Now, here's one for the gear brigade: I was sent the first issue of the
new science journal, Soft Matter, at work (presumably to get me lobbying
my library to subscribe to it). It's not really my area of materials
science, but I had a quick flick through before binning it and found a
review paper by Mathilde Callies and David Quere from the College de
France, Paris, titled "On water repellency". It was quite readable to
someone not familiar with this science. If you have access to a
university library with a subscription, you can get the article at
http://www.rsc.org/Publishing/Journals/SM/article.asp?doi=b501657f
(M Callies, D Quere; Soft Matter 1 (2005) 55).
They describe how texturing the surface of a chemically hydrophobic
(water repellent) material can make it superhydrophobic. For example,
you need to tilt a teflon surface by 10-30 degrees before droplets
run off, but if the same material is suitably textured, 1 degree tilt
is sufficient.
Hydrophobicity is measured in terms of the contact angle, ie the angle
between the macroscopic surface of the material underneath the water
drop and the tangent to the drop where its surface intersects with
the material's surface. The bigger the contact angle, the closer the
drop is to a spherical shape, ie the less it wets the material.
They explain that making the surface rough can have two effects. The
drop may fill up the roughness, which will increase the measured contact
angle because some of the drop is actually below the material's surface
as defined above (the macroscopic surface is what you get if you put
a ruler across all the bumps in the rough surface).
The other effect occurs if the drop does /not/ fill up the roughness
but is supported only by the tops and ridges sticking out from the
surface. Then, most of the drop is actually not in contact with the
material at all but with air in the channels and pockets between the
bits sticking out from the surface. Therefore, its shape is hardly
affected by the material and remains close to spherical. This is the
situation we want when out in the rain.
The transition between the two states depends on (a) the chemical
hydrophobicity of the material and (b) the radius of the droplets,
which is why (a) water repellency needs to be restored by chemical
treatment and (b) what works in heavy rain doesn't necessarily work
in a fine spray.
Water-repellent structures in nature often have a texture on two
different length scales, which seems to work around the problem of
the radius dependence, although it is not quantitatively understood
yet how this works.
Technological applications suffer as yet from the problem that carefully
engineered surface textures tend not to be sufficiently robust.
--
Rudi Winter, Aberystwyth, Wales