| dc.description.abstract | The aims of this thesis are investigating the swarming dynamics of Vibrio alginolyticus and studying the collective motions and individual dynamics of self-propelled long bacteria. Self-propelled particle (SPP) has been studied both in theoretical and experimental approaches. However, theoretical works lead experimental works because of the limitation of manufacturing ideal experimental SPP systems. Bacterial swarm is a good candidate for building an experimental SPP system. Here, we survey the swarming behavior of Vibrio alginolyticus and build up swarming protocol to perform self-propelled particle experiments.
Vibrio alginolyticus is a marine bacterium with dual flagellar motor systems, a Na^+-driven polar flagellum and multiple H^+-driven lateral flagella. While growing on agar plate, they elongate without division into swarming phase. Three strains of Vibrio alginolyticus, 138-2 (wild-type), VIO5 (Pof^+), and YM19 (Laf^+) were first examined in both agar plate and aqueous environments. We found that YM19 strain shows very strong surface swarm motility and is an ideal experimental system for both bacterial swarming and self-propelled particle experiments.
During the swarm colony development, YM19 cells shows rich dynamical patterns such as turbulence, edge waving, edge streaming, jamming and mono-layer. We calculate the velocity fields by particle image velocimetry (PIV) and characterize the dynamics of the velocity by power spectra and vorticity calculation. The cells show different dynamical patterns and movement in different cell-densities and boundary conditions. With confined edge, a band of cells anchored on the non-moving contact-line show periodic waving about 0.8 Hz. The driving force is not only from the external turbulence region but also the self-propelled cells in the band. During the colony expansion, the contact-line is not marching together but protruding by a group of cells behind it. Both cell motility and collected motion are critical for the colony expansion.
At mono-layer, single cell can be tracked by microscope and high speed camera. In this region, some long cells are interacting with large amount of short cells and showing very dynamical configurations. The cell wall is rigid to maintain the bacterial shape. However, long cells can be bended by the collisions of short cells. Short cells are like a rigid rod while long cells are bendable and the two ends move independently. Long cells can be self-assembled into rotating spiral coils. Four types of spiral coils can be found featuring by rotational direction and the center structure: single clockwise (SCW), single counterclockwise (SCCW), double clockwise (DCW), and double counterclockwise spiral coil (DCCW) spiral coils. The length of spiral coils cells are between 32 to 296 μm. The rotational speeds are between 2.22 to 22.96 rad/s. These spiral coils rotate stably and the displacement are strongly influenced by the collision of short-cells clusters. The folding and unfolding of spiral coils are the results of the interaction between cells and the change of local moving direction of long cells. The finding of stable spiral coils in the active system indicating the existence of a local stable structure in the non-equilibrium background. The self-assembled process of spiral coil may inspire a new micro manufacturing technology. | en_US |