Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session T06: Low Reynolds Number Locomotion: General |
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Chair: Henry Fu, University of Utah Room: 133 |
Monday, November 21, 2022 4:10PM - 4:23PM |
T06.00001: Implications of flow field modes for transport by swimming microalgae Jeffrey Guasto, Richard J Henshaw, Douglas R Brumley Flagellar and ciliary propulsion are ubiquitous among swimming microorganisms, whose resultant flow fields dictate the organisms’ interactions with the physical environment, including resource uptake, predation, and interactions with particulates. While theoretical models of microbial swimming abound, compact comparisons with experiments are lacking and the essential flow field features that determine transport remain unresolved. Here, we use proper orthogonal decomposition (POD), in conjunction with micro-PIV measurements, to examine flagellar-generated flow fields of individual somatic cells and colonies of the model microalga Volvox carteri. Hierarchical POD modes of flow fields from individual cells reveal a striking similarity to a multipole expansion of the Stokes equations. For cell colonies, modal analysis provides a compact description of the complex spatio-temporal flow fields resulting from metachronal waves. Flow fields reconstituted from POD modes are used to computationally quantify uptake efficiency, including the implications of swimming modes and contrasting symplectic and antipletic waves. |
Monday, November 21, 2022 4:23PM - 4:36PM |
T06.00002: Active particles crossing a sharp viscosity gradient Jiahao Gong Swimming microorganisms and other active particles often move through and even exploit complex and inhomogeneous environments. These active particles tend to perform taxis in such environments by reorienting to move up or down gradients in (chemical or material) properties of their environments. Recent experiments show that the alga Chlamydomonas reinhardtii displays complex refraction and scattering behavior when encountering sharp viscosity gradients. Motivated by these experiments, we have modeled an active particle as a spherical squirmer and analyzed the motion of it near and across similar sharp viscosity gradients. We find these active particles will either cross the interface or scatter off it in accordance with the experiments. Scattering only happens when the initial orientation relative to the interface normal is large and the particle is crossing from low to high viscosity. Otherwise, the particle just crosses the interface by undergoing some reorientation. The law governing the reorientation of neutral swimmers resembles ray optics, while the reorientation process of the pusher/puller is qualitatively similar. |
Monday, November 21, 2022 4:36PM - 4:49PM |
T06.00003: Taxis in density gradients Vaseem A Shaik, Gwynn J Elfring Microorganisms living in oceans, lakes, or ponds (e.g., planktonic organisms) often move through density gradients caused by spatially varying temperature or salinity. These 'active particles' usually perform vertical migration in search of food and this motivated the earlier researchers to address the influence of vertical gradients on the vertical motion of particles. Here we discuss our analysis on the effect of density gradients on particle reorientation which, among other things, determines whether vertical migration is stable. Our analysis reveals that active squirmer-type particles reorient to swim either vertically or horizontally in vertical density gradients. Pushers like E. coli reorient to swim horizontally, whereas pullers, like Chlamydomonas reinhardtii, reorient to swim vertically but up vs down gradients depends on their initial orientation. These results point towards applications of active matter control through prescribed density gradients. |
Monday, November 21, 2022 4:49PM - 5:02PM |
T06.00004: Elastic paddle design with three-dimensional reconfiguration for drag-based underwater locomotion Minho Song, Junkyu Ham, Daegyoum Kim Manipulating the surface area of a paddle during the power stroke and recovery stroke is one of the locomotive strategies to enhance cyclic thrust in drag-based propulsion. Motivated by the unique shape and motion of the arms of a crinoid living in the ocean, the design of an elastic paddle consisting of multiple side flaps is proposed. The array of side flaps along the longitude of the paddle is allowed to undergo uni-directional deflection, resulting in a significant difference in the projection area of the paddle between the power and recovery strokes, which is beneficial to enhancing net thrust. To characterize the propulsive mechanism of the elastic paddle, its three-dimensional reconfiguration by fluid flow and drag generation is investigated using experimental measurements and theoretical analysis, and governing model parameters are derived. |
Monday, November 21, 2022 5:02PM - 5:15PM |
T06.00005: A hybrid Eulerian-Lagrangian algorithm for soft slender structures immersed in viscous flows Mattia Gazzola, Yashraj R Bhosale Structures encountered in biological and robotic domains are often constituted of slender elastic elements that are distributed, heterogeneous, and hierarchically organized. Their interaction with surrounding fluids is often tedious and computationally expensive to resolve. Here we mitigate these issues via a hybrid Eulerian-Lagrangian algorithm that combines Cosserat rod theory and remeshed vortex methods. The resulting elastohydrodynamic solver is tested against a battery of benchmarks, and further extended to the context of active swimmers, multi-body contact, magnetic actuation, and viscous streaming. |
Monday, November 21, 2022 5:15PM - 5:28PM |
T06.00006: Gait switching and targeted navigation of microswimmers via deep reinforcement learning Zonghao Zou, Yuexin Liu, Yuan-Nan Young, On Shun Pak, Alan Cheng Hou Tsang Swimming microorganisms switch between locomotory gaits to enable complex navigation strategies such as run-and-tumble to explore their environments and search for specific targets. This ability of targeted navigation via adaptive gait-switching is particularly desirable for the development of smart artificial microswimmers that can perform complex biomedical tasks such as targeted drug delivery and microsurgery in an autonomous manner. Here we use a deep reinforcement learning approach to enable a model microswimmer to self-learn effective locomotory gaits for translation, rotation and combined motions. The Artificial Intelligence (AI) powered swimmer can switch between various locomotory gaits adaptively to navigate towards target locations. The multimodal navigation strategy is reminiscent of gait-switching behaviors adopted by swimming microorganisms. We show that the strategy advised by AI is robust to flow perturbations and versatile in enabling the swimmer to perform complex tasks such as path tracing without being explicitly programmed. Taken together, our results demonstrate the vast potential of these AI-powered swimmers for applications in unpredictable, complex fluid environments. |
Monday, November 21, 2022 5:28PM - 5:41PM |
T06.00007: Numerical investigation of wake dynamics of tail-first "wriggling" swimmers Chhote Lal Shah, Karthick Dhileep, Sridhar Ravi, Sunetra Sarkar Natural invertebrate swimmers, such as mosquito larvae, move tail-first, which is unique among animals. In this study, the relationship between kinematics, stiffness of the body, and the resulting fluid-structure interaction (FSI) on swimming dynamics is investigated using a computational model consisting of a flexible body attached to a rigid head. The body stiffness and angular oscillation amplitude of the head were varied systematically, while the flow profile and forces were analyzed using an in-house two-way coupled IBM-based FSI solver. We found that the combination of the low bending rigidity of the body and small amplitude oscillation of the head resulted in anguilliform motion matching previous observations in the literature. However, for cases where bending rigidity of the body was low and pitching amplitude of the head was high, successive vortices are produced, first to one side and then to the other of the mean swimming path. The vortices convected anteriorly (from tail to head), exerting a net downward force. As a result, the net swimming direction of a body was backward, and the resulting kinematics mimicked the wriggling motion profile of mosquito larvae. The findings of this study corroborate the observations and measurements of natural aquatic swimmers. |
Monday, November 21, 2022 5:41PM - 5:54PM |
T06.00008: Generalized squirming motion in a Brinkman medium Devanayagam Palaniappan, Herve Nganguia, On Shun Pak Many microorganisms swim in complex media with porous structures such as biological mucus, soils, and aquifers. The squirmer model has been widely used as a tool to probe how properties of these complex media affect the swimming behavior of microorganisms. Here we generalize previous works by considering the non-axisymmetric squirming motion in a porous medium described by the Brinkman equation. We derive an analytical solution to the flow surrounding the squirmer and characterize its propulsion performance in terms of swimming speed, power dissipation, and efficiency. We draw comparisons with the case of generalized squirming motion in the Stokes limit and discuss the implications of our findings on locomotion in porous media. |
Monday, November 21, 2022 5:54PM - 6:07PM |
T06.00009: Load-dependent resistive-force theory Pyae Hein Htet, Eric Lauga The passive rotation of rigid helical filaments is the strategy employed by flagellated bacteria (and some artificial microswimmers) to swim at low Reynolds numbers. In his classical 1976 paper, Lighthill calculated, for the force-free swimming of a rotating helix with no load attached (e.g. with no cell body), the 'optimal' resistance coefficients that, in a local resistive-force theory, most closely reproduce predictions from the nonlocal slender-body theory. These resistance coefficients have since been used ubiquitously in the literature, regardless of whether the conditions under which they were originally derived hold. Here we revisit the problem in the case where a load is attached to the rotating helical filament. We show that the optimal resistance coefficients depend in fact on the size of the load, and highlight and improve upon the growing inaccuracy of Lighthill's coefficients as the load increases. We also provide a physical explanation for the origin of this surprising load-dependence. |
Monday, November 21, 2022 6:07PM - 6:20PM |
T06.00010: Periodic trajectories of rotating micro-cylinders in a confining geometry Hanliang Guo, Yi Man, Hai Zhu Cylindrical-shaped structures are ubiquitous in nature. Many micro-swimmers such as bacteria and algae utilize the microtubule-based flagella and cilia for locomotion. These cylindrical organelles almost never live in free space, yet their motions in a confining geometry can be counter-intuitive. For example, one of the intriguing (yet classical) results in this regard is that a rotating cylinder next to a plane wall does not generate any net force in Newtonian fluids and therefore does not translate. In this work, we employ both numerical and analytical tools to investigate the motions of micro-cylinders under prescribed torques in a confining geometry. We start by studying the self-induced translations of a single cylinder bounded by a circular confinement. We then show that a cylinder pair can form a variety of periodic trajectories depending on the relative position to the confinement. Potential physical mechanisms will be discussed at the end of the talk. |
Monday, November 21, 2022 6:20PM - 6:33PM |
T06.00011: Control strategies for magnetically driven artificial microswimmers Jake Buzhardt, Phanindra Tallapragada Artificial microswimmers have received significant experimental and theoretical research attention due to their promising potential biomedical applications, such as targeted drug delivery, minimally invasive surgery, and micro-particle manipulation. While various means of propulsion have been considered, swimming bodies driven by externally applied magnetic fields seem particularly promising, as they present the capability of controlling the swimmers remotely. While previous works have controlled such microswimmers using simplified models for control or by aligning the rotation direction of the magnetic field with the desired steering direction, for complex swimming tasks, more advanced control strategies are needed. Here, we consider a micro-robot composed of three rigidly connected spheres, driven by a torque induced through an externally applied magnetic field. Through Stokesian dynamics simulations, we examine model-based and data-driven control strategies for various swimming scenarios, such as path tracking for a single swimmer, manipulation of a passive particle, and control of multiple swimmers using a single, global magnetic field. |
Monday, November 21, 2022 6:33PM - 6:46PM |
T06.00012: Modelling ciliary flows in confined geometries Siluvai Antony Selvan, Linda Blackall, Peter Duck, Draga Pihler-Puzovic, Douglas R Brumley Ciliary flows are typically studied in-vitro using microfluidics. Existing theoretical studies capture the flow generated by collective ciliary motion (including Ramirez-San Juan et al. 2020), but fail to resolve the flow generated by individual cilia, which play a crucial role in mixing and nutrient exchange. We propose a new model based on a point torque (i.e., a rotlet) to capture such flows between two parallel plates. The flow field correction due to the bounded domain is resolved using the method of images and Fourier transforms. We observe that a rotlet captures both near- and far-field features of the flow field generated by the individual cilia as opposed to a point force (i.e., stokeslet). The flow generated by an array of cilia is modelled using a superposition of point torques, a framework which enables us to quantify the effects of hydrodynamics confinement, and is employed to study micromixing and nutrient exchange at the ciliary surface. |
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