Emergent behavior across scales: locomotion, mixing, and collective motion in active swimmers

Tuesday, June 15 at 05:45pm (PDT)
Wednesday, June 16 at 01:45am (BST)
Wednesday, June 16 09:45am (KST)

SMB2021 SMB2021 Follow Tuesday (Wednesday) during the "MS09" time block.
Note: this minisymposia has multiple sessions. The second session is MS15-MFBM (click here).

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Robert Guy (University of California Davis, United States), Arvind Gopinath (University of California Merced, United States)


This minisymposium focuses on emergent and novel multi-scale behavior in active swimmer systems. The first part of the minisymposium focuses on the locomotion of single swimmers. Many living micro-organisms move by coordinated movements of flagella. While some artificial microswimmers mimic these movements, alternative designs that leverage instabilities of the ambient media are possible. Synchronous flagellar beats result from the coupling between flagella elasticity, ambient fluid properties and internal motor driven activity. Synthetic swimmers may similarly be realized by coupling external driving forces to body elasticity and fluid rheology. Research is presented on how fluid rheology enables flagellar beats, novel swimming strategies that exploit symmetry breaking, and methods for studying swimmers. The second part of the mini-symposium features talks on the collective behavior of microswimmers and associated mixing flows. Mixing fluids at small scales is challenging given the lack of inertia, yet mixing is needed in many microfluidic settings. Research is presented on sorting (unmixing) in bacterial suspensions, mixing flows originating from microorganism interactions or instabilities in complex fluids, and on chemical reactions related to the emergence of life in microfluidic experiments.

Henry Fu

(University of Utah, United States)
"Symmetry breaking propulsion of magnetically rotated spheres in nonlinearly viscoelastic fluids"
Symmetries have long been used to understand when propulsion is possible in microscale systems. Currently, artificially propelled magnetic micro- and nanoparticles are being utilized in a variety of techniques including hyperthermia, drug delivery, and magnetic resonance imaging. Rotation of rigid magnetic particles by an external magnetic field is a promising category of such artificial propulsion. Propulsion would seem to be prohibited by geometries with fore-aft symmetry along their rotation axis, such as a rotating sphere. We have shown that in nonlinearly viscoelastic fluids, a symmetry breaking propulsion is possible for rotating microspheres. We show that this propulsion occurs in both mucin and polyacrylamide solutions, and propose that it results from rod-climbing-like effects which squeeze the sphere and reinforce its translation. A perturbative analysis of the forces on a rotating sphere in a nonlinear polymeric fluid corroborates this mechanism.

Kathryn Link

(University of California Davis, United States)
"Emergent Properties of Flagellar Waveforms in Viscoelastic Fluids"
Eukaryotic cells move in rheologically complex environments via deformations of their flagella, which are slender threadlike structures that are powered by internal molecular motors. It is an ongoing scientific pursuit to determine how flagellar beat emerges from the coordination of the mechanics of the flagella, the interactions with the external fluid environment, and the mechano-chemical feedback of the molecular motors. Existing theories have shed light on the origins of this behavior in a viscous fluid, however, due to the inherent nonlinearity and mathematical complexity involved in modeling viscoelastic fluids, both analytical and numerical predictions require nonstandard approaches. In this work we propose an extension to the current models to make a prediction about how viscoelasticity changes the beat frequency of the emergent waveform.

Rudi Schuech

(Tulane University, United States)
"Viscoelastic Network Remodeling by Microswimmers"
Microorganisms often navigate a complex environment composed of a viscous fluid with suspended microstructures such as elastic polymers and filamentous networks. These microstructures can have similar length scales to the microorganisms, leading to complex swimming dynamics. Some microorganisms are known to remodel the viscoelastic networks they move through. In order to gain insight into the coupling between swimming dynamics and network remodeling, we use a regularized Stokeslet boundary element method to compute the motion of a microswimmer consisting of a spherical body and rotating helical flagellum. The viscoelastic network is represented by a cloud of points with virtual Maxwell element links. We consider two models of network remodeling in which (1) links break based on their distance to the microswimmer body, modeling enzymatic dissolution by bacteria or microrobots, or (2) links break based on a threshold tension force. We compare the swimming performance of the microbes in each remodeling paradigm as they penetrate and move through the network.

Sookkyung Lim

(University of Cincinnati, United States)
"Simulations of microswimmers propelled by multiple flagella"
Peritrichously flagellated bacteria swim in a fluid environment by rotating motors embedded in the cell membrane and consequently rotating multiple helical flagella. We present a novel mathematical model of a microswimmer that can freely run propelled by a flagellar bundle and tumble upon motor reversals. Our cell model is composed of a rod-shaped rigid cell body and multiple flagella randomly distributed over the cell body. These flagella can go through polymorphic transformations. We demonstrate that flagellar bundling is influenced by flagellar distribution and hence the number of flagella. Moreover, reorientation of cells is affected by the number of flagella, how many flagella change their polymorphisms within a cell, the tumble timing, different combinations of polymorphic sequences, and random motor reversals. Our mathematical method can be applied to numerous types of microorganisms and may help to understand their characteristic swimming mechanisms.

Hosted by SMB2021 Follow
Virtual conference of the Society for Mathematical Biology, 2021.