Dynamics of confined microswimmers inside a droplet: From microactivity to macromovement
In this thesis we investigate three experiments where bacterial suspensions are encapsulated in droplets. The aim of these experiments is to understand how the microactivity at the local scale, when bacteria organize collectively, can create a macromovement at the containing droplet scale, which is...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2020 |
| País: | Chile |
| OAI Identifier: | oai:repositorio.anid.cl:10533/246251 |
| Acceso en línea: | https://hdl.handle.net/10533/246251 |
| Access Level: | acceso abierto |
| Palabra clave: | Ciencias Naturales Ciencias Físicas Otras Especialidades de la Física |
| Sumario: | In this thesis we investigate three experiments where bacterial suspensions are encapsulated in droplets. The aim of these experiments is to understand how the microactivity at the local scale, when bacteria organize collectively, can create a macromovement at the containing droplet scale, which is about 100 times the size of a bacterium. In other words, how we can extract useful work from these encapsulated bacterial suspensions. First, we confine a dense suspension of motile Escherichia coli inside a spherical droplet in a water-in-oil emulsion, creating a "bacterially" propelled droplet. We show that droplets move in a persistent random walk, with a persistence time τ ~ 0.3 s, a long-time diffusion coefficient D ~ 0.5 μm2/s, and an average instantaneous speed V ~ 1.5 μm/s when the bacterial suspension is at the maximum studied concentration. Several droplets are analyzed, varying the drop radius and bacterial concentration. We show that the persistence time, diffusion coefficient and average speed increase with the bacterial concentration inside the drop, but are largely independent of the droplet size. By measuring the turbulent-like motion of the bacteria inside the drop, we demonstrate that the mean velocity of the bacteria near the bottom of the drop, which is separated from a glass substrate by a thin lubrication oil film, is antiparallel to the instantaneous velocity of the drop. This suggests that the driving mechanism is a slippery rolling of the drop over the substrate, caused by the collective motion of the bacteria. Our results show that microscopic organisms can transfer useful mechanical energy to their confining environment, opening the way to the assembly of mesoscopic motors composed of microswimmers. In a second experiment, we show that under the application of a constant magnetic field, motile magnetotactic bacteria confined in water-in-oil droplets self-assemble into a rotary motor exerting a torque on the external oil phase. A collective motion in the form of a large-scale vortex, reversable by inverting the field direction, builds-up in the droplet with a vorticity perpendicular to the magnetic field. We study this collective organization at different concentrations, magnetic fields and droplets radii and reveal the formation of two torque-generating areas close to the droplet interface. We characterize quantitatively the mechanical energy extractable from this new biological and self-assembled motor. Finally, we study a bacterial suspension of E.coli encapsulated in a double emulsion, where an oil droplet gets trapped inside a water-in-oil emulsion. We show that the inner oil droplet performs a persistent random walk in the horizontal plane with a persistence time τ ~ 0.3 s. The diffusion coefficient in the horizontal plane depends inversely on the inner droplet radius, and we compare it with the thermal diffusion coefficient. It allow us to compute an active temperature, which has a value of 2.7×10^4 K, two orders of magnitude larger than room temperature, consistent with the experiment of bacterially propelled droplets and previous works. The vertical plane was also studied, revealing that the diffusion coefficient in the vertical axis is smaller than in the horizontal axis, due to the geometric trapping. |
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