Figure 3.

BV is geometrically captured in orbital motion around spherical inert beads. Active BV are mixed with inert CL beads between a microscope slide and a coverslip (A). Sample trajectories show BV swimming along a wall (constructed as explained in the Materials and Methods) (B), and being geometrically captured around beads (C). Each colored line indicates a separate bacterial trajectory, with the points along each line indicating the position of the bacterium at each interval. From 84 trajectories, we collected a density histogram (see the Supporting Material for details of the calculation) of BV showing how BV tightly localizes in orbital motion around beads (D). Density enhancement is computed with respect to the center of the bead (r₀) as explained in the Supporting Material and Fig. S7. Analysis of beads of decreasing size reveal how BV’s capture time decreases for smaller beads (E). Each data point is the mean trajectory’s duration within the capture region (5 μm from the bead surface) for a corresponding bead after dropping $5\%$ of outliers from each side (that is, bacteria that stayed stuck to the bead or bacteria that only grazed the bead). The error bar is 1 SD. The data point corresponding to R_bead = ∞ shows the trajectory duration expected for an infinite radius bead (obtained by averaging the trajectory duration on the surface of the coverslip and slide combined; from Fig. 2, B and D). More data are provided in Figs. S6 and S7. Simulations of BV trajectories are shown around beads with radii of 2 (F), 20 (G), and 40 (H), measured in units of BV’s bacterial body length with identical initial conditions. Model details are found in the Fig. S2 and the Supporting Material. The capture probability increases (from ∼0 to ∼1) as the bead size increases from 2 (F) to 40 (H). Strong interactions with the surface on which the bead rests contribute to BV’s eventual detachment from the bead in experiments. To see this figure in color, go online.