheology of these networks, it is still not clear how the behavior of individual building blocks (ants) affect their emerging response. In this presentation, we will discuss an alternative way to think about these materials by seeing them as a collection of individual building blocks connected by elastic chains that can associate and dissociate over time. From the knowledge of these connections (elasticity, activity), we will construct a statistical description of the chain stretch and derive an evolution equation of the corresponding distribution. This time-evolving distribution is then used to determine important macroscopic measures such as stress, energy storage, and dissipation in the network. In this context, we will show how the physical characteristics and activity of single ants can explain the elasticity, rheological properties, and behavior of the aggregation. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X54.00006: Non-linear mechanical properties of fire ant aggregations Michael Tennenbaum, Alberto Fernandez-Nieves Fire ant aggregations are inherently active materials. Each ant converts its own chemical energy into motion, and it is the overall motion of all individual ants that contributes to the bulk material properties of the aggregation. However, the activity level in fire ant aggregations is not constant in time. This lets us measure the material properties of this active material at different activity levels. Here we investigate the non-linear mechanical properties of fire ant aggregations using large amplitude oscillatory shear rheology at varying activity levels. For each oscillatory experiment we generate Lissajous curves and calculate common non-linear moduli used to characterize the non-linear behavior. Our data suggest that strain stiffening correlates with activity. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X54.00007: How topology influences force chains in networks of epithelial cells Meryl Spencer, Jesus Lopez-Gay, Yohanns Bellaïche, David Lubensky In some epithelial tissues under external mechanical stress cells are arranged in a stationary brick-like pattern where the cells form rows parallel to the high stress axis. This facilitates the creation of multicellular action-myosin cables which oppose the external stress. However, when an external mechanical stress causes calls to flow past one another they exchange neighbors in a way that forms rows of cells perpendicular to the high stress axis. We propose a metric for distinguishing between these two orientations for cells in a disordered tissue based on geometric properties of the cells as they are strained with fixed edge length. We also describe the different types of coordinated topological changes that cells can undergo to move between the two distinct patterns. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X54.00008: Actuation in simple gel and elastomer systems Laura Hanzly, Adair Maynard, Justin Barone Bending can be realized in polymer bilayer sheets when there is a mismatch in properties between the 2 layers. For gelatin hydrogels stimulated by swelling in water, the sheet curvature can be described by the mismatch in gel crosslink density, which directly influences swelling and modulus. For thermoplastic elastomer bilayer sheets stimulated by deformation, the curvature can be described by the elastic recovery difference between the 2 layers. The elastic recovery difference arises from the size and number of polymer crystal “crosslinks” in each layer to realize a “mechanically-programmable” material. In this manner, the curvature can be predicted from molecular level and small scale structural information and can aid in the design of polymer actuators and soft machines that can do work upon stimulation. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X54.00009: Cell Driven Buckling of Active Collagen Microbeams Cameron Morley, Sarah Ellison, Tapomoy Bhattacharjee, Tristan Hormel, Christopher O'Bryan, Sean Niemi, W. Sawyer, Thomas Angelini Collective cell migration and multi-cellular force generation has been explored in recent years in order to gain fundamental insight into the role mechanics plays in tissue development, health, and disease. Great progress has been made studying 2D monolayers, with an emerging understanding of the collective cell response to substrate stiffness and cell density. To more broadly model and investigate mechanical processes in living tissues, it is critical to develop the 3D counterparts of these 2D systems. Here we 3D print simple structures from fibroblasts dispersed in collagen-I networks, which are gently supported within a jammed microgel growth medium. For the case of long, thin microbeams, we observe a self-induced mechanical buckling that can be predicted from classical Euler-Bernoulli beam theory. By varying the beam aspect ratio and cell number density, we can control the transition between stable contracting beams and unstable buckling beams. This transition also allows the indirect measurement of cell generated contractile stress in 3D. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X54.00010: Numerical Simulations of Activity-Driven Mechanical Instabilities in Gels Nicholas Derr, Christoph Weber, L Mahadevan, Christopher Rycroft Biological systems allow for the generation of active stresses, which can lead to instability and pattern formation. These phenomena often stem from material fluxes induced by active stress, and hence many systems are frequently described using a single, fluid-like phase. However, this description is not sufficient for modeling poroelastic systems such as cartilage, bone, gels, or developing tissue, and the effect of differential activity on the stability of biphasic systems such as these remains largely unstudied. In this talk, we describe a generic hydrodynamic description of active poroelastic media and apply it to an incompressible mixture of solid- and fluid-like phases with varying levels of active stress generation. Simulations are carried out using a custom implementation of Chorin's projection method to enforce incompressibility, and the large linear system resulting from the projection is solved at each time step using a geometric multigrid method. We find this differential activity can drive mechanical instabilities leading to condensation of solid patches and macroscopic contractions. Our results indicate that differential activity, like differential adhesion, size and shape, acts as a generic mechanism for instability. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X54.00011: Time-Dependent Microscale Mechanics of Actomyosin Networks During Triggered Activity Bekele Gurmessa, Rae Anderson Networks of semiflexible actin filaments with their associated motor protein, myosin, are critical elements of the cytoskeleton that enable cellular motility and other key mechanical processes. By varying environmental conditions, such as ATP and salt concentrations, cells can dynamically alter the activity and mechanics of actomyosin networks. However, how changing chemical conditions map to time-varying mechanical properties of actomyosin networks remains to be understood. Here, we couple optical tweezers microrheology with microfluidics to measure the time-dependent viscoelastic response of in vitro crosslinked actomyosin networks while modulating ATP and salt concentrations. In particular, we use optical tweezers to sinusoidally oscillate microspheres embedded in networks and measure the resulting force at set time-intervals as we change buffer conditions via passive exchange of small molecules.We also image labeled actin filaments to characterize corresponding network mobility and structure. Our measurements shed important new light onto how actomyosin networks tune their mechanical activity in response to varying environmental conditions, and how their mechanical properties evolve as they transition from one state to the next. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X54.00012: Active Plastisity Daniel Goldstein, Bulbul Chakraborty, Sriram Ramaswamy Active nematics - systems made of biological filaments and motors - show a wide verity of interesting behaviors. These system transition form a passive gel to flowing states when supplied with ATP. To capture this change in rheological properties we propose a minimal model of the stress organization in these system where the activity is captured by self-extending force dipoles that are part of a cross linked network. This network can reorganize itself through buckling of extending filaments and cross linking events that alter the topology of the network. Mean field calculations and simulations of this network reveal that these force dipoles build up stress with time, coupled with an average dissociation time of these force dipoles this give a typical yield stress similar to a yielded plastic solid. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X54.00013: The cell cytoskeleton and the cell nucleus: A tale of two interacting active environments Jennifer Schwarz, Kuang Liu The cell nucleus is very much an active environment with molecular motors such as RNA polymerase, helicase, and topoisomerase all manuevering to facilitate DNA transcription. The cell cytoskeleton is also an active environment with molecular motors such as myosin, kinesin, and cytoplasmic dynein working to give the cell shape and rigidity. On the other hand, the underlying backbone of the active environment of the cell nucleus is a single, compacted polymer, i.e. DNA, while for the cell cytoskeleton, it is a composite semiflexible polymer network involving actin, microtubules, and intermediate filaments. It turns out that these two active environments are not independent of each other and are, not suprisingly, mechanically coupled via LINC complexes, though details of the coupling remain poorly understood. We, therefore, computationally study the shape of the interface between these two active environments as a function of the nature of their mechanical coupling to ultimately work towards a framework for quantitative prediction of nuclear shape fluctuations in cells. |
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