|
06/27/2007
Footpads - Hairy and Smooth
Computer animation has reached amazing heights in movies,
video games and TV commercials. Take, for example, the
erudite GEICO gecko with its cultured accent - my favorite.
Aside from selling insurance, the gecko is of scientific interest
because of its “sticky feet”. In two earlier columns (7/11/2000
and 11/21/2002), we talked about those feet and their unusual
structure that permits the gecko to climb smooth walls and even
traipse across ceilings upside down without falling.
The cover of the June issue of the MRS (Materials Research
Society) Bulletin caught my attention with pictures of a beetle, a
frog and a gecko heralding the Bulletin’s seven papers on
“Sticky Feet: From Animals to Materials”. The three creatures
on the cover all have the ability to navigate, if not on a ceiling,
on steeply inclined surfaces. Scientists study these animals’
sticky feet hoping to apply their findings to the design of
products ranging from tires to robots capable of scaling walls.
Let’s first review the Gekko gecko, otherwise known as the
Tokay gecko, one of over a thousand gecko species. In the
earlier columns, we discussed the work of biologist Kellar
Autumn and his colleagues. They pinned down the secret of the
gecko’s sticky feet and have a patent on synthetic adhesives
based on their work. Autumn authored one of the MRS Bulletin
articles. The gecko’s foot has five toes; each toe has about 20
leaf-like layers of tissue known as scansors. From these scansors
spring foot hairs known in the trade as setae. There are a lot of
setae on a gecko foot – over 14 thousand per square millimeter!
If you think that’s impressive, each of these setae in turn ends in
a spread of hundreds of so-called spatular structures. These
spatulas are teensy stalks with thin roughly triangular plates at
the ends. So, when the gecko walks, there are hundreds of
thousands, possibly a million or more tiny nano hairs contacting
the surface. A key question was whether the stickiness of the
gecko’s feet was “dry” or “wet”. The gecko’s setae are strongly
hydrophobic (water repellant). In order to check if water was
involved in making the sticky contacts, Autumn and his
coworkers carried out experiments with the geckos on water-
attracting (hydrophilic) surfaces and on water-rejecting
(hydrophobic) surfaces.
The gecko performed essentially the same on both hydrophobic
and hydrophilic surfaces. The fact that the dry gecko setae
adhere to the dry hydrophobic surface leads to the conclusion
that the force holding the gecko to the surface is the so-called
van der Waals force. The relatively weak van der Waals force is
well known to chemists as a force responsible for molecules
attracting each other at very close range. (You get an idea of
how weak the force is every time you write with a pencil and the
layers of graphite slide onto the paper. It’s the van der Waals
force that holds the layers of carbon together in graphite.) Those
hundreds of thousands or millions of spatulas plonking down on
the surface, each attracted to the surface by a small force, add up
to enough force to hold the lizard on the wall or ceiling!
What about other animals, such as the frog? In another article,
W. Jon P. Barnes discusses various ways an animal overcomes
gravity. One method is “interlocking”. A bear climbs a tree by
using its claws to lock onto irregularities in the bark and/or to dig
into the tree to haul itself up. Another prime method, the one
used by the frog, is “bonding”, one form of which is the dry
adhesion used by the gecko. Other animals, such as certain bats,
use suction cups on their wings that work by reducing the
internal pressure so that the atmospheric pressure holds the
animal in place. But the frog, as well as many insects, uses a
form of bonding that involves “wet adhesion”.
In wet adhesion, a film of liquid is involved in the contact
between the foot/toe and the surface. Wet adhesion can involve
a hairy pad such as the ones we see in the gecko. However, the
frog, as well as many insects, employs a “smooth adhesive pad”
(SAP). Let’s take a look at a frog’s toe pad with a scanning
electron microscope. The smooth adhesive pad is not exactly
smooth but looks to me more like a mud flat or a sloppy
honeycomb. The surface is a collection mostly of more or less
hexagonal columns roughly 5-10 microns (a fraction of the width
of a human hair) in size with channels surrounding each
columnar cell. There are mucous glands interspersed among the
cells and the channels presumably help to spread the mucous
over the surface of the cells. The SAPs from different species of
tree frogs and “torrent “ frogs (they clamber over rocks in
streams) all show very much the same pattern.
This similarity in various species of frogs is cited as an example
of “convergent evolution”. In spite of different evolutionary
histories, the various species of frogs have arrived at the same
pad structure. But even more interesting is a comparison of the
structure of the frog SAP with the SAP of Tettigonia viridissima,
a cricket. They’re practically identical! Talk about convergent
evolution. Nature seems to like a good design when it finds one.
One of the salient features of an SAP is its extreme softness.
This softness allows the SAP to conform closely to the
irregularities of the surface upon which the animal treads. The
researchers are of course interested in what the force of adhesion
is, that is, how strongly is our frog held to the surface by its
SAPs. It turns out that the adhesive force is pretty easy to
measure. Just put your frog on a platform and raise the platform
gradually from horizontal to vertical (90 degree angle) until the
platform is upside down (180 degrees). Note when the frog falls
off.
If you know the mass of the frog, the adhesive force is simply the
mass times the acceleration of gravity (a well known constant)
times the cosine of the angle at which the frog falls off the
platform. The stickiness of the frogs’ feet for 13 different frog
species was measured. The toe pad areas of the different size
frogs varied significantly and on a log-log plot the data fell on a
straight line. Roughly, a toe pad with an area ten times that of
another species had ten times the adhesive force holding that frog
on the platform. The data indicate the adhesive force arises from
capillary action and surface tension but I won’t go into that.
There doesn’t seem to have been as much excitement about
SAPs as about the hairy gecko story. However, the tire industry
has noted the similarity between the slits and grooves called
sipes in a tire and the structure of the animal SAPs. A tire wants
to drain the water quickly on a wet road in order to grip the road.
One tire made by Continental now has a honeycomb structure
tire especially designed for winter driving. Apparently, the
honeycomb-patterned sipes provide more gripping surfaces on
curves than other designs.
Some of you may be thinking, “If the force holding the frog or
gecko to the surface is so great, how does the animal walk?”
Hey, I have enough problems understanding the stickiness
without having to worry about the “release” aspect! Autumn and
crew have shown that the gecko peels its feet from the surface in
a relatively gradual manner so it doesn’t have to overcome the
adhesive force all at once. Let’s settle for that explanation for
this year, or until those wall-climbing robots are a reality.
Allen F. Bortrum
|
