Effect of boundaries on swimming of Paramecium multimicronucleatum

Microorganisms swimming in their natural habitat interact with debris and boundaries, which can modify their swimming characteristics. However, the boundary effect on swimming microorganisms have not been completely understood yet, and is one of most active areas of research. Amongst microorganisms, unicellular ciliates are the fastest swimmers and also respond to a variety of external cues. We choose Paramecium multimicronucleatum as a model system to understand the locomotion of ciliates.

First, we explore the effects of boundaries on swimming modes of Paramecium multimicronu- cleatum by introducing them in 2D films and 1D channels. The geometric confinements cause the Paramecia to transition between: a directed, a meandering and a self-bending behaviors. During the self-bending mode the cell body exerts forces on the walls; which is quantified by using a beam bending analogy and measuring the elasticity of the cell body. The first inves- tigation reveals the complicated swimming patterns of Paramecium caused by boundaries.

In the second study we investigate the directed swimming of Paramecium in cylindrical capillaries, which mimics the swimming of ciliates in the pores of soil. A finite-sized cell lo- comoting in extreme confinements creates a pressure gradient across its ends. By developing a modified envelop model incorporating the confinements and pressure gradient effects, we are able to predict the swimming speed of the organisms in confined channels.

Finally we study how Paramecium can swim and feed efficiently by stirring the fluid around its body. We experimentally employ "-Particle Image Velocimetry to characterize flows around the freely swimming Parameicum and numerically use Boundary Element Method to quantify the effect of body shapes on the swimming and feeding process. Results show that the body shape of Paramecium (slender anterior and bulky posterior) is hydrodynamically optimized to swim as well as feed efficiently.

The dissertation makes significant advances in both experimentally characterizing and the- oretically understanding the flow field and locomotion patterns of ciliates near solid bound- aries.