Curved Tails in Polymerization-Based Bacterial Motility

The curved actin ``comet-tail'' of the bacterium Listeria monocytogenes is a visually striking signature of actin polymerization-based motility. Similar actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae, the Vaccinia virus, and vesicles and microspheres in related in vitro systems. We show that the torque required to produce the curvature in the tail can arise from randomly placed actin filaments pushing the bacterium or particle. We find that the curvature magnitude determines the number of actively pushing filaments, independent of viscosity and of the molecular details of force generation. The variation of the curvature with time can be used to infer the dynamics of actin filaments at the bacterial surface.Comment: 8 pages, 2 figures, Latex2

Graded sediment transport modelling

Due to natural processes and human interference, the coastal zone is constantly changing. In order to understand and predict these morphological changes, the study of sediment transport under currents and waves is of great importance. Graded sediment is found everywhere within the natural environment and each of the grain sizes behave differently under the same flow conditions. Early research into sediment transport attempted to simplify the system by relating the sediment transport rate to specific attributes of the sediment, such as the median grain diameter. However, this simplification can lead to a significant underestimation of the transport rate, especially in the case of there being a broad spectrum of grain sizes available for transport. Accordingly, the grading of sediments in modelling sediment transport should be taken into account. By doing a detailed analysis of the composition of sediments, important information with regard to the sediment transport processes can be obtained. Therefore, the first objective of this thesis was to do a literature reviewon the most current research analysis on graded sediment transport. The second objective was the implementation of the physics of graded sediment into an existing 2 DH depth averaged sediment transport model. Numerical experiments similar to those in the literature review were carried out and the results compared, showing good agreement. The third objective was to test the numerical model against a benchmark laboratory experiment, this test case came from the ongoing EC funded SANDPIT project. The laboratory set - up was a wave current flume with a trench, the aim of the experiment was to simulate the morphodynamic evolution of the trench over a 10 hour period. The result of the numerical model for this lab experiment was encouraging as it reproduced the bed evolution and sediment flux fairly accurately, and gave significantly better results than the case with a single grain size. Having applied the model to a controlled lab experiment, the fourth and fInal objective was to test the model against a field case. The selected field case was the Teignmouth site that was extensively monitored and modelled as part of the EC funded COAST 3D project, and represents a site with strong three dimensional characteristics. Although the numerical model is able to simulate graded sediment spatially over the model domain, it is not able to be more positive, i.e. an improvement over using a single grain size, this then highlights areas for further research