Suspensions of aerobic bacteria often develop flows from the interplay chemotaxis and buoyancy. We find in sessile drops that related to those Boycott effect sedimentation carry bioconvective plumes down slanted meniscus concentrate cells at drop edge, while pendant such self-concentration occurs bottom. On scales much larger than a cell, concentrated regions both geometries exhibit transient, reconstituting, high-speed jets straddled by vortex streets. A mechanism for large-scale coherence...

Turbulence is ubiquitous, from oceanic currents to small-scale biological and quantum systems. Self-sustained turbulent motion in microbial suspensions presents an intriguing example of collective dynamical behavior among the simplest forms life important for fluid mixing molecular transport on microscale. The mathematical characterization turbulence phenomena active nonequilibrium fluids proves even more difficult than conventional liquids or gases. It not known which features phases living...

Bacterial processes ranging from gene expression to motility and biofilm formation are constantly challenged by internal external noise. While the importance of stochastic fluctuations has been appreciated for chemotaxis, it is currently believed that deterministic long-range fluid dynamical effects govern cell-cell cell-surface scattering - elementary events lead swarming collective swimming in active suspensions biofilms. Here, we report first direct measurements bacterial flow field...

Aerobic bacteria often live in thin fluid layers near solid–air–water contact lines, which the biology of chemotaxis, metabolism, and cell–cell signaling is intimately connected to physics buoyancy, diffusion, mixing. Using geometry a sessile drop, we demonstrate suspensions Bacillus subtilis self-organized generation persistent hydrodynamic vortex that traps cells line. Arising from upward oxygentaxis downward gravitational forcing, these dynamics are related Boycott effect sedimentation...

At concentrations near the maximum allowed by steric repulsion, swimming bacteria form a dynamical state exhibiting extended spatiotemporal coherence. The viscous fluid into which locomotive energy of individual microorganisms is transferred also carries interactions that drive concentration dependence correlations in collective probed here with novel technique herds condensed populations adjustable concentration. For particular thin-film geometry employed, correlation lengths vary smoothly...

Self-sustained turbulent structures have been observed in a wide range of living fluids, yet no quantitative theory exists to explain their properties. We report experiments on active turbulence highly concentrated 3D suspensions Bacillus subtilis and compare them with minimal fourth-order vector-field for incompressible bacterial dynamics. Velocimetry bacteria surrounding fluid, determined by imaging cells tracking colloidal tracers, yields consistent results velocity statistics...

In contexts such as suspension feeding in marine ecologies there is an interplay between Brownian motion of nonmotile particles and their advection by flows from swimming microorganisms. As a laboratory realization, we study passive tracers suspensions eukaryotic swimmers, the alga Chlamydomonas reinhardtii. While cells behave ballistically over short intervals, diffusively, with time-dependent but self-similar probability distribution function displacements consisting Gaussian core robust...

All Together Now (Sometimes) Motile cilia and flagella protrude from the surface of many eukaryotic cells. Understanding how operate is important for understanding ciliated cells in metazoans, ecology behavior motile microorganisms, mechanisms molecular motors signal transduction. Using very-high-speed video microscopy, Polin et al. (p. 487 ; see Perspective by Stocker Durham ) discovered that biflagellated single-cell alga Chlamydomonas rheinhartii switch between synchronous beating, which...

Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of suspensions. While extensively studied theoretically, these have not been measured in detail around any freely-swimming microorganism. We report such measurements for microphytes Volvox carteri Chlamydomonas reinhardtii. The minute ~0.3% density excess V. over water leads to a strongly dominant Stokeslet contribution, with widely-assumed stresslet flow only correction subleading source...

Confining surfaces play crucial roles in dynamics, transport and order many physical systems, but their effects on active matter, a broad class of dynamically self-organizing are poorly understood. We investigate here the influence global confinement surface curvature collective motion by studying flow orientational within small droplets dense bacterial suspension. The competition between radial confinement, self-propulsion, steric interactions hydrodynamics robustly induces an intriguing...

Concentrated suspensions of swimming microorganisms and other forms active matter are known to display complex, self-organized spatio-temporal patterns on scales large compared those the individual motile units. Despite intensive experimental theoretical study, it has remained unclear extent which hydrodynamic flows generated by cells, rather than purely steric interactions between them, drive self-organization. Here we utilize recent discovery a spiral-vortex state in confined \textit{B....

The spherical alga Volvox swims by means of flagella on thousands surface somatic cells. This geometry and its large size make it a model organism for studying the fluid dynamics multicellularity. Remarkably, when two nearby colonies swim close to solid surface, they attract one another can form stable bound states in which ``waltz'' or ``minuet'' around each other. A surface-mediated hydrodynamic attraction combined with lubrication forces between spinning, bottom-heavy explains formation,...

Interactions between swimming cells and surfaces are essential to many microbiological processes, from bacterial biofilm formation human fertilization. However, despite their fundamental importance, relatively little is known about the physical mechanisms that govern scattering of flagellated or ciliated solid surfaces. A more detailed understanding these interactions promises not only new biological insights into structure dynamics flagella cilia but may also lead microfluidic techniques...

A major puzzle in biology is how mammalian sperm maintain the correct swimming direction during various phases of sexual reproduction process. Whilst chemotaxis may dominate near ovum, it unclear which cues guide spermatozoa on their long journey towards egg. Hypothesized mechanisms range from peristaltic pumping to temperature sensing and response fluid flow variations (rheotaxis), but little known quantitatively about them. We report first quantitative study rheotaxis, using microfluidic...

Flows generated by ensembles of flagella are crucial to development, motility and sensing, but the mechanisms behind this striking coordination remain unclear. We present novel experiments in which two micropipette-held somatic cells Volvox carteri, with distinct intrinsic beating frequencies, studied high-speed imaging as a function their separation orientation. Analysis time series shows that interflagellar coupling, constrained lack connections between be hydrodynamical, exhibits spatial...

Vitamin B12 (cobalamin) is a dietary requirement for humans because it an essential cofactor two enzymes, methylmalonyl-CoA mutase and methionine synthase (METH). Land plants fungi neither synthesize or require cobalamin they do not contain mutase, have alternative B12-independent (METE). Within the algal kingdom, approximately half of all microalgal species need vitamin as growth supplement, but there no phylogenetic relationship between these species, suggesting that auxotrophy arose...

Dense suspensions of swimming bacteria are known to exhibit collective behaviour arising from the interplay steric and hydrodynamic interactions. Unconfined transient, recurring vortices jets, whereas those confinedin circular domains may order in form a spiral vortex. Here we show that confinement into long narrow macroscopic `racetrack' geometry stabilises bacterial motion steady unidirectional circulation. This is reproduced simulations discrete swimmers reveal crucial role...

In the past decade volvocine green algae, spanning from unicellular $Chlamydomonas$ to multicellular $Volvox$, have emerged as model organisms for a number of problems in biological fluid dynamics. These include flagellar propulsion, nutrient uptake by swimming organisms, hydrodynamic interactions mediated walls, collective dynamics and transport within suspensions microswimmers, mechanism phototaxis, stochastic synchronization. Green algae are well suited study such because their range...

Significance In areas as diverse developmental biology, physiology, and biomimetics, there is great interest in understanding the mechanisms by which active hair-like cellular appendages known flagella or cilia are brought into coordinated motion. The prevailing theoretical hypothesis over many years that fluid flows driven beating provide coupling leads to synchronization, but this surprisingly inconsistent with certain experimentally observed phenomena. Here we demonstrate insufficiency of...

The Korteweg--de Vries, modified and Harry Dym hierarchies of integrable systems are shown to be equivalent a hierarchy chiral shape dynamics closed curves in the plane. These purely local conserve an infinite number global geometric properties curves, such as perimeter enclosed area. Several techniques used study these have simple differential-geometric interpretations. Connections with incompressible, inviscid fluid flow two dimensions suggested.

A stiff one-armed swimmer in glycerine goes nowhere, but if its arm is elastic, exerting a restorative torque proportional to local curvature, the can go on way. Considering this happy consequence and principles of elasticity, we study hyperdiffusion equation for shape elastica viscous flow, find solutions impulsive or oscillatory forcing, elucidate relevant aspects propulsion. These results have application variety physical biological contexts, from dynamic biopolymer bending experiments...