Figure 1

(a) Two dimensional trajectories of 50 swimming cells. Trajectory duration is 20 seconds. Trajectories starting positions have been all shifted to the origin. (b) Measured mean square displacement (MSD) of cells as a function of time. Averages have been extracted from 9 Hz sequences of about 700 cells trajectories. MSD fits well to a persistent random walk process with a correlation time $t_v=3.5\pm 0.1\mathrm{s}$ and a coefficient $D=995\pm 20\mu {\mathrm{m}}^2{\mathrm{s}}^{-1}$ (see text). Inset: Log-Log plot. (c) Measured mean velocity $V$ of cells as a function of the inversed viscosity of the medium. The line represents a linear fit. Inset: Viscous drag force should be proportional to the product of the medium viscosity ${\eta }_0$, the mean radius $R$, and the velocity $V$ of the cells. This product ${\eta }_{0}RV$ is shown to be independent of the medium viscosity. Error bars are smaller than the symbols.Reuse & Permissions

Figure 2

(a) Effective viscosity of chlamydomonas suspensions as a function of shear rate. Data are shown for different volume fractions of the suspension, and star symbols represent the viscosity ${\eta }_0$ of the culture medium. The lines are shown for ease of viewing only. (b) Relative viscosity of microswimmer suspensions (measured at shear $\mathrm{\text{rate}}=5{\mathrm{s}}^{-1}$) as a function of volume fraction. Solid symbols represent live cell data and crossed symbols represent dead cell data. Measurements are fitted to Eq. (1) using ${\varphi }_m=0.62$ and $\alpha =2.5$ (dead cells) and 4.5 (swimming cells).Reuse & Permissions

Figure 3

(a) Dead cell in a $10{\mathrm{s}}^{-1}$ shear flow experiencing tumbling around the $x$ axis. (b) Live chlamydomonas swimming in the same shear flow. Arrows indicate cells flips. Time between pictures is 20 ms. Cell diameter is about 10 micrometers. (c) Schematic view of the flow cell, where $Oz$ is the direction of observation, $yOz$ is the shear plane.Reuse & Permissions