FIG. 6.

(a) Observations of the hydrodynamic bound state of Volvox. (i) View through the top of a glass chamber showing superimposed images taken 4 s apart, graded in intensity for clarity. (ii) Side and (iii) top views of a colony swimming against a coverslip, with fluid streamlines from particle image velocimetry. (iv) A linear cluster viewed from above. (b) Data and analysis of the hydrodynamic bound state of Volvox. Geometry of (i) two interacting Stokeslets (side view) against an upper no-slip surface and (ii) the gap between nearby spinning colonies. (iii) Radial separation r of infalling colonies, normalized by the mean colony radius, as a function of rescaled time for 60 events (black). The running average (green) compares well with predictions of the singularity model (red). (Inset) Orbiting frequency Ω versus mean spinning frequency $\stackrel{‒}{\omega }$, and the linear fit. Figure adapted with permission from Drescher et al. (2009b). Copyright 2009 by the American Physical Society.