Self-motile colloidal particles

Moving in the nano-world poses some interesting technical challenges. To small objects, fluids like water behave as very viscous liquids, and collisions with fast water molecules jostle them, making them stagger about drunkenly.

In this experiment uniform polystyrene beads were tracked as they moved in water or dilute hydrogen peroxide solutions under the influence of Brownian motion. Some of the beads, however, were half coated with platinum, which acts as a catalyst decomposing hydrogen peroxide.

The decomposition reaction yields two molecules of water, and a molecule of oxygen for every two molecules of hydrogen peroxide. This expansion pushes the coated particles, such that for short times they are seen to move in a consistent direction.

In the movie, coloured spheres follow the tracks taken by the real particles observed in the experiment. You can start new particles of either plain polystyrene or half coated polystyrene by clicking on the start button. The particles are all followed for the same amount of time, allowing you to observe how the different particles behave.

References

"Self-motile colloidal particles: from directed propulsion to random walk" J.R. Howse, R.A.L. Jones, A.J. Ryan, T. Gough, R. Vafabakhsh, and R. Golestanian, Phys. Rev. Lett. 99 (2007), 048102.

Nanoscale swimmers on the Soft Machines blog by Richard Jones (has a link to the full PDF of the article).