Some things just won”t go away, but stick around longer than
you”d like. You may remember my last December”s $2500
estimated tax payment that was miscoded as $25 (mentioned here
a few weeks ago). Supposedly, my bank corrected the matter
back in May. Two days ago I received a call from the IRS
saying they still haven”t gotten the $2475! On the other hand,
where is stickiness when you need it? I”m approaching the
second anniversary next month of breaking my leg on the golf
course, caused by plastic “spikes” not sticking to a wet railroad
tie bordering a cart path. For those who may not already know, I
was pacing off the tee-to-green distance at the time so as to
repeat my hole-in-one on the same hole a couple years earlier!
One way to make something stick, for example, a thermometer
on a windowpane, is to provide it with a rubber or plastic suction
cup. You press the rubber cup (possibly wetted with some
water) firmly against the glass and it sticks for years. The reason
it sticks is that you”ve pushed out virtually all the air and now
you have atmospheric pressure pushing against the square inch or
so of surface of the suction cup. This is like having a14- to 15-
pound weight sitting on top of the cup.
Did you ever take a few microscope slides out of the box and
find that they tend to stick together pretty well? You”re in good
company. It seems that Galileo, back in the early 1600s, also
found that flat glass plates tend to stick together. I”ve just been
reading an article in the Summer issue of The Electrochemical
Society”s journal Interface. The article is on “wafer bonding”.
Having been out of the semiconductor field for a couple decades,
I apparently missed a new field of device fabrication. Wafer
bonding depends on the same effect noted by Galileo. Suppose
you want a device involving a silicon layer on an insulating
substrate. This is known in the trade as a silicon-on-insulator
(SOI) device, appropriately enough. I won”t go into detail on the
various types of SOI devices.
However, let”s suppose the insulator is glass. Unfortunately glass
doesn”t have a particularly crystalline structure, actually being
somewhat more like a liquid. This means we couldn”t use the
glass as a base for growing a single crystal layer of silicon, the
usual approach when silicon is grown on silicon. However, by
polishing a thin disk of silicon and a disk of glass until they”re
both very flat, the two disks will hold together quite firmly. A
little heating may then make the bonding even stronger for
making actual devices. According to the article, the forces
holding the two plates together are van der Waals forces.
What are these van der Waals forces? Suffice to say here that
when you bring molecules close enough together, there is a weak
attraction due to the way the electrons in the atoms interact. In
the same way, when you bring two polished plates of just about
any materials together there is this same kind of van der Waals
attraction. You are all quite familiar with van der Waals forces,
though you might not know it. The layers of graphite in your
lead pencil are also held together primarily by van der Waals
forces. These forces are so weak, however, that the layers of
graphite slide past each other onto your paper as you press down
on your pencil to write.
Those of you who live in or have visited southern climes, e.g.,
Florida, are quite familiar with those ubiquitous little lizards that
scurry up walls and even upside down on ceilings. Even if
you”re squeamish at first, you eventually learn to accept and even
appreciate their antics and perhaps also their taste for insects.
But you probably didn”t know (I certainly didn”t) that these little
lizards are also well acquainted with van der Waals forces. You
may have seen recent articles in the paper about a particular
lizard, the Tokay gecko. This little gecko is the subject of work
done by Kellar Autumn and Robert Full and their colleagues at
the University of California, Berkeley. My information cited
here is based on a Science article reporting on their work
published in the June 8 issue of Nature.
Not only can this gecko run around vertically, horizontally or
upside down, but also it can hang by a single toe pad. The
Science article likens this to hanging in midair by your fingertip!
If I had been asked to guess how the gecko accomplishes these
feats, I would have guessed that he or she had little suction cups
on their feet. Not so! Instead they have something that I
understand those of you of the feminine gender would give
anything to avoid – split ends! At least that”s what I would gather
from the TV commercials for various women”s hair care
products. How do I tell if I myself suffer from this dreadful
malady?
The Berkeley crew not only found that the gecko has about a half
million little hairs lined up in rows on it”s toe pads, but also that
each hair has around a thousand split ends! If my math is
correct, that works out to be half a billion little wispy fibers
sticking out of the feet of each gecko. Ok, now I”m guessing that
each little fiber has a mini suction cup, but again I”m wrong.
According to the article, various researchers have ruled out
suction, glues or electrostatic forces. Of course, the answer
seems to be van der Waals forces. The ends of the split ends are
attracted to the surface by these weak forces.
Amazingly, they”ve been able to remove a single hair from a
gecko”s foot and measure the force exerted by a single hair on a
sensing device. The hairs are too small to be seen with an
ordinary optical microscope. At first they didn”t find much force
but then they pushed down on the hair against the sensor surface
and dragged the hair along, much like the gecko itself would do.
Now they found forces that surprised them, being about 10 times
more than expected. With its thousand or so split ends, they
calculated a single hair could hold up an ant. That might not
sound too impressive but if they scaled up to a million hairs they
figured they could hold up a “small child”! I”m not sure how
much a “small child” weighs. However, based on my own
observations of the lizards in Florida, I guess I shouldn”t be
surprised that a few hundred thousand hairs are more than
enough to hold the gecko.
In actuality, not all the hairs are oriented in the so-called
”adhesive” direction but those that are do the job. The gecko
researchers are now trying to apply their results to the fabrication
of more lifelike robots and to develop new materials that could
mimic the gecko hairs. They envision a gecko-type tape, for
example, that would not employ an adhesive and would leave no
residue on untaping. Unfortunately the problem of trying to
come up with a fibrous material capable of forming a thousand
split ends is a formidable task. Apparently, the Berkeley workers
aren”t optimistic that gecko climbing gloves or shoes are in our
near future, if at all. Just think, if they were, we would have no
need for ladders, mountain climbing gear or the like. The
possibilities are endless!
WARNING: I was all set to post this column when I decided to
search “Tokay gecko” on the Internet. Whoa! This is not your
little Florida lizard. The adults get to be a foot long and are meat
eaters, with a tendency to bite! Their usual hangout is Southeast
Asia and these lizards are unusual in that they “bark”, the bark
sounding like their name, To-kay. None of this was mentioned
in the Science article. If you decide you”d like to have one of
these critters for a pet, I suggest you research their characteristics
a little more thoroughly! You”ll certainly need a goodly supply
of crickets to feed them.
Allen F. Bortrum
