Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A48: GSNP Student & Postdoc Prize SessionFocus Prize/Award
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Sponsoring Units: GSNP Chair: David Egolf, Georgetown Univ Room: LACC 510 |
Monday, March 5, 2018 8:00AM - 8:36AM |
A48.00001: Dissertation Award in Statistical and Nonlinear Physics Talk: Life in Suspense: Particle dynamics in suspensions of swimming bacteria Invited Speaker: Alison Koser Patteson Cells and microorganisms move in diverse environments that range from pond and ocean water to complex biological fluids, such as mucus. These environments frequently contain passive particles, such as macromolecules, flexible polymers, or colloids, which can influence cell motility and behavior. Interactions between cells and particles underlie many aspects of medicine, biology, and engineering, including the spread of bacterial infections, the formation of biofilms, and the design of swimming micro-robots. Identifying means to relate the microstructural features of the swimmer to macrostructural properties and bulk phenomena of the fluid would yield ways to control, manipulate, and direct biological systems. In this talk, I will explore the use of colloidal particles and polymer molecules to experimentally probe the emergent non-linear flows in suspensions and swarms of bacteria. The results identify new avenues of transport and highlight how active bacterial fluids differ from their passive counterparts. I will discuss applications to particle sedimentation and boundary propagation in microbial environments. |
Monday, March 5, 2018 8:36AM - 8:48AM |
A48.00002: Designing Gripper-like Architectures from Self-folded Bilayers Arif Abdullah, K Jimmy Hsla Motivated by the environmentally responsive actuation mechanisms of non-muscular plants, research efforts have been made toward the development of smart reconfigurable devices that can sense and autonomously respond to their surroundings. Tetherless self-folding microgrippers - extensively used for micro/ nano-manipulation - are examples of such engineered systems as they could be actuated on-demand by external stimuli. Being multilayer in nature, these structures rely on spatially patterned hinges for their morphing behavior, and hence they require extensive fabrication efforts. The goal of this work is to establish a design paradigm so that gripper-like architectures could be realized from simple stimuli-responsive planar bilayers (one layer expanding isotropically due to a stimulus while the other remains passive). Through a combination of finite element modeling and experiment, we investigated the stimulus-responsive behavior of bilayer stars and established the principles to achieve stable axisymmetric gripper-like shapes in a programmable manner. Thus, in addition to proposing a simpler route to achieve microgrippers, our research contributes to the rapidly emerging multidisciplinary field of stimuli-responsive self-assembly. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A48.00003: Phononic Frequency Combs Adarsh Ganesan, Ashwin Seshia Optical frequency combs, by its manifestation as a frequency ruler, has revolutionized optical frequency metrology and spectroscopy. Only very recently, the phononic analogue of such frequency combs was experimentally demonstrated1 in a microelectromechanical resonator confirming predictions made by numerical simulations of a Fermi-Pasta-Ulam system2. This initial experimental demonstration was followed up in further studies3-5 where variants of phononic frequency combs have been systematically studied using similar device testbeds over a range of drive conditions. The generation of phononic frequency combs in each case is inherently related to nonlinear modal coupling in such structures with exquisite control of geometry, material properties and conditions required for engineering application. In addition to the general scientific interest, phononic frequency combs also enable a unique approach to tracking the resonant frequency of a micromechanical resonator without an external feedback loop with the potential for improvements in frequency stability compared to conventional feedback oscillators6. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A48.00004: Identity crises in hard polyhedral glass-formers Erin Teich, Greg Van Anders, Sharon Glotzer Colloidal systems are capable of self-assembling into a wide variety of ordered structures, ranging from the simple to the exceedingly complex. Often, however, no such assembly occurs, and the system instead displays dynamical characteristics of glass-formation. Here, we computationally investigate assembly failure in a family of monodisperse, one-component systems, composed of colloidal particles of polyhedral shapes with no interactions aside from those of excluded volume. We study the role that local structure plays in dynamical arrest in these entropic systems, and find that assembly failure arises from an ``identity crisis” experienced on a local level and manifested in shape space. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A48.00005: Surges of Collective Human Activity Emerge from Simple Pairwise Interactions Christopher Lynn, Evangelia Papadopoulos, Daniel Lee, Danielle Bassett Collective human behavior drives a wide range of social, political, and technological phenomena in the modern world. However, while the correlated activity of one or two individuals is partially understood, it remains unclear if and how these simple low-order interactions give rise to the complex large-scale patterns characteristic of human experience. Here we show that a network of email correspondence exhibits surges of collective activity, which cannot be explained by assuming humans act independently. Intuitively, this collective behavior could arise from complicated correlations between large groups of users, or from shared daily and weekly rhythms. Instead, we find that the network is quantitatively described by a maximum entropy model that depends only on simple pairwise interactions, making it equivalent to the Ising model. Remarkably, we find that the learned Ising interactions, which are inferred exclusively from the timing of sent emails, are closely related to the ground-truth topology of email correspondence. Together, these results suggest that patterns of collective activity emerge from simple pairwise correlations, which, in turn, are largely driven by direct inter-human communication. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A48.00006: Exploiting Nonlinear Dynamics for Programmable Behavior in Microfluidic Networks Daniel Case, Jean-Régis Angilella, Adilson Motter The development of microfluidic systems that do not rely on abundant external hardware, yet retain controllable, complex behavior has been a primary research goal for the past decade. Microfluidic systems are composed of a network of flow channels and are generally considered to be linear systems; however, by creating nonlinearities in the network we are able to produce a diverse range of flow dynamics. Here, I present a simple microfluidic network that exhibits spontaneous oscillations in the flow rate. It is possible to switch between an oscillating and steady flow state by only changing the driving pressure. This functionality does not rely on external hardware and may even serve as an on-chip memory or timing mechanism. Further, I demonstrate the ability to control the direction of flow though intermediate channels not directly connected to the controlled terminal. I use analytic models and rigorous fluid dynamics simulations to show these results. We expect this work to advance built-in control mechanisms in microfluidic systems towards the goal of designing portable systems that are as controllable as microelectronic circuits. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A48.00007: Dense packing of cell monolayers: Jamming of deformable polygons Armand Boroman Collective motion in dense packings of cells occurs in wound healing, embryonic development, and cancerous tumor growth. Most current computational models of dense cell packings either treat the system as collections of spherical particles or assume that the system is confluent, with no extracellular space. We have developed a new model for dense cell packings in two spatial dimensions, where the cells are modeled as deformable particles that have a preferred area and perimeter. We measure the packing fraction, φj at jamming onset as a function of the asphericity, α, which is the ratio of the perimeter square to the area of the particle. We find that the jammed packing fraction increases monotonically with α and that the system becomes confluent with φj = 1 for α > 1.16. Using surface Voronoi analysis, we show that this value for α corresponds to the case when the cells completely fill their Voronoi-tesselated regions. We also demonstrate that the free area per cell obeys a k-gamma distribution, which has been found for jammed packings of non-deformable particles. Finally, we will describe results from our model concerning the mobility of deformable particles subjected to applied forces, as well as diffusion of deformable particles subjected to active forces to discuss the effect of geometry in active jamming of deformable particles. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A48.00008: Flocking from a quantum analogy: Spin-orbit coupling in an active fluid Benjamin Loewe, Anton Souslov, Paul Goldbart Quantum analogues have proven to be valuable tools in the study of both equilibrium and non-equilibrium statistical systems. At their core, these analogies allow one to explore some complex classical systems in terms of simpler quantum ones, thus facilitating the use of the powerful toolkit of quantum mechanics. We enlarge on the well-known relationship between the Schrödinger equation and the diffusion equation in order to incorporate self-propulsion, and thus, to build quantum analogues of systems of two-dimensional self-propelled particles. Crucially, we show how, on the quantum side, spin and spin-orbit coupling capture both a particle’s orientation and self-propulsion. Interestingly, the microscopic active system that stems from this analogy is characterized by a coupling of translational and rotational noises, which resembles the Heisenberg uncertainty principle. Finally, by coarse-graining the microscopic model, we obtain explicit expressions for the coefficients in the Toner-Tu equations, which describe the hydrodynamic limit of the system. The connection between self-propelled particles and quantum spins may help realize exotic phases of matter using active fluids via analogies with systems composed of strongly correlated electrons. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A48.00009: Modeling multicomponent phase behavior inspired by membraneless compartmentalization in cells Sheng Mao, Mikko Haataja, Andrej Kosmrlj Recent evidence shows that intracellular phase separation can drive the formation of membraneless liquid-like droplets composed of protein RNA and other biomolecules. Gibbs rule suggests that the number of possible coexisting phases scales linearly with the number of components, which is on the order of hundreds in cells. However, in typical biological systems only a small number of phases are observed and they are often assembled in highly organized structures. To resolve this puzzle, we first employ Flory-Huggins theory to examine the global phase structure of many components, whose interaction energies are randomly drawn from a Gaussian distribution. We find that the typical number of coexisting phases is primarily determined by the composition and the mean strength of interaction. However, the enhanced variance of interaction increases the range of parameters, where small number of coexisting phases are observed. In order to see, how different phases evolve in time and arrange in space, we used the Cahn-Hilliard formalism. We investigate the nucleation and growth of domains as well as their relative packing for 3, 4, 5 and more components. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A48.00010: Insight into Shear Thickening Suspensions using Boundary Stress Microscopy Vikram Rathee, Daniel Blair, Jeffrey Urbach The bulk rheological response of shear thickening (ST) suspensions is well documented but the microscopic origin of ST remains poorly understood. Using direct measurement of spatially resolved surface stresses in ST using boundary stress microscopy (BSM), we recently reported the existence of clearly defined dynamic regions of substantially increased stress that appear intermittently at stresses above a critical value. Here we present measurements using a smaller system and lower magnification so that the whole suspension is visualized during shear and show that the localized high stress events observed in steady shear have a finite lifetime. In large amplitude oscillatory measurements, we do not observe any localized high stress events if the peak shear rate is less than a critical shear rate, but the events appear when the peak shear rate is large enough. At very high values, the heterogeneous events become well established. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A48.00011: Unexpected Tissue Surface Tension in Simple Models of Dense Biological Tissues Daniel Sussman, Jennifer Schwarz, M Cristina Marcetti, M Manning Tissue surface tension, the effective interfacial tension between two tissues composed of different cell types, has been a useful paradigm for explaining cell sorting, compartmentalization, and boundary maintenance in vivo or in co-culture. Different experimental techniques have been developed to measure tissue surface tension by analogy with the equilibrium behavior of molecular fluids. An important open question is whether these different techniques probe the same thing, since tissues are far from equilibrium and cells have different degrees of freedom and interactions compared to molecules. We extend existing vertex-based models for confluent tissues by allowing individual cells to regulate the tensions between neighboring cells of like or unlike type, and then numerically measure the effective surface tension between coexisting cell populations. Strikingly, we find that different methods which yield identical values of the tension in molecular fluids differ by more than an order magnitude in confluent models for tissue. We demonstrate that this difference stems from the topological nature of the interactions between cells, and speculate that similar interfacial sharpening may occur in other systems with topological interactions. |
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