Figure 1

Identification of the swimmer populations by tracking active swimmers (red tracks) and random swimmers (blue tracks), ${\varphi }_A$ is the corresponding fraction of active swimmers. Panels (a),(b),(g) correspond to E. coli see inset in (c)] and panels (d),(e) correspond to Au-Pt rods [see inset in (f)]. The round black circles in (a),(b) are $2\mu \mathrm{m}$ latex beads; the white small circles in (d),(e) are $1\mu \mathrm{m}$ Dynal beads. Panels (c),(f) display the relation between the active swimmer’s mean velocity $V_A$ and ${\varphi }_A$. In (c) are 6 independent experiments with E. coli: 1N cells (brown, red, and green), mixture of 1N and 2N cells (black), and 2N cells (pink, blue). Labels (1) and (2) are for $⟨N⟩=100$ and $⟨N⟩=200$ cells in the observation field, respectively. (f) Au-Pt rods with varying proportions of inactive Au rods (2 independent experiments). (g) density probability of the observed bacterial tracks in the $(N_c,⟨|\theta |⟩)$ space. The color map goes from blue for vanishing probability to red for high probability. Two clusters are identified, centered at $(0.9,0.3)$ and $(0.17,\pi /2)$, corresponding, respectively, to the active and random swimmers.Reuse & Permissions

Figure 2

Enhanced diffusivity $D_P$ of passive tracers as a function of $J_A$. Squares and circles represent tracer of 2 and $1\mu \mathrm{m}$ diameters, respectively. Panel (a) corresponds to the bacterial suspensions. Each color represents an experiment performed over a range of bacterial dilution. Panel (b) corresponds to suspensions of Au-Pt rods: green, blue, and black circles correspond to various ${\mathrm{H}}_2{\mathrm{O}}_2$ concentrations (from 2.5% to 20%). The dashed lines are linear fits and the error bars are standard deviations (the rest of the colors defined in Fig. 1 and text).Reuse & Permissions