Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R03: Collective Behavior and Active Matter II |
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Chair: William Uspal, University of Hawai'i at Manoa Room: Ballroom C |
Monday, November 25, 2024 1:50PM - 2:03PM |
R03.00001: Abstract Withdrawn
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Monday, November 25, 2024 2:03PM - 2:16PM |
R03.00002: Odd Viscodiffusive Fluids Alhad Deshpande, Cory M Hargus, Karthik Shekhar, Kranthi K Mandadapu We present a class of materials termed "odd viscodiffusive fluids," which exhibit odd transport by coupling diffusive fluxes to concentration gradients and reciprocally, stresses to concentration gradients. While other odd transport phenomena, such as odd viscous dissipation and odd diffusion, are observed in chiral active systems, they are limited to two-dimensions. In contrast, we showcase odd viscodiffusive coupling to be an instance of odd transport in three-dimensions. By analyzing microscopic fluctuations in the steady state, we use the recently proposed "flux hypothesis" to develop Green-Kubo and reciprocal relations for the governing transport coefficients. These relations suggest that only parity symmetry, and not time-reversal symmetry, must be broken at the microscopic scale to observe these couplings, allowing for these materials to be either passive or active. We then discuss a few analytically tractable cases that illustrate the nature of viscodiffusive cross-coupling in chiral matter, and enable the experimental measurement of the novel transport coefficients. Finally, we make the case for chiral bacterial suspensions to be odd viscodiffusive fluids, and use our theory to understand prior experimental studies involving bacterial migration in response to shearing flows. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R03.00003: Stability of a passive viscous droplet in a confined active nematic liquid crystal Tanumoy Dhar, Michael J Shelley, David Saintillan The transport and shape deformations of a passive viscous Newtonian droplet immersed in an active nematic liquid crystal under circular confinement are analyzed using a linear stability analysis. We focus on the case of a sharply aligned active nematic in the limit of strong elastic relaxation in two dimensions. Using an active liquid crystal model, we employ the reciprocal theorem for Stokes flow to study the growth of interfacial perturbations as a result of both active and elastic stresses. Instabilities are uncovered in both extensile and contractile systems, for which growth rates are calculated in terms of the dimensionless ratios of active, elastic and capillary stresses, as well as the viscosity ratio between the two fluids. We also extend our theory to analyze the inverse scenario, namely the stability of an active droplet surrounded by a passive fluid layer. The instabilities uncovered here may be relevant to a plethora of biological active systems, from the dynamics of passive droplets in bacterial suspensions to the organization of chromatin compartments in the differentiated nucleus. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R03.00004: Enhancing Physical Realism in Mean-Field Continuum Models through Maximum Capacity Constraints on Concentration Fields in Active Matter Cayce Julian Fylling, Arvind Gopinath, maxime theillard Mean-field continuum models are commonly used to simulate systems of multiple agents in suspension, such as bacteria or engineered micro-machines. Ensuring realistic physical constraints in computational models of concentration fields is essential for accurately representing such systems. We present a novel method to enforce a maximum capacity constraint on a computed concentration field within a specified domain, restricting the concentration values to a physical range. Our approach involves formulating a partial differential equation (PDE) for a concentration correction field that preserves the overall mass while ensuring the concentration remains within the physical range of capacity, and minimizes loss of energy. This projection step is incorporated into a mean-field continuum model constructed from the first three orientational moments of a probability density function that solves the Smoluchowski equation. This method enhances the physical realism of the simulation results, making them more applicable to real-world scenarios. We demonstrate the efficacy of our approach through various applications, highlighting its potential to improve the fidelity of simulations in fields ranging from biology to materials science. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R03.00005: Flow behavior and odd viscosity of a suspension of chiral brownian particles Giuseppe Gonnella, Demian Levis, Giuseppe Negro, Lucio M Carenza, Claudio Caporusso We consider a chiral fluid in two dimensions composed of Brownian disks interacting via a Lennard-Jones potential and a nonconservative transverse force, mimicking colloids spinning at a given rate. The system exhibits a phase separation between a chiral liquid and a dilute gas phase that can be characterized using a thermodynamic framework. We compute the equations of state and show that the surface tension controls interface corrections to the coexisting pressure predicted from the equal-area construction. Transverse forces increase surface tension and generate edge currents at the liquid-gas interface. This induces a complex response to external shear driving, also depending on the intrinsic chirality of the system with respect to the shear direction. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R03.00006: Control of collective effects in chiral active colloidal systems Jaideep Katuri, Navneet Kaur, William E Uspal, Allison Cornelius, Jamel Ali Chiral active systems represent a class of active matter systems where the constituent particles break time reversal symmetry by orbiting or spinning, rather than self-propulsion. Similar to the emergence of flocking states in self-propelled systems, novel collective effects such as active crystallization and circulating clusters have been observed in biological chiral active systems. Here we experimentally study chiral colloidal systems that are driven by external fields. We show that in a system of spinning colloidal magnets driven by a rotational magnetic field the system phase separates into circulating clusters with unidirectional edge flows. The emergent behavior is sensitive to the particle shape and by using particles of different geometries we demonstrate the formation of different collective phases. In addition to the hydrodynamic interactions between spinning particle, we introduce diffusiophoretic interactions by suspending the hematite colloids in H2O2 where they generate phoretic flows in the presence of UV light. The added interfacial flows lead to the formation of bound states between spinning colloids that are stabilized through near-field hydrodynamic and chemical interactions and at a collective level cause a loss in structural cohesion of the circulating clusters. Finally, we use two-photon-polymerization lithography to fabricate geometrically anisotropic colloids that exhibit controllable chiral motion in uniform electric fields, even in the absence of any external rotating field. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R03.00007: Extended Stokesian dynamics for the calculation of active particles dynamics in an arbitrary confined space Yu Kogure, Toshihiro Omori, Takuji Ishikawa The analysis of active particle dynamics in their confined suspensions provides valuable insights into a range of fields, including biology and medical engineering; it would be useful in understanding bacterial distribution in the gut and in optimizing the design of microrobots for use in drug delivery systems. In the present study, a novel methodology combining the Stokesian dynamics method (SDM) and the boundary element method (BEM) was developed for calculating the motion of active particles in a confined boundary. The SDM was adopted to calculate the motion of the particles, while the BEM was integrated into the SDM to calculate the fluid dynamics in the presence of a boundary wall. The accuracy of the developed method was validated and guaranteed by comparing the velocities of an active particle moving near a flat wall obtained by the present method with those obtained only by the BEM. It was also demonstrated that the number of particles that can be handled can be increased from 10 to 100 times that of BEM for the same calculation time, and the calculation time can be reduced by one-tenth for the same number of particles. The proposed method is applied to some problem settings where many active particles swim in a complex closed geometry, which illustrates the strength of the proposed methodology. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R03.00008: Heat-thickening in active particle suspensions Sowmya Kumar, Yikai Zhao, Daniel S Tam The rheology of active suspensions is rich, complex and counter-intuitive due to the presence of self-propelled particles. In our study, the shear viscosity of puller suspensions of several volume fractions has been experimentally measured at different temperatures and shear rates. Firstly, we report that puller suspensions show an order of magnitude increase in viscosity with temperature. Complementing the rheology measurements with microscopic observations, we find that the beating frequency of the propulsion apparatus of single micro swimmers increase with temperature. Since the motility of the microswimmer leads to the addition of stresses into the fluid, also called as 'active' stresses, we are able to use the acquired beating frequency data to explain the surprising effect of temperature on the rheology of the suspension. Secondly, we also observe that puller suspensions are shear-thinning. We find that shear-thinning emerges because the added 'active' stress is independent on the imposed shear rate. Further, a simple model relating the viscosity of the suspension to the motility of the microswimmer satisfactorily describes both features of our experimental data. This work highlights the cruical role of motility in active-suspension rheology. |
Monday, November 25, 2024 3:34PM - 3:47PM |
R03.00009: Collective Dynamics of Self-avoidant, Secreting Particles Sang-Eun Lee, Ricardo Cortez, Lisa J Fauci Motivated by autophoretic droplet swimmers that move in response to a self-produced chemical gradient, here we examine the collective dynamics of individual motile agents using a reaction-diffusion system. The agents have an unlimited supply of a chemical, secrete it at a given rate, but are anti-chemotactic so move at a given speed in the direction of maximal decrease of this chemical. In both one- and two-dimensional periodic domains, we find intriguing long-time behavior of the system. Depending upon a non-dimensional parameter that involves secretion rate, agent velocity, domain size and diffusion, we find that the position of the agents either relax to regularly spaced arrays, approach these regular arrays with damped oscillation, or exhibit undamped, periodic trajectories. We examine the progression of particles that are initially seeded randomly, and we also examine the stability of the steady and periodic states. In addition, we present results when these agents are embedded in an incompressible fluid, thus adding advection of the chemical field to the dynamics of this complex system. |
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