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

Vast numbers of microorganisms are suspended in temperate aqueous environments. Oceans and rivers, puddles and droplets, the fluid interiors of animals, all host an array of splendidly varied creatures. Although their presence is usually not casua1\y obvious, they constitute the major part of the world's biomass. Their population dynamics-replication and decay, accumulation and dispersal-modulates and regulates their own life, the life of the larger creatures that feed on them, and even the climate (Charlson et al 1987). Microorganisms interact with each other and with the world, at length scales that vary upward from the size of an individual, say 10-4 cm, to the dimensions of the entire body of fl uid in which they live. Fluid mechanics, in concert with the organisms' behavior, governs the dynamics of many of these interactions. However, we are concerned here with relatively localized, small-scale phenomena. We consider only single celled microorganisms which are motile, i.e. self-propelled, and so small that inertial effects can be ignored in describing their locomotion. Examples to be discussed include species of algae, bacteria, and protozoa, not to mention spermatozoa. The trajectories along which individual cells swim are determined by thc

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 the coherence. The concentration dependence of correlations in the collective state is probed here with a novel technique that herds bacteria into condensed populations of adjustable concentration. For the particular thin-film geometry employed, the correlation lengths vary smoothly and monotonically through the transition from individual to collective behavior.

Aerobic bacteria often live in thin fluid layers near solid-air-water contact lines, in which the biology of chemotaxis, metabolism, and cell-cell signaling is intimately connected to the physics of buoyancy, diffusion, and mixing. Using the geometry of a sessile drop, we demonstrate in suspensions of Bacillus subtilis the self-organized generation of a persistent hydrodynamic vortex that traps cells near the contact line. Arising from upward oxygentaxis and downward gravitational forcing, these dynamics are related to the Boycott effect in sedimentation and are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics. The vortex is shown to advectively enhance uptake of oxygen into the suspension, and the wedge geometry leads to a singularity in the chemotactic dynamics near the contact line.bioconvection ͉ chemotaxis ͉ singularity ͉ Bacillus subtilis T he interplay of chemotaxis and diffusion of nutrients or signaling chemicals in bacterial suspensions can produce a variety of structures with locally high concentrations of cells, including phyllotactic patterns (1), filaments (2), and concentrations in fabricated microstructures (3). Less well explored are situations in which concentrating hydrodynamic f lows actually arise from these ingredients. Here we report a detailed experimental and theoretical study of an intriguing mechanism termed the ''chemotactic Boycott effect.'' Described brief ly before (4), it is intimately associated with buoyancy-driven f lows, metabolite diffusion, and slanted air-water menisci. The ubiquity of contact lines and their transport singularities (5) suggest importance of these observations in biofilm formation (6). The large-scale stirring created by these f lows illustrate important advective contributions to intercellular signaling, as in quorum sensing (7).The chemotactic Boycott effect takes its name from a phenomenon in sedimentation (8) that occurs when the chamber containing a fluid with settling particles is tilted from vertical. Settling depletes the fluid near the upper wall, making it buoyant relative to nearby fluid, whereupon it rises. This boundary flow stirs up the entire medium, greatly accelerating the settling process. In the chemotactic version, negatively buoyant aerobic bacteria swim up to the free surface of a sessile drop and slide down the slanted meniscus, producing high concentrations of cells near the three-phase contact line. In earlier work where this was observed (4), the detailed nature of hydrodynamic flows near the contact line was unclear. Here, by direct visualization and particle-imaging velocimetry (PIV), we show that the sliding surface layer drives a circulating hydrodynamic vortex in the meniscus region that is central to the microecology. Although counterintuitive in viscous flows, persistent circulation driven by forcing at the free surface is consistent with the classic analysis for vortex generation in wedge geometry (9).The initial discussion of the chemota...