Drag reduction: Plastron and air lubrication
What would happen if an object could retain a thin layer of air ("a plastron") when falling in water? Would bouyancy slow its fall?
Our experiments used superhydrophobic spheres falling in water (click here to see the experimental apparatus). We coated acrylic spheres in sieved hydrophobic sand and dropped them in a large tube of water. To compare spheres with and without plastrons we used a trick from insect physiologists (immersion in ethanol) to remove the plastron. Vakarelski et al went further and created a perfect superhydrophobic sphere using a Leidenfrost effect Videos.
But can plastron drag reduction really make boats go faster? For a debate about the possible relevance to Olympics 2012 and sailing, see the RCUK/Royal Institution Cutting Edge 2012 Debate: Behind Sailing.
Terminal velocity and drag reduction measurements on superhydrophobic spheres,
Appl. Phys. Lett. 94 (6) (2009) art. 064104.
View postprint pdf
Drag reduction by Leidenfrost vapor layers,
Physical Review Letters 106 (21) (2011) art 214501. View abstract