Bio-convection

Credit: Otti Croze

Bioconvection pattern formed by the swimming alga Dunaliella salina, a commercial beta-carotene producer. This species, like many others, is biased by a gravitational torque to swim up. In a flow, an additional viscous toque causes swimming to downwelling fluid (gyrotaxis).

The pattern is the result of overturning (similar to Rayleigh-Bernard convection, but without external energy input!) and gyrotactic instabilities. Similar physics needs to be considered in photobioreactors to grow algae.

Flow field of swimming nematode

Credit: Tom Montenegro-Johnson and Eric Lauga

The nematode worm C. elegans swims by generating travelling undulations along its body, driving a complex, three-dimensional flow that is revealed by our boundary element simulations, based on experimentally-extracted kinematics. Here, we see flow streamlines captured at an instant where the swimmer's body forms an 'S'-shape, where the colour bar gives the flow speed in millimetres per second. Using theoretical fluid dynamics, experimental planar flow fields extracted from particle tracks can be corrected for this three-dimensional motion [preprint].