Swimming of \emph{Chlamydomonas reinhardtii} in weakly elastic fluids

The swimming behavior of the algae \emph{Chlamydomonas reinhardtii} in weakly elastic fluids is investigated in experiments using microscopy and tracking methods. The effects of fluid viscosity and elasticity on the swimming speed, flagellar shape, beating frequency, and efficiency are examined. Here, the fluid viscosity is varied using water and sucrose solutions, while fluid elasticity is introduced by adding flexible polymer CMC (carboxymethyl cellulose) to the buffer solution. Swimming experiments are performed in a thin-film apparatus equipped with a microscope and high-speed camera. We find that even small amounts of fluid elasticity can have a significant effect on the swimming kinematics and dynamics of \emph{Chlamydomonas} because of the relatively high beating frequency of its flagella (50-60 Hz). For example, the \emph{Chlamydomonas} swimming speed is hindered by fluid elasticity compared to Newtonian fluids. In addition, the algae swimming speed decreases as the fluid elasticity is increased.