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 buoyancy slow its fall?

Experimental apparatus

Our experiments used superhydrophobic spheres falling in water. 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:

Publications

  • Terminal velocity and drag reduction measurements on superhydrophobic spheres
    G. McHale, N.J. Shirtcliffe, C.R. Evans and M.I. Newton
    Appl. Phys. Lett. 94 (6) (2009) art. 064104.
  • Drag reduction by Leidenfrost vapor layers
    I.U. Vakarelski, J.O. Marston, D.Y.C. Chan and S.T. Thoroddsen
    Physical Review Letters 106 (21) (2011) art. 214501.

Leidenfrost Videos

Leidenfrost layer break-up (Vakarelski et al)
Race between spheres (Vakarelski et al)