http://www.physorg.com/news101665888.html

Catching Waves: Measuring Self-Assembly in Action

Schematic of the monolayer self-assembly process studied by the NIST/NCSU
team. The silicon substrate is approximately 1 x 5 cm in dimensions. The
source (left) is a mixture of organosilane (OS) molecules and parafin oil
(to control the evaportion rate). The whole system is enclosed in a Petri
dish. The concentration of OS molecules is higher near the source and the
ordering process initiates near this region. Molecules behind the advancing
self-assembly front are relatively ordered, while molecules ahead of the
front are engulfed and incorporated as the front reaches them. The molecules
at the leading edge of the front are less ordered and this region becomes
broader as the front advances -- this is the key phenomenon measured in the
experiment. Credit: National Institute of Standards and Technology
By making careful observations of the growth of a layer of molecules as they
gradually cover the surface of a small silicon rectangle, researchers from
the National Institute of Standards and Technology and North Carolina State
University have gained basic insights into how self-propagating
self-assembly wave fronts develop and have produced the first experimental
verification of recently improved theoretical models of such systems.
In addition, the researchers say, the results reported in this week's
Proceedings of the National Academy of Science should be important to
understanding self-propagating chemical reactions and ordering and
self-assembly phenomena in situations involving confinement, such as thin
films and the porous internal geometries of many materials, such as rocks
and cement.

Systems that are transformed by so-called 'self-propagating' or
'autocatalytic' wave fronts actually are very common. Diverse molecular
processes, and even social processes and population dynamics, often can be
described in terms of fundamental 'entities' that undergo changes randomly
on an individual level, but at large scales exhibit some regular motion or
pattern formation as they collectively move from some unstable situation to
a relatively stable state. Mathematicians have developed a highly successful
basic model for such processes, called mean field theory.

The same basic equations describe, for example, the spread of advantageous
genes in an animal population, the growth of brain tumors, wound healing,
flame propagation, the spread of contagious epidemics, the spread of
Neolithic farming techniques, and chemical reaction fronts and nerve
propagation-all phenomena that grow outward on waves of change.

In recent years, simulations and theoretical arguments have suggested that
small fluctuations can significantly influence the advance of these
wavefronts as they grow. In the simple case of growth of a layer of
molecules by self-assembly, this would lead to a progressive roughening of
the interface between the ordered and disordered regions. This phenomenon is
completely missed in the classical mean field theory, raising important
questions about the applicability of these and philosophically similar
models to describe propagating fronts under general conditions.


To provide experimental verification of this phenomenon in a real system,
the NIST/NCSU team examined the spontaneous assembly of organosilane
molecules into a monolayer film on an oxidized silicon surface. If a supply
of the carbon-silicon-based molecules is placed along one edge of a treated
silicon wafer, under controlled conditions, the organosilane molecules
spontaneously organize themselves into a well-ordered layer, creating a
carpet-like layer on the silicon that advances from the edge of the wafer at
a constant velocity.

The technique involved was developed in the 1990s as a simple way to create
substrates with a gradually changing surface-energy gradient, a useful
experimental tool for surface scientists. The system lends itself to
high-resolution measurement because the process is slow enough to allow
highly precise, quantitative measurements of the layer as it advances using
a high-resolution synchrotron X-ray technique. The team found wavelike
ordering, as expected from classical theory, but with the interface of the
growing front broadened in time, as predicted by the recent theoretical
modeling, but in contrast with classical theory.

Citation: J.F. Douglas, K. Efimenko, D.A. Fischer, F.R. Phelan and J.
Genzer. Propagating waves of self-assembly in organosilane monolayers.
Proceedings of the National Academy of Science, 2007 104: 10324-10329.

Source: National Institute of Standards and Technology

-- 

Ken

"Buddhism elucidates why we are sentient."
"Buddhism follows thought throughout the Universe."
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