Fig. 1.
Experimental design. (A) Shapes of some types of gears used in this study. (B) Scheme of the experimental setup. The film is suspended between four wires attached to a moveable frame. Film thickness is controlled by the linear motion of the frame (indicated by the red arrow). The experimental apparatus is placed in a transparent chamber with controlled atmosphere and mounted on the moving stage of inverted microscope. (C) Velocity distribution function P(V) for small fluorescent tracers (2.5 μm polystyrene microspheres, Spherotech, Inc.) advected by Bacillus subtilis bacteria in the regime of collective swimming at concentration 2 × 1010 cm-3. Corresponding rms velocity, Vrms, of the tracers is about 13 μm/ sec. Red line gives the best fit to the Gaussian law, and blue line is a stretched exponential fit with the exponent 1.16.