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

Wednesday, June 16 at 05:45pm (PDT)
Thursday, June 17 at 01:45am (BST)
Thursday, June 17 09:45am (KST)

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

Maxime Theillard

(University of California Merced, United States)
"Multi-scale multi-species modeling of emergent flows and active mixing in confined bacterial swarms"
Autonomous collective motion of disparate agents in nonequilibrium is fundamental to many biological and engineering systems. An example from biology is bacterial swarms, that are prototypical dense multi-phase active fluids. Here we present a new method for modeling such fluids under confinement. We use a continuum multiscale mean-field approach to represent each specie by its first three orientational moments, and couple their evolution with those of the suspending fluid. The resulting coupled system is solved using parallel hybrid level-set based discretization on adaptive cartesian grids for high computational efficiency and maximal flexibility in the confinement geometry. Motivated by recent experimental work, we employ our method to study emergent flows in bacterial swarms. Our computational exploration demonstrate that we can reproduce the observed emergent collective patterns including active dissolution. This work lays the foundation for a systematic characterization of natural and synthetic systems such as bacterial colonies, bird flocks, fish schools, colloidal swimmers, or programmable active matter.

Paulo Arratia

(University of Pennsylvania, United States)
"Bacteria hinder stretching and large-scale transport in time-periodic flows"
In this talk, I will show recent experiments on the mixing of a passive scalar (dye) in dilute suspensions of swimming textit{Escherichia coli} in time-periodic flows. Results show that the presence of bacteria hinders large scale transport and reduce overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.

Bin Liu

(University of California Merced, United States)
"Anomalous size-dependent active transport in structured environments"
Variations of transport efficiency in structured environments between distinct individuals in actively self-propelled systems is both hard to study and poorly understood. Here, we study the transport of a non-tumbling Escherichia coli strain, an active-matter archetype with intrinsic size variation but fairly uniform speed, through a periodic pillar array. We show that long-term transport switches from a trapping dominated state for shorter cells to a much more dispersive state for longer cells above a critical bacterial size set by the pillar array geometry. Using a combination of experiments and modeling, we show that this anomalous size-dependence arises from an enhancement of the escape rate from trapping for longer cells caused by nearby pillars. Our results show that geometric effects can lead to size being a sensitive tuning knob for transport in structured environments, with implications in general for active matter systems and, in particular, for the morphological adaptation of bacteria to structured habitats, spatial structuring of communities and for anti-biofouling materials design.

Nick Cogan

(Florida State University, United States)
"Modeling the Origin of Life Reaction in Microfluidic Chambers"
The origins of life are rooted in the organization from small molecules to larger molecules into self-assemblies. This organization requires energetic input that appears to have been driven by temperature and pressure differentials near hydrothermic vents. It has been hypothesized that the building blocks of life originated at the crossroads of high temperature water exacting into the oceans via these vents. Many different chemical reactions have been proposed to study the dynamics of self assemblies across steep chemical gradients. In our study, we focus on the development of a solid membrane via a simplified chemical precipitate reaction. The aims are to understand the physical interaction between the precipitating solid and the fluid dynamics as the membrane barrier is formed. Mathematically, we use a multiphase framework that is highly customizable and addresses the transitions between solids and liquids in a variety of settings. We introduce a slight change in the standard formulation and show that this model is compatible with Darcys’ law and standard porous media equations in different limits. We also provide numerical and linearized results indicating the affect of a developing solid within a flowing liquid.

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