Evaluation of the Mass Transfer Effect of the Stalk Contraction-Relaxation Cycle of <i>Vorticella convallaria</i>

Advisor: Sangjin Ryu Vorticella convallaria is a genus of protozoa living in fresh water. It has a bell-shaped zooid equipped with adoral ciliary bands and a contractile stalk tethering the zooid to a substrate. Vorticella is regarded as a biological spring because its contractile stalk can pull the zooid towards the substrate at a remarkably high speed and then relaxes to its extended state much more slowly. However, reasons for Vorticella's stalk contraction are still unknown. It is presumed that the flow field induced by the contraction-relaxation cycle of Vorticella would augment mass transfer effect near the substrate. We investigated this hypothesis with an experimental Vorticella model with particle tracking velocimetry and a computational fluid dynamics model. In both approaches, Vorticella is modeled as a solid sphere translating perpendicular to a solid surface in water. After having been validated by the experimental model and verified by grid convergence index test, the computational model simulated water flow caused by Vorticella during the cycle, based on the measured time course of stalk length changes of Vorticella. We calculated the trajectory of particles, which were uniformly distributed near Vorticella initially, based on the simulated flow field, and then evaluated the mass transfer effect of Vorticella's stalk contraction-relaxation by spatial point analysis. The results illustrated that the uniformly distributed particles were transferred to the area away from the Vorticella radially, and became more clustered against a statistical model of purely random point pattern. The results indicate the mass transfer effect has been enhanced in the vicinity of Vorticella. Acknowledgements I would like to thank my advisor Dr. Ryu for introducing me into the research field on fluid mechanics and computational fluid dynamics, thanks for guiding me how to manage and conduct a whole research project independently. Besides the research, I also learned a lot of things from him such as philosophy, humanity, education and language, which makes my research life like a spiritual study. Also, I would like to thank Dr. Admiraal for helping me with the experimental part of this research. And I would like to thank my thesis committee members: Dr. Neganban and Dr. Gogos for giving me pertinent suggestions on my thesis.