A water bottle
that fills itself
Few creatures can survive in Africa’s
Namib Desert, located on the southwest
coast of Africa, with an average annual
rainfall of a mere 1.3 centimeters. But the
Namib Desert beetle (pictured above) is
the exception. It has an ingenious method
of obtaining water. The shell that covers its
wings is covered with many tiny bumps.
The tip of each bump is hydrophilic (
water-attracting) while the sides of these bumps
are hydrophobic (water-repelling).
Hydrophilic substances are polar, while
hydrophobic substances are nonpolar.
When it comes to solutions, “like attracts
like.” So, water, being polar, is attracted
to other polar substances. The Namib is a
coastal desert off the Atlantic Ocean. Short-
lived early-morning fogs pass over the
desert sands only about six times per
month. The Namib Desert beetle survives
by using its bumpy shell to draw drinking
water from periodic fog-laden winds.
As morning fog sweeps across the desert
floor, the water sticks to the peaks of the
hydrophilic bumps on the beetle’s wings,
eventually forming droplets. When the droplets become large and heavy enough,
they roll down from the top of the
peaks and are channeled to a spot
on the beetle’s back that leads
straight to its mouth.
This beetle’s ingenious method
of obtaining water has served as
a prototype for a self-filling water
bottle, which shows real promise
for use in arid regions of the world
that have a lot of mist or fog and
not much access to fresh water. Any ordi-
nary water bottle may someday be converted
into a self-filling bottle by attaching an insert
that fits into the cap. Like the beetle’s wings,
this bottle insert contains regions of polar
and nonpolar surfaces, with the polar regions
attracting water vapor molecules.
The insert is composed of layers of polar
aluminum oxide and non-polar polypropylene
that are sand-blasted to create a rough struc-
ture. It is pointed so that when enough tiny
droplets collect from mist or fog, they can
aggregate into a bigger drop, falling down
into the bottle.
Although early prototypes have not col-
lected as much water as initially predicted,
substitution of better-quality hydrophilic
and hydrophobic surfaces may someday
significantly increase water
ChemMatters | FEBRUARY/MARCH 2016 7
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Hennighausen, A.; Roston, E. 14 Smart Inventions Inspired by Nature: Biomimicry. Bloomberg, Aug 19,
biomimicry.html [accessed Nov 2015].
Forbes, P. Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology. Scientific American, July 30, 2008:
http://nano.z9i.com/files/Scientific-American-Self-Cleaning-Materals-Lotus-Effect.pdf [accessed Nov
Clark, L. This Self-Filling Water Bottle Mimics a Desert Beetle. Wired: United Kingdom, Nov 26, 2012:
http://www.wired.com/2012/11/namib-beetle-bottle/ [accessed Nov 2015].
Brian Rohrig is a science writer who lives in Columbus, Ohio. His most recent ChemMatters article, “Safety
Data Sheets: Information that Could Save Your Life,” appeared in the December 2015/January 2016 issue.
The surface of the leaf of the sacred lotus
plant (Nelumbo nucifera) is similar to the
wings of the Namib Desert beetle. Both are
covered with tiny microscopic bumps. The
lotus plant, a native of Asia, has long been
known for its water-repellent properties, as
it always has a neat and clean appearance.
Water rolls right off its waxy leaves. Dirt and
mud roll off just as easily.
When a water droplet strikes the surface of
the lotus leaf, it sits atop these bumps. Each
bump is covered with tiny hairs, which trap a
layer of air underneath. This impressive ability
to repel water was dubbed the “lotus effect”
by Wilhelm Barthlott, of the University of Bonn
in Germany, a leader in the field of biological
and technical interfaces.
A big reason for this material’s superior
hydrophobic properties involves the contact
angle of water droplets with the surface.
Normally, when a water droplet lands on a
surface, it becomes somewhat flattened, with
a contact angle less than 30° (Fig. 1a). When
a drop of water lands on a lotus leaf, the drop-
let is nearly completely spherical, forming a
contact angle greater than 150° (Fig. 1b). A
similar effect can be observed when you place
a drop of water on a piece of wax paper. This
minimal contact enables water to roll off the
leaves easily, carrying dirt with it.
The lotus effect has been put to a wide variety of uses. One of the first was in the 1990s
with the honey spoon, a nonstick spoon that
allowed even a substance as sticky as honey
to drip off. Fabrics that repel water and stains
are commonplace today, using nanotechnology that embeds tiny hydrophobic fibers on
the surface of the fabric.
Lotusan self-cleaning paint uses this same
effect. Even though it goes on smooth and
looks like normal paint, on a microscopic
level its surface resembles the bumpy surface
of the lotus leaf, keeping painted surfaces
clean and dry as water and dirt roll right off.
Super-hydrophobic coatings applied to win-
dows, mirrors, airplanes, and boats all use the
Nature provides a wonderful chemistry
lab for us, and we can learn something from
every living organism. Keep your eyes open—
you could make the next great discovery that
could revolutionize our world!
Figure 1. (a) When a water droplet lands on a solid surface, it spreads out to a degree that
depends on how water-repellent the surface is. Shown here is a typical water droplet on a glass
surface, with a contact angle of 30˚. (b) When a water droplet lands on a lotus leaf, it is nearly
completely spherical, forming a contact angle greater than 150˚.