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Dr. Bortrum

 

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11/27/2001

Pull Yourself Together

I''m sitting at my desk that I purchased a few years ago at a local
Staples store. After signing the credit card slip for the purchase,
I was surprised to find that the desk would be delivered in a box
rather than as a finished product. It was up to me to assemble the
box''s contents to approximate the desk on the showroom floor.
Of course, my surprise is a dead giveaway to my age, which
corresponds to a generation that expected things to be delivered
all in one piece. In the end, I opted to pay an extra sum of
money to have the desk assembled by a professional, thinking
that I wasn''t strong enough to get the box upstairs to my office,
let alone put it together.

I later purchased a two-drawer portable file to wheel next to the
desk. This time I felt confident that I could perform the self-
assembly required. Indeed, I was quite proud that it only took
me about four hours to accomplish the task. Ok, it took the pro
just one hour to put together the more complicated desk! These
days, self-assembly is the norm for all kinds of consumer items,
not just furniture. In recent years, self-assembly also has
blossomed in various fields of science and technology. This is
notably the case in the burgeoning field of nanotechnology, the
field of really tiny particles, devices and structures.

Just what is self-assembly? I found a definition on the MITRE
Corporation Web site that says it all: "Self-assembly is
coordinated action of independent entities under distributed (i.e.,
non-central) control to produce a larger structure or to achieve a
desired group effect." Had I known that self-assembly was so
complicated, I never would have attempted to self-assemble that
file cabinet!

Fortunately for us, Nature is not deterred by this definition.
Carbon and its compounds have this ability to self-assemble into
such useful items as cells, tissues and organs of the body. We
would be hard pressed to exist without these examples of self-
assembly! Of course, to accomplish something like the growth
of such biologically complex things as cells or organs requires
having just the right mix of chemicals and the right growth
conditions. Much of today''s stem cell research is aimed at
duplicating Nature''s accomplishments in self-assembly.

Understanding such complicated biological self-assembly is too
much for my feeble brain so I set out to find a simpler example.
I found an elegantly simple example on the Georgia Tech Web
site, which referred to work by Mohan Srinivasarao and
coworkers published in the April 6, 2001 issue of Science. I
thought to myself, "Great, I''ve probably thrown that issue out,
this being November." However, my procrastination paid off
when I found it right on top of my unread stack of journals. And,
when I picked it up, it opened spontaneously to page 79 and the
very article I was looking for. Obviously, Fate had decreed that
I should write about this subject!

The title of the article is "Three-Dimensionally Oriented Arrays
of Air Bubbles in a Polymer Film". Why should anyone care
about polymer films with orderly arrays of air bubbles? A very
simple practical application would be to use the holes left by the
air bubbles as very tiny beakers. Picture a sheet of plastic with
these tiny ''beakers" containing micro-drops of solutions to be
analyzed or drugs to be tested. With today''s sensitive analytical
techniques and micropipettes, this could well be a valuable
application. With bubble dimensions the wavelength of light, the
bubble arrays could also be of use for optical applications such as
using them to steer beams of light carrying data or other forms of
information. One might also use them to make porous metals by
depositing metal in the voids.

How to make such a structure? It''s amazingly simple. First,
dissolve a polymer such as polystyrene in a volatile solvent such
as benzene. (Polystyrene is a very common polymer - you''ve no
doubt used or come into contact with many polystyrene items.)
Next, take some of your solution and put a drop of it on a glass
slide. Now blow on the slide! I should have mentioned it also
has to be a humid day. When you''ve blown away all the
benzene, what''s left is a film with holes in it, the holes arranged
in rows in what''s called a hexagonal array. In what might be my
first attempt at graphics in these columns, the arrays look like

O O O O O O O
O O O O O O O O
O O O O O O O

If you look closely, you can spot groups of six holes forming
hexagons with another hole in the centers of the hexagons. Now
picture the holes as our little beakers.

I have to admit that I may have misled you. I''m not really sure
that just blowing on the slide will give you such an orderly array
but the principle is the same. Actually, the researchers control
the airflow more precisely and they also control the humidity in
the atmosphere around the glass slide. It only takes a few
seconds for the benzene to vaporize. Why the need for a humid
atmosphere? According to the Science article, the answer traces
back to observations by Lord Rayleigh back more than a hundred
years ago. And it does have something to do with blowing, or at
least exhaling.

What happens when you blow lightly on a cold windowpane or
on your glasses? They fog up with condensed moisture. Lord
Rayleigh studied those so-called "breath figures". What does
that have to do with our hexagonal array of holes? When the
benzene evaporates it takes up heat from the glass slide, which
cools as a result. The surface may cool as much as 40 degrees
Fahrenheit or so. The researchers propose that the cold surface
in the humid atmosphere causes water to condense on the
surface. The water self-assembles into hexagonal arrays of water
droplets. Why the droplets self-assemble in this way is
apparently not totally pinned down yet and a key problem still
being studied is why the droplets don''t come together to form
bigger drops. Some possible explanations are given in the paper
but I won''t belabor you with them now.

What about the holes? Remember the reason the water droplets
formed was the cooling of the surface by evaporating benzene.
When the benzene is all gone, the surface heats back up again
and the water itself evaporates, leaving behind the holes in the
polystyrene. The solid polystyrene, incidentally, forms when the
benzene solvent evaporates.

You can play tricks with the experimental conditions. For
example, by blowing the air faster or slower, you can change the
size of the water droplets and hence the size of the holes. Since
benzene is lighter (less dense) than water, the water droplets will
sink in the benzene and you can make films with layers of
bubbles in three dimensions, not just on the surface. If you use a
heavier solvent than benzene, the water droplets won''t sink and
you''ll only have holes on the surface of the polymer film. How
can they be so sure that water is involved at all? They''ve done
the same experiments under dry conditions and the polystyrene
forms in a solid layer with no holes.

This is just one of many examples of self-assembly that are being
pursued in labs all over the world these days. Just last week,
New Jersey''s Star Ledger had an article about Bell Labs
scientists Zhenan Bao and Hendrik Schon and their "teeny-tiny"
transistor that "builds itself". The article describes a transistor
in which the "channel", the gap between electrodes, is just one
molecule wide. That''s about a hundred thousand times thinner
than a human hair and is 10 or 20 times thinner than another
teeny transistor recently announced by Intel. The molecule is an
organic compound known as a thiol. Bao and Schon have
demonstrated that the thiol device can be switched on and off and
can be used as an amplifier, qualifying it as a transistor. How do
you construct a transistor like this? You don''t, you let it self-
assemble. Indeed, they call it a SAMFET, Self-Assembled
Molecular Field Effect Transistor.

These molecular transistors aren''t going to replace the good old
silicon chip for quite some time. For one thing, there''s the little
problem of wiring up circuits on a molecular level. It''s getting
hard enough on today''s silicon chips. However, so much work is
being spent on transistors made from organic molecules, DNA is
another example, that they just might be feasible in another
decade or two. If so, they could rescue Moore''s Law from
extinction when silicon transistors get so small they just can''t get
any smaller.

Well, it''s time for lunch and I think I''ll go downstairs and self-
assemble a meatloaf sandwich.

Allen F. Bortrum



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-11/27/2001-      
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Dr. Bortrum

11/27/2001

Pull Yourself Together

I''m sitting at my desk that I purchased a few years ago at a local
Staples store. After signing the credit card slip for the purchase,
I was surprised to find that the desk would be delivered in a box
rather than as a finished product. It was up to me to assemble the
box''s contents to approximate the desk on the showroom floor.
Of course, my surprise is a dead giveaway to my age, which
corresponds to a generation that expected things to be delivered
all in one piece. In the end, I opted to pay an extra sum of
money to have the desk assembled by a professional, thinking
that I wasn''t strong enough to get the box upstairs to my office,
let alone put it together.

I later purchased a two-drawer portable file to wheel next to the
desk. This time I felt confident that I could perform the self-
assembly required. Indeed, I was quite proud that it only took
me about four hours to accomplish the task. Ok, it took the pro
just one hour to put together the more complicated desk! These
days, self-assembly is the norm for all kinds of consumer items,
not just furniture. In recent years, self-assembly also has
blossomed in various fields of science and technology. This is
notably the case in the burgeoning field of nanotechnology, the
field of really tiny particles, devices and structures.

Just what is self-assembly? I found a definition on the MITRE
Corporation Web site that says it all: "Self-assembly is
coordinated action of independent entities under distributed (i.e.,
non-central) control to produce a larger structure or to achieve a
desired group effect." Had I known that self-assembly was so
complicated, I never would have attempted to self-assemble that
file cabinet!

Fortunately for us, Nature is not deterred by this definition.
Carbon and its compounds have this ability to self-assemble into
such useful items as cells, tissues and organs of the body. We
would be hard pressed to exist without these examples of self-
assembly! Of course, to accomplish something like the growth
of such biologically complex things as cells or organs requires
having just the right mix of chemicals and the right growth
conditions. Much of today''s stem cell research is aimed at
duplicating Nature''s accomplishments in self-assembly.

Understanding such complicated biological self-assembly is too
much for my feeble brain so I set out to find a simpler example.
I found an elegantly simple example on the Georgia Tech Web
site, which referred to work by Mohan Srinivasarao and
coworkers published in the April 6, 2001 issue of Science. I
thought to myself, "Great, I''ve probably thrown that issue out,
this being November." However, my procrastination paid off
when I found it right on top of my unread stack of journals. And,
when I picked it up, it opened spontaneously to page 79 and the
very article I was looking for. Obviously, Fate had decreed that
I should write about this subject!

The title of the article is "Three-Dimensionally Oriented Arrays
of Air Bubbles in a Polymer Film". Why should anyone care
about polymer films with orderly arrays of air bubbles? A very
simple practical application would be to use the holes left by the
air bubbles as very tiny beakers. Picture a sheet of plastic with
these tiny ''beakers" containing micro-drops of solutions to be
analyzed or drugs to be tested. With today''s sensitive analytical
techniques and micropipettes, this could well be a valuable
application. With bubble dimensions the wavelength of light, the
bubble arrays could also be of use for optical applications such as
using them to steer beams of light carrying data or other forms of
information. One might also use them to make porous metals by
depositing metal in the voids.

How to make such a structure? It''s amazingly simple. First,
dissolve a polymer such as polystyrene in a volatile solvent such
as benzene. (Polystyrene is a very common polymer - you''ve no
doubt used or come into contact with many polystyrene items.)
Next, take some of your solution and put a drop of it on a glass
slide. Now blow on the slide! I should have mentioned it also
has to be a humid day. When you''ve blown away all the
benzene, what''s left is a film with holes in it, the holes arranged
in rows in what''s called a hexagonal array. In what might be my
first attempt at graphics in these columns, the arrays look like

O O O O O O O
O O O O O O O O
O O O O O O O

If you look closely, you can spot groups of six holes forming
hexagons with another hole in the centers of the hexagons. Now
picture the holes as our little beakers.

I have to admit that I may have misled you. I''m not really sure
that just blowing on the slide will give you such an orderly array
but the principle is the same. Actually, the researchers control
the airflow more precisely and they also control the humidity in
the atmosphere around the glass slide. It only takes a few
seconds for the benzene to vaporize. Why the need for a humid
atmosphere? According to the Science article, the answer traces
back to observations by Lord Rayleigh back more than a hundred
years ago. And it does have something to do with blowing, or at
least exhaling.

What happens when you blow lightly on a cold windowpane or
on your glasses? They fog up with condensed moisture. Lord
Rayleigh studied those so-called "breath figures". What does
that have to do with our hexagonal array of holes? When the
benzene evaporates it takes up heat from the glass slide, which
cools as a result. The surface may cool as much as 40 degrees
Fahrenheit or so. The researchers propose that the cold surface
in the humid atmosphere causes water to condense on the
surface. The water self-assembles into hexagonal arrays of water
droplets. Why the droplets self-assemble in this way is
apparently not totally pinned down yet and a key problem still
being studied is why the droplets don''t come together to form
bigger drops. Some possible explanations are given in the paper
but I won''t belabor you with them now.

What about the holes? Remember the reason the water droplets
formed was the cooling of the surface by evaporating benzene.
When the benzene is all gone, the surface heats back up again
and the water itself evaporates, leaving behind the holes in the
polystyrene. The solid polystyrene, incidentally, forms when the
benzene solvent evaporates.

You can play tricks with the experimental conditions. For
example, by blowing the air faster or slower, you can change the
size of the water droplets and hence the size of the holes. Since
benzene is lighter (less dense) than water, the water droplets will
sink in the benzene and you can make films with layers of
bubbles in three dimensions, not just on the surface. If you use a
heavier solvent than benzene, the water droplets won''t sink and
you''ll only have holes on the surface of the polymer film. How
can they be so sure that water is involved at all? They''ve done
the same experiments under dry conditions and the polystyrene
forms in a solid layer with no holes.

This is just one of many examples of self-assembly that are being
pursued in labs all over the world these days. Just last week,
New Jersey''s Star Ledger had an article about Bell Labs
scientists Zhenan Bao and Hendrik Schon and their "teeny-tiny"
transistor that "builds itself". The article describes a transistor
in which the "channel", the gap between electrodes, is just one
molecule wide. That''s about a hundred thousand times thinner
than a human hair and is 10 or 20 times thinner than another
teeny transistor recently announced by Intel. The molecule is an
organic compound known as a thiol. Bao and Schon have
demonstrated that the thiol device can be switched on and off and
can be used as an amplifier, qualifying it as a transistor. How do
you construct a transistor like this? You don''t, you let it self-
assemble. Indeed, they call it a SAMFET, Self-Assembled
Molecular Field Effect Transistor.

These molecular transistors aren''t going to replace the good old
silicon chip for quite some time. For one thing, there''s the little
problem of wiring up circuits on a molecular level. It''s getting
hard enough on today''s silicon chips. However, so much work is
being spent on transistors made from organic molecules, DNA is
another example, that they just might be feasible in another
decade or two. If so, they could rescue Moore''s Law from
extinction when silicon transistors get so small they just can''t get
any smaller.

Well, it''s time for lunch and I think I''ll go downstairs and self-
assemble a meatloaf sandwich.

Allen F. Bortrum