Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A09: Active Materials I |
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Sponsoring Units: DSOFT Chair: Janet Sheung, Scripps College Room: Room 132 |
Monday, March 6, 2023 8:00AM - 8:12AM |
A09.00001: Directed Collective Synchronization in Active Vortex Lattices Andreas Glatz, Alexey Snezhko, Koohee Han, Andrey Sokolov Collective dynamics of active matter systems holds great promise for the next generation of advanced tunable materials with high adaptability. However, because of their complex out-of-equilibrium nature, it has been challenging to precisely control the spatio-temporal organization and collective synchronization in active matter. Here, we show that, when energized by an external magnetic field, an ensemble of active magnetic rollers dispersed in micro-wells spontaneously evolves into vortex lattices. The presence of micro-wells facilitates not only the self-organization of active rollers into multi-vortex lattices but also the synchronization between the emergent vortices. In particular, we study the correlation between neighboring vortex-antivortex pair, resulting in an antiferromagnetic arrangement of the lattice, depending on the geometry of the wells. In addition it is shown that these synchronized vortex lattices have the ability to self-heal and to switch its chirality on-demand. Here we focus on the numerical simulations of such system using a contiuum model for the particle density which is coupled to shallow water fluid dyanmics. |
Monday, March 6, 2023 8:12AM - 8:24AM |
A09.00002: Data-driven model discovery using SINDy on particle based simulations of dry active nematics Chris Amey, Yingyou Ma, Raymond Adkins, Paul Kreymborg, Zvonimir Dogic, Aparna Baskaran, Michael F Hagan Active nematic liquid crystals exhibit a wide range of exotic phenomena that cannot be observed in equilibrium systems. However, due to their complex non-equilibrium nature, the theoretical tools available to probe active nematics are quite limited. In order to better understand these systems, we use a tensor form of "Sparse Identification of Nonlinear Dynamics" (SINDy), which is a regression technique that takes raw data as an input and returns a parsimonious governing partial differential equation. Using tensor SINDy greatly reduces the complexity of the input library and makes it simple to include terms up to a desired order in the Q, velocity, and density fields. Furthermore, the output is more easily interpretable to humans. In addition to introducing the tensor SINDy framework, I will discuss results that we have obtained by applying tensor SINDy to particle-based dry active nematic simulations, where we control the microscopic parameters of the system. Using SINDy, we are able to connect the emergent hydrodynamic equations to the microscopic properties of our model system. |
Monday, March 6, 2023 8:24AM - 8:36AM Author not Attending |
A09.00003: Cooperative transport in a swarm of aligning active particles Matan Yah Ben Zion From humans hauling an oversized sofa to ants foraging a large leaf, cooperative transport can be found across the animal kingdom. Yet, achieving cooperative transport by design remains a great challenge for physicists, mechanical engineers, and roboticists. In my talk, I will present the ``transporton'', a new kind of self-propelled particle whose mechanical design gives rise to a novel dynamical response. When subjected to an external force, a transporton aligns and propagates in a direction opposite to the force. This unique force response allows these active particles to spontaneously coordinate transport of much larger objects, alleviating the dependence on a complex circuitry, sensors, or communication. Using both experiments and simulations, we show that the transport propensity of a swarm of transportons is super-linear with group size, a hallmark of cooperation. Finally, we derive an analytical model for the interaction of a transporton with an obstacle, revealing the role of the different parameters at play. Our results both shed light on the mechanical origin of cooperative transport observed in nature, as well as offer engineers a generic design rule for future applications in swarm robotics. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A09.00004: Geometrical and chemical sensing of an active droplet Shiva Dixit A self-propelled droplet of 1-pentanol was found to sense its geometrical and chemical asymmetry in its environment. Experimentally, it is found that in its environment, the drop was able to sense geometrical asymmetry with 80% probability and chemical asymmetry with 100% probability. In our liquid-on-a-liquid system, the main reason behind this geometrical sensing is the unbalanced forces induced because of the geometrical imbalance in the environment while in chemical asymmetry, a fixed drop of 1 pentanol renders asymmetry in the concentration profile in the surroundings. For a better physical understanding of a drop in a Y-shaped channel, we developed a minimal numerical model considering suitable Marangoni forces caused by the surface tension gradient and Yukawa-type interaction. The numerical model and statistical analysis corroborated the experimental findings qualitatively as well as quantitatively. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A09.00005: The Mechanical Theory of Active Crystallization Daniel Evans, Ahmad K Omar The equilibrium crystallization transition of hard spheres is among the most familiar examples of an entropically-driven order-disorder phase transition. Recently, it has been shown that crystallization can also be driven by activity in models of active Brownian hard spheres. The breaking of detailed balance, and the resulting absence of Boltzmann statistics, motivates the development of a theory of symmetry-breaking transitions that is independent of the underlying distribution of microstates. Here, we develop such a theory, leveraging mechanics, conservation laws, and symmetries to describe crystal-fluid coexistence in both equilibrium and nonequilibrium systems. We apply our framework to active crystallization, deriving the coexistence criteria for active crystal-fluid coexistence in terms of bulk equations of state. We perform particle-based simulations to obtain these equations of state, allowing for a complete description of the phase diagram of active Brownian hard spheres. Our predicted phase diagram quantitatively recapitulates the crystal-fluid coexistence curve as well as other key features of active phase behavior, including the triple point. Our findings offer a concrete path forward towards the development of a general theory for coexistence when both conserved and nonconserved order parameters are coupled. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A09.00006: Non-symmetric pinning of topological defects in Living Liquid Crystals Nuris Figueroa Morales, Mikhail Genkin, Andrey Sokolov, Igor Aranson Topological defects, such as vortices and disclinations, play a crucial role in spatiotemporal organization of equilibrium and non-equilibrium systems. The defect immobilization or pinning is a formidable challenge in the context of the out-of-equilibrium system, like a living liquid crystal, a suspension of swimming bacteria in lyotropic liquid crystal. Here we control the emerged topological defects in a living liquid crystal by arrays of 3D printed microscopic obstacles (pillars). Our studies show that while −1/2 defects may be easily immobilized by the pillars, +1/2 defects remain motile. Due to attraction between oppositely charged defects, positive defects remain in the vicinity of pinned negative defects, and the diffusivity of positive defects is significantly reduced. Experimental findings are rationalized by computational modeling of living liquid crystals. Our results provide insight into the engineering of active systems via targeted immobilization of topological defects. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A09.00007: Modeling Active Composites Layne B Frechette Soft materials embedded in an active fluid exhibit emergent dynamics and self-organization unseen in their passive counterparts. However, the behaviors of these "active composites" result from processes acting over a wide range of length and time scales and are hence challenging to understand. We create multiscale models of such active composites and use theory and computer simulations to explore their behaviors. Our framework sheds light on pattern formation in active composites and may inform efforts to design and control such materials. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A09.00008: Optimal control of active nematics in bulk and confined geometries. SAPTORSHI GHOSH Being intrinsically nonequilibrium, active materials have control to perform functions that would be thermodynamically forbidden in passive materials. However, active systems |
Monday, March 6, 2023 9:36AM - 9:48AM |
A09.00009: Do active liquid crystals exist after all? Luca Giomi, Livio N Carenza, Dimitrios Krommydas, Josep-Maria Armengol-Collado Quasi-long ranged order is the hallmark of two-dimensional liquid crystals. At equilibrium, this property implies that the correlation function of the local orientational order parameter decays with distance as a power law: i.e. C(r) ∼ |r|–η, with η a temperature-dependent exponent. While in general non-universal, η = 1/4 universally at the Kosterlitz-Thouless transition, where orientational order is lost due to disclination unbinding. Does this definition of liquid crystal order in two dimensions also apply to active liquid crystals? And if not, are these more or less ordered than their passive counterpart? In this talk, I will share the outcome of a survey of experimental data and I will discuss it in the light of recent numerical simulations and analytical work. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A09.00010: Mucus viscoelasticity controls bacterial active matter correlations Wentian Liao, Igor S Aronson Active matter is the system of self-propelled particles converting the chemical energy of a nutrient into mechanical motion. Bacteria swimming in biological fluids is a simple representative of active matter. Collective motion and swarming emerge in concentrated bacteria suspension. While the emergence, mechanism, and physical properties of the collective motion of bacteria swarming in Newtonian fluid have been intensively studied, many fundamental questions in the case of complex non-Newtonian fluid, such as viscoelastic mucus, are unexplored. Here, we probe the physical properties of the collective motion of bacteria in viscoelastic mucus. Velocity temporal correlation time and spatial correlation length were studied to characterize the influence of the complex non-Newtonian response of mucus on the collective motion of bacteria. Our study shows that the collective motion's scale increases with the viscoelasticity of the mucus. Our results imply the possibility of manipulation of bacterial active matter through the solution's viscoelasticity. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A09.00011: Smarticles 2.0 – Robotic Modules Designed for 3D Entanglement Danna Ma Smart robotic matter consists of aggregates of programmable modules able to actuate, sense, and respond intelligently to environment. Design challenges involve scalable fabrication and operation, and coordination algorithms for reliable autonomy. We present a new ‘Smarticles’ iteration capable of unprecedented 3D entanglement [1]. We discuss the design which supports low cost, low weight, low barrier-of-entry, and ease of operation. We characterize the platform in terms of actuation repeatability and longevity, lifting and holding strength, sensing modalities, and battery life. Finally, we demonstrate tactile and acoustic coordination, and show exploratory collective behaviors with up to 10 modules, including static entanglement and self-disassembly. Building on this work, we are now performing stress tests of our original platform to enable bigger collectives the future. We are also comparing emergent behaviours of this physically entangled collective with predictions in simulation [2]. We look forward to use Smarticle 2.0 to study global emergent behaviours that arise from local individual interactions. We expect this robophysical platform will help promote insights on materials and biological swarms, as well as taskable robotic systems that can change shape and properties. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A09.00012: Using Active Matter to Model Different Types of Epidemic Behavior Cynthia Reichhardt, Peter Forgacs, Andras Libal, Nick Hengartner, Charles M Reichhardt Active matter describes self motile systems including particle based systems that undergo run-and-tumble or driven diffusive motion. Here we show how particle-based active matter systems can be used to model and tune between Susceptible-Infected (SI) and Susceptible-Infected-Recovered (SIR) regimes. The motility-induced phase separation that occurs in active matter permits the easy introduction of spatial heterogeneity, quenched disorder, and mobility changes among the active agents, which can infect each other with some probability upon contact. When we encode the standard SIR model into active particles interacting with quenched disorder, we show when the infection rate is low and the spread of the infection is heterogeneous, the quenched disorder has a strong impact on the epidemic, whereas when the infection rate is high, the impact of the quenched disorder is reduced and the epidemic spreads via well defined waves [1]. For the case where spontaneous recovery is absent, as in the zombie outbreak model [2], we show that by introducing two species of susceptible agents and giving only one species the ability to cure the zombies, it is possible to induce a novel tunable transition between SI and SIR behavior. This type of model could address situations such as HIV in which there are limited resources for reducing infection. We discuss how active matter systems could be used to produce table top epidemic model experiments. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A09.00013: Topological Entropy in Simulations of Active Nematics Md Mainul Hasan Sabbir, Kevin A Mitchell Microtubule-kinesin-based active nematic is a non-equilibrium system composed of rod-like subunits that can generate self-driven flows. The spontaneous emergence of the topological defects occurs due to the active flow, and the movement of defects generate a complex braiding pattern. Experimental observation suggests that the local fluid stretching, and global mixing can be quantified by Lyapunov exponent, and topological entropy. However, there has been no systematic study to verify experimental observations using theory. In this study, we simulate nematohydrodynamics equation with periodic boundary conditions. Using the simulation data, we compute topological entropy from defect braiding as well as curve extension rates. Then from the microtubule velocity field, we compute Lyapunov exponent to quantify local fluid stretching. This study represents theoretical verification of topological chaos in active nematics. |
Monday, March 6, 2023 10:36AM - 10:48AM |
A09.00014: Stiffening of an active solid Mario Sandoval This work deals with the mechanical properties of an active elastic solid defined as a two-dimensional network of active stochastic particles interacting by nonlinear hard springs. It is numerically found that when activity in the system is turned on, the active solid stiffens. Interestingly, the active forces individually acting along the solid are stochastic; thus no preferred direction is imposed. This effect could be potentially used to construct novel active materials whose mechanical properties could be tuned according to their needs. Additionally, a collective behavior and density fluctuation analysis to the active solid in the absence of an external stress, is also carried out. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A09.00015: Nonlinear plastic modes reveal defects in solids with pressure gradients Julia A Giannini, Edan Lerner, M Lisa L Manning Glasses are characterized by structural disorder and unique thermal, mechanical, and dynamical behavior. Several studies have shown that these features are governed by populations of defects in the system's disordered microstructure. In contrast to crystalline defects, glassy defects are difficult to identify from local structural information. This challenge is further complicated in active disordered solids, such as human crowds, cell collectives, and active colloids, which have internally generated stresses, strong gradients in pressure, and noisy dynamics. In recent work, we examined the structure and dynamics of a simulated active particle model for human crowds, where stable disordered reference configurations with pressure gradients are formed in the limit of persistent self-propulsion. We discovered that normal mode analysis, derived from a harmonic approximation of the system's potential energy, is not sufficient to predict rearrangements in dense regions of these packings, in contrast to previous findings. Here, we utilize anharmonic approximations of the potential energy to identify populations of defects in a similar class of particle packings. Further, we seek to compare the capability of harmonic and anharmonic structural predictors in forecasting plastic motion in disordered solids with global pressure gradients. The methods we develop in this work may be generalizable to predict dynamics in more complex non-hamiltonian active materials. |
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