Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y54: Active Mechanics of Networks and Gels II |
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Sponsoring Units: GSOFT DBIO Chair: Itamar Kolvin, Hebrew Univ of Jerusalem Room: LACC 514 |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y54.00001: A Hierarchy of Instabilities in an Active Material Peter Foster, Sebastian Fürthauer, Bezia Laderman, Che-Hang Yu, Stephanie Ems-McClung, Claire Walczak, Zvonimir Dogic, Michael Shelley, Dan Needleman
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Friday, March 9, 2018 11:27AM - 11:39AM |
Y54.00002: Dynamic Balance between Force Generation and Relaxation Facilitates Pulsatile Contraction of Actomyosin Jing Li, Qilin Yu, Taeyoon Kim Actomyosin contractility regulates various biological processes including cell migration, muscle contraction, and tissue morphogenesis. Cell cortex underlying a membrane, which is a representative actomyosin network in eukaryote cells, exhibits dynamic contractile behaviors. Interestingly, the cell cortex shows reversible aggregation of actin and myosin called pulsatile contraction in diverse cellular phenomena. While contractile behaviors of actomyosin machinery have been studied extensively in several in vitro experiments and computational studies, the pulsatile contraction of actomyosin networks observed in vivo has not been recapitulated well. Here, we employed an agent-based computational model based on Brownian dynamics to identify critical factors facilitating the pulsatile contraction of actomyosin networks. We found that the strong pulsatile contraction of actomyosin networks only occurs when there is subtle balance between force generation from motors, global force relaxation via actin turnover, and local force relaxation via angle-dependent actin severing. Our study provides critical insights into understanding the mechanisms and roles of the pulsatile contraction observed in cells. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y54.00003: Probing fluctuations and active dynamics of the cellular actomyosin cortex with magnetic micropost arrays Yu Shi, John Crocker, Daniel Reich Actomyosin networks inside living cells are an archetypical example of an active matter system. Here we use poly(dimethylsiloxane) (PDMS) micropost arrays with embedded nickel nanowires to measure the dynamical fluctuations and the local rheology of cellular actomyosin networks simultaneously. By characterizing their fluctuations using mean square displacements (MSDs), we found that the cellular cortex and stress fibers showed different dynamics. We also found large dispersion in the MSD exponent as well as the processivity time of the fluctuations in the cortex, indicating the presence of significant stochasticity in the myosin motors’ contractile dynamics. From correlations between individual posts’ deflections, we have identified specific large contractile and relaxational events in the cortex. The structure of these events is consistent with the presence of giant fluctuations in the active actomyosin system, and not with the response expected from models of step-like forces from single myosin force generators. These results thus suggest the presence of cooperativity in cortical myosin thick filament binding and unbinding. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y54.00004: Three-dimensional modeling and simulation of the cell cortex: bringing remodeling, elasticity and active forces together Alejandro Torres-Sánchez, Marino Arroyo The cell cortex is a thin layer of cross-linked actin filaments lying just beneath the plasma membrane of animal cells. The cell cortex plays a key role in biological processes involving mechanical and active forces, such as cytokinesis or migration. The structure of the cortex undergoes a turnover in about one minute, consisting in the rearrangement of the filament network. This rearrangement results from the polymerization and depolymerization of actin and the binding and unbinding of cross-linkers to the network. Due to the turnover, the cell cortex behaves as an elastic network at short time-scales but it resembles a viscous fluid at longer time-scales. Furthermore, the cortex is crowded with myosin motors, which generate a source of active tension. Here we bring together all these different ingredients in a three-dimensional and non-linear model of the cortex. We investigate the relevance of cortex viscoelasticity in processes such as cytokinesis or migration. Furthermore, we perform computer simulations of our model in different mechanical assays resembling experiments, which allow us to extract they key parameters of the model. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y54.00005: Mechanisms Of Selective Transport Through Nuclear Pore Complex Mimics Laura Maguire, Michael Stefferson, Katherine Rainey, Nathan Crossette, Eric Verbeke, Meredith Betterton, Loren Hough Few cellular processes require such intricate active control as transport through the nuclear envelope. The nuclear pore complex facilitates transport, preventing most macromolecules from crossing the envelope while allowing the passage of transport factors and their cargo. While the basic biochemical interactions of transport are well-understood, the detailed mechanism remains a topic of significant debate. We developed a theoretical framework to test models of selective transport. The results suggest that the flexible nature of the disordered FG Nups that line the nuclear pore in vivo and the transient, multivalent nature of FG Nup – transport factor interactions are together sufficient for selectivity. In order to test our model predictions, we created tunable hydrogel mimics of the nuclear pore selective barrier and aim to use the model to tune the mimic's parameters to maximize selectivity. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y54.00006: Enhanced water transport in 8Å diameter carbon nanotube porins. Aleksandr Noy Living systems move ions and small molecules across biological membranes using protein pores that rely on nanoscale confinement effects to achieve efficient and exquisitely-selective transport. I will show that carbon nanotube porins—pore channels formed by ultra-short carbon nanotubes assembled in a lipid membrane—can exploit similar physical principles to transport water, protons, and small ions with efficiency that rivals and sometimes exceeds that of biological channels. I will discuss the role of molecular confinement in these pores, and show how it can enhance water transport efficiency, and influence the mechanisms of ion selectivity in these pores. Overall, carbon nanotube porins represent a simple and versatile biomimetic membrane pore that is ideal for studying nanoscale transport phenomena, and building the next generation of separation technologies and biointerfaces. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y54.00007: Membrane-embedded nanoparticles can modulate lipid membrane permeability Mukarram Tahir, Alfredo Alexander-Katz Lipid bilayers possess a characteristic hydrophobic interior that indiscriminately blocks passive transport of polar molecules across biological membranes. Membrane proteins controllably modify this barrier property by enabling selective transport in response to environmental stimuli. We have previously demonstrated that gold nanoparticles functionalized with a monolayer of amphiphilic ligands can non-disruptively insert into lipid membranes and induce perturbations of functional significance. In this work, we use molecular dynamics simulations to investigate how the properties of these membrane-bound particles can be exploited for altering the permeability of small molecules across lipid membranes in a manner that is analogous to the behavior of membrane proteins. We perform free energy calculations to systematically investigate how membrane permeability is modified by morphology, surface chemistry, and embedding state of the nanoparticle. Using unbiased simulations, we also suggest possible strategies for utilizing these nanoparticles as biomimetic membrane channels that are responsive to external stimuli such as membrane tension and ionic imbalance. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y54.00008: Nonequilibrium Dissipation in Living Oocytes Wylie Ahmed Living organisms are inherently out-of-equilibrium systems. We employ recent developments in stochastic energetics and rely on a minimal microscopic model to predict the amount of mechanical energy dissipated during active vesicle motion in mouse oocytes. Our model includes complex rheological effects and nonequilibrium stochastic forces due to molecular motors. By performing active microrheology and tracking micron-sized vesicles in the cytoplasm of living oocytes, we quantify the spectrum of dissipated energy. We show that our model is consistent with the experimental data, and we use it to offer predictions for the injection and dissipation energy scales involved in active fluctuations in the oocyte cytoplasm. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y54.00009: Non-equilibrium scaling behavior in active biological networks Federica Mura, Grzegorz Gradziuk, Chase Broedersz Recent experiments indicate non-equilibrium activity in a host of biological systems, including chromosomes, cell membranes, and the cytoplasm. Measuring and quantifying non-equilibrium dynamics in such systems is a major challenge in biophysics, due to their many-body nature and the limited number of variables accessible in an experiment. We investigate what information concerning the system’s non-equilibrium state can be extracted by non-invasively tracking a subset of degrees of freedom. To this end, we develop a general, yet simple stochastic model of soft elastic networks with a heterogeneous distribution of activities, representing internal enzymatic force generation. With this model, we determine the scaling behavior of non-equilibrium dynamics from the phase space currents of tracer particles with varying spatial separations in the system. Our results provide insight into how internal driving by enzymatic activity generates non-equilibrium dynamics on different length scales in a variety of biological systems, including polymers, membranes and networks. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y54.00010: Abstract Withdrawn
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Friday, March 9, 2018 1:15PM - 1:27PM |
Y54.00011: Insect Respiratory Network-Inspired Microfluidic Devices Act as Hydrodynamic Ratchets Krishnashis Chatterjee, Philip Graybill, Anne Staples, Rafael Davalos Many entomological respiratory systems appear to be actively ventilated by the periodic buckling of localized sections of the tubes that comprise the tracheal network. These tubes range in diameter from approximately 500 microns at the openings to the respiratory system, depending on the insect, to a fraction of a micron at the distal end of the tracheal system, near the tissue. Often the buckling occurs in a directional manner. We fabricated nine different 3-layer PDMS insect-inspired microfluidic devices to determine if a device that actuates flow in the channel via directional partial collapse of the channel ceiling could produce a unidirectional flow. In all the devices, the flow channel ceiling was collapsed via a symmetric, periodic input forcing signal. We found that many of the designs produced unidirectional flows, and that the flow amount depends on the forcing frequency. Furthermore, we found that the ability to produce a unidirectional flow depends on spatial asymmetries in the hydraulic resistance of the channel. Hence, the successful devices translated a symmetric periodic input forcing into a unidirectional flow, acting as a type of hydrodynamic ratchet. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y54.00012: Viscous dipping, application to the capture of fluids in living organisms Amandine Lechantre, Pascal Damman Various animals (among insects, birds and mammals) use flower nectar as their energy supply. They then developed specific skills to ingest highly viscous fluids. Depending on the sugar content in the nectar, different strategies are observed: hummingbirds have a tongue made from two thin flexible sheets that bend to form a tube when immersed in the nectar; other animals exhibit in contrast a specific papillary structure. Bees and some bats possess a tongue decorated with complex structures that, according biologists, are optimized for fluid capture. In this talk, we first make an extensive investigation of the viscous drag/drainage with smooth rods. For high capillary numbers, a switch from the classical LLD model to a visco-gravitational regime is observed. In a second stage, the influence of micro-structures that mimic biological morphologies is investigated. The micro-structures has several purposes: to trap fluid in the controlled roughness, and to alter the viscous drag process. Interestingly, it appears that increasing the size of protuberances increases the amount of trapped fluid but to the detriment of the dragged fluid. There should thus be an optimum of the structure determined by the viscosity of nectar and the characteristics of the tongue. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y54.00013: Superresolution Microscopy of Individual and Densely Packed pNIPAM Microgels Gaurasundar Marc Conley, Philippe Aebischer, Sofi Nöjd, Marco Braibanti, Peter Schurtenberger, Frank Scheffold Microgels made from poly(N-isoprolylacrylamide) (pNIPAM) change their conformation in response to changes in temperature or solvent composition. Below ~32°C they are swollen and have a core-shell structure, while above they collapse and are akin to hard spheres. Their core-shell structure allows them to be packed far beyond jamming. An exhaustive description of dense suspensions relies on independent characterization of individual microgels. However, non-isotropic shape deformations are hardly accessible via scattering techniques and single particle observations in situ have so far been hindered by insufficient resolution, with optical microscopy, or contrast, with cryo-TEM. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y54.00014: The world’s smallest flash mob: Modulating colloidal interactions using light Emily Gehrels, Vinothan Manoharan We present a new approach to dynamically control systems of DNA-coated colloids using light-modulated interactions. To achieve this control, we infiltrate particles with dyes that cause local heating upon illumination with the appropriate wavelength, thereby affecting the DNA binding. With this new control, we rapidly and reversibly switch between binding and unbinding, and we demonstrate how this modulation can be used to understand the behavior of driven, out-of-equilibrium systems. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y54.00015: Why won't these balls stop squeeking and jumping!? (i.e. the transition from elastic to equilibrium Leidenfrost with hydrogel spheres) Scott Waitukaitis, Kirsten Harth, Martin Van Hecke When hydrogel spheres are dropped onto hot surfaces, they harvest mechanical energy and squeek and bounce for several minutes. This commotion is related to the Leidenfrost effect, whereby a liquid drop above a hot surface floats on a cushion of vapor. Why can't hydrogels also do this "normal" floating Leidenfrost? In this talk, I will show that they can and explain how. Aside from shushing these squeeky hot bouncers, creating hovering hydrogels opens up the possibility to direct their dynamics by molding them into smart shapes. |
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