Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session L31: Biological Fluid Dynamics: Micro-swimmer Experimental |
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Chair: Nuris Figueroa-Morales, Pennsylvania State University Room: 613 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L31.00001: Bacterial ridesharing: swimming and rolling of a sessile-motile aggregate Bin Liu, Yu Zeng While motile microorganisms disperse actively in aqueous environments by exploiting locomotory organelles, sessile cells are deficient in locomotory organelles and disperse passively through flow entrainment. Beyond these two discrete classes of motility, we explore an assembly containing both motile and sessile cells, a rosette aggregate of \textit{Caulobacter crescentus}. Despite its predominantly sessile cells, the \textit{C. crescentus} rosette exhibits surprisingly active motility, powered by as little as a single flagellar motor. In addition, proximity to a solid surface promotes rolling movements along the solid-liquid interface. This rolling mechanism emerges from a division of labor between sessile and motile compartments that respectively function as structural and powering modules, which can be extended to a wide range of natural and engineered microbial systems. [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L31.00002: Navigating through complex networks by sniffing gradients: diffusiophoresis vs. chemotaxis Jinzi Mac Huang, Tanvi Gandhi, Antoine Aubret, Desmond Li, Sophie Ramananarivo, Massimo Vergassola, Jeremie Palacci At the beginning of life, searching for food and evading hazards are two essential activities for microorganisms to survive, and the way they navigate is through chemotaxis. The optimal chemotaxis in complicated terrains determines the fate of living creatures, and natural selection ensures the existence of such an optimization. In our study, we investigate the navigation of inert particles in a network that has multiple junctions. In micro-networks manufactured through photolithography, a background gradient of salt is established as the signal of chemoattractant by placing a source and a sink of salt. Colloidal particles then follow this signal through diffusiophoresis and move towards the source. Through stochastic modeling, we show that particles prefer to exit each junction at the end with higher concentration gradient. This preference is further enhanced when the particle size is larger, which leads to a way to magnify small signals in a network so that the colloidal particles larger than a critical size can always move towards the source of salt through the shortest path. Ultimately, we compare the navigation schemes of inert particles and living organisms, aiming to understand biological chemotaxis and shed light on future manufacturing of navigable microswimmers. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L31.00003: Alleviation of hypoxia by biologically generated mixing from aggregations of centimeter-scale swimmers Isabel Houghton, John Dabiri Daily vertical migrations of zooplankton have been shown to affect nutrient distributions and dissolved gas concentrations in ocean and lake environments. Additionally, laboratory experiments have demonstrated the potential for mixing generated by these migrations to alter the physical structure of a water column by mixing different density water. In this work, we investigate the importance of biologically generated mixing relative to other processes in determining the biogeochemical structure of a water column inhabited by migrating zooplankton. Specifically, we consider oxygen, a highly ecologically relevant scalar, and the competition between metabolic consumption and biogenic mixing in a stably stratified water column with a hypoxic layer. We illustrate the potential for migrating animals to alleviate hypoxia, introducing complex feedbacks between the presence of animals and the biogeochemical state of their surroundings. Furthermore, we demonstrate the feasibility of oxygen as a potential indicator of biogenic mixing for future in situ investigation given its low diffusivity and higher signal-to-noise. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L31.00004: Surfing the Wave: How Bacteria Migrate through Porous Media Tapomoy Bhattacharjee, Daniel Amchin, Felix Kratz, Sujit Datta While bacterial motility is well-studied for motion on flat surfaces or in unconfined liquid media, most bacteria are found in heterogeneous porous media, such as biological gels and tissues, soils, and sediments. However, it is unknown how pore-scale confinement alters bacterial motility, because the opacity of typical 3D media precludes direct visualization. Here, we reveal that the paradigm of run-and-tumble motility is dramatically altered in a porous medium. By directly visualizing individual \textit{E. coli}, we find a new form of motility in which cells are intermittently and transiently trapped as they navigate the pore space; analysis of these dynamics enables prediction of single-cell transport over large length and time scales. Moreover, we show that concentrated populations can collectively migrate through a porous medium—despite being strongly confined—by chemotactically “surfing” a self-generated nutrient gradient. This behavior depends sensitively on pore-scale confinement, initial colony density, and nutrient consumption, providing a means to control collective migration in bacterial populations. Our work provides a revised picture of active matter transport in complex media, with implications for healthcare, agriculture, and bioremediation. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L31.00005: Controlling topological defects in Living Liquid Crystals Nuris Figueroa-Morales, Andrey Sokolov, Mikhail M. Genkin, Igor S. Aranson A realization of active nematics has been conceived by combining swimming bacteria and a lyotropic liquid crystal. The complex dynamics of such active material arises from the non-trivial interplay between hydrodynamic flows and elastic forces: while bacteria are guided by the local director field, the local alignment of the liquid crystal is disturbed by the swimming bacteria. At high bacterial concentration, the domination of bacterial activity leads to creation of motile topological defects, which alter bacterial distribution. Here, we experimentally explore the possibility of controlling and pinning of emerged topological defects by artificially created microstructures. The microstructures were printed using a state-of-art multiphoton 3D lithography system and mimicked the shape of defects cores. While -1/2 defects may be easily pinned to the created pattern, +1/2 defects remain motile. Due to an attraction between opposite defects, positive defects remain in the vicinity of pined negative defects, significantly diminishing their diffusivity. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L31.00006: Experimental investigation of hydrodynamic interactions between motile green algae Junaid Mehmood, Abel-John Buchner, Koen Muller, Daniel Tam The motility of micro-organisms plays a crucial role in many biological processes, such as reproduction and biofilm formation. Mechanical interactions between swimming cells can lead to collective phenomena, e.g., bio-convention or pattern formation. Hydrodynamic interactions between swimming cells have been the focus of several numerical studies, but experimental evidence of intercellular hydrodynamic interaction remains scarce. Here, we use a unique multi-camera microscopy set-up to track a dilute suspension of the model green alga Chlamydomonas reinhardtii. The cells are free to swim within a flow cell, which does not constrain their dynamics. The resulting three-dimensional trajectories provide data by which to examine pair-wise interactions between the motile cells. Hydrodynamic interactions can lead to a change in direction or velocity magnitude. This information is used to find out the length and time scale for which interaction is occurring between the pair of swimmers. We also study the velocity correlations between all the swimmers to find out the extent of length and time scale associated with these interactions. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L31.00007: Self-organization of active spinners in liquid metamaterials Jean-Baptiste Gorce, Hua Xia, Nicolas Francois, Horst Punzmann, Michael Shats The new concept of liquid metamaterials opened new ways of engineering surface flows with adjustable properties at microscales. The liquid-interface metamaterials are generated by the superposition of two orthogonal standing surface waves. Fluid particles exhibit different type of trajectories depending on the phase shift between the two orthogonal waves. Circular orbits can be created when the two orthogonal waves are phase shifted by 90 degrees. The liquid metamaterials are periodic arrays of unit cells, somewhat analogous to optical lattices. The similarity with optics triggered recent investigation into the trapping of surface magnetic spinners within the liquid metamaterials. A single spinner exhibits stable orbits in a unit cell and can be guided by changing the spinning frequency and the spin direction. The spinner's motion arises from the coupling between the spinner angular momentum and the wave angular momentum. Multiple spinners self-organize into stable configurations around the centre of the unit cells. The results offer novel methods of manipulation and confinement of actively moving surface particles at the fluid interfaces using waves and suggest new analogies between surface wave physics and confinement of nano- and micro-particles and atoms by optical fields. [Preview Abstract] |
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