Drag reduction

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?

To explore this, we set up a large transparent tube of water (see above). We then compared the motion of 1½ inch diameter acrylic spheres with three different surface coatings (below). 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, which results in plastron formation. 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. (Note that insect physiologists use immersion in ethanol to remove plastrons, which is why we knew to use the same trick here.)

It is clear that the sphere with the plastron falls fastest. Read more about this experiment in the publications below.

Theoretical calculations can also be carried out that confirm the experimental results.

The image above (taken from the Soft Matter article below) shows calculated streamline patterns in the plastron layer (the shaded contours) for different values of the plastron thickness (h/b).

We have seen that plastron drag reduction can make acrylic spheres fall faster. An obvious question is whether plastron formation can be utilised on boat hulls to make them go faster. For a debate about the possible relevance to the 2012 Olympics, see the RCUK/Royal Institution Cutting Edge 2012 debate 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) 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