Drag reduction: Changing terminal velocity

Voted into the top 10 “most beautiful experiments” of all time (Robert Crease, Physics World, 2003), falling-body experiments intrigue and delight readers. From visions of Galileo climbing the Leaning Tower of Pisa to the video of the Apollo 15 Astronaut Dave Scott dropping a hammer and feather on the moon (see below and click here for more details), non-scientist preconceptions that heavier objects fall faster are confounded. But in the real world hammers do fall faster than feathers and each object attains its own terminal velocity as every parachutist knows.

If an object falling through water retains a film of air due to a superhydrophobic surface, shouldn’t buoyancy dictate that it falls slower and not faster?

The animation above shows a comparison of a 1½ inch diameter acrylic sphere with three different surface coatings settling in water in a large transparent tube (right). The object on the left is an acrylic sphere with a thin rough coating. The object in the centre is the same acrylic sphere with its thin layer coating having an additional coating of a superhydrophobic paint. The object on the right is the same as in the centre, but with an ethanol pre-treatment to suppress the formation of a plastron.

Which seems to fall fastest? Read more about this experiment in the publications below.

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.
  • Plastron induced drag reduction and increased slip on a superhydrophobic sphere
    G. McHale, M.R. Flynn and M.I. Newton
    Soft Matter 7 (21) (2011) 10100-10107.