Research Projects

Chlamydomonas reinhardtii (CR) is a model biological organism that has been used extensively to understand fluid flow and swimming at low Reynolds number. This single celled alga swims in a fluid using its two flagella that beat in a breast-stroke manner and result in a flow field of the puller type. The individual and collective behaviour of wild type CR cells is well understood in unconfined fluids. However, the effect of spatial confinement has largely been unexplored, even though tightly confined spaces like porous rocks are natural habitats of these cells. This proposal will compare and contrast the behaviour of wild type and CC2377 mutant strain of CR when they are confined between two plates separated by approximately cell body diameter. CC2377 is a recently identified mutant strain of CR whose flow field is of pusher type instead of the conventional puller type associated with the wild type strain. Therefore, by using these two kinds of cells we will be able to explore both the role of hydrodynamics and steric interactions in governing the phase behaviour of the confined cell suspensions. Specifically, we will measure the beat waveform and flow fields of both wild type and CC2377 cells under strong confinement. We will measure density dependent motility, scaling of number fluctuations and velocity correlations as the concentration of cell increases. We will characterize the mesoscopic flow set up by collective swimming of cells by analyzing the tracer displacement statistics. Finally, we will study mixtures of wild type and CC2377 cells to explore if phase separation between two kinds of swimmers takes place and if the overall behaviour of 1:1 mixtures approaches that of non-interacting swimmers. The novelty/significance of this proposal is two-fold. This study will be the first one to elucidate the effect of confinement on collective motion of CR cells. Currently, 1 micron long E.Coli and 10 micron long wild type CR cells are the only known systems which set up pusher and puller flow fields, respectively. Their size difference and other system specific details prevent a robust experimental study of mixtures of pushers and pullers. Our proposal to use mutant strain CC2377 of CR as a pusher alleviates this issue because both wild type CR and CC2377 have similar sizes and hence our study of mixtures of these cell types will provide the first unambiguous results on phase behaviour of mixtures of pushers and pullers.