June 25, 2007

NC State Engineers Provide Insight Into the Dynamics of Molecular Self-Assembly

  — Information may help to explain propagating wavefronts in many diverse fields, including materials sciences, spreading of diseases and language, and even urban sprawl

Bacterial growth, Crystallization, and Cancerous tumor growth. (Reprinted with permission from: E. Ben-Jacob, O. Shochet, A. Tenenbaum, I. Cohen, A. Czirok, T. Vicsek, “Response of bacterial colonies to imposed anisotropy”, Physical Review E 53, 1835-1843 (1996); V. Ferreiro, J.F. Douglas, J. Warren, A. Karim, “Growth pulsations in symmetric dendridic crystallization in thin polymer blend films,” Physical Review E 65, 051606 (2002); E. Khain, L.M. Sander, “Dynamics and pattern formation in invasive tumor growth”, Physical Review Letters 96, 188103 (2006).  Copyright 1996, 2002, and 2006 by the American Physical Society. ) Polymer dissolution. (Reprinted with permission from T. Ribar, R. Bhargava, J.L. Koenig, “FT-IR imaging of polymer dissolution by solvent mixtures: 1. Solvents,”Macromolecules 33, 8842-8849 (2000).  Copyright 2000 by the American Chemical Society.) Tissue growth & wound healing. (Reprinted with permission from P.K. Maini, D.L.S. McElwain, D. Leavesley, “Travelling waves in a wound healing assay,”Applied Mathematics Letters 17, 575-580 (2004).  Copyright 2004 by Elsevier.) RNA & virus replication: (Reprinted by permission from Y. Lee, J. Yin, “Detection of evolving viruses,” Nature Biotechnology 14, 491-493 (1996).  Copyright 1996 by Macmillan Publishers Ltd.) Chemical reaction waves. (Source:Apple.  Copyright Felice Frankel (Harvard University)) Spreading of language, disease & farming practices: source. (Source:Wikipedia.) City growth & urban sprawl. (Source: Cary Roberts, US Environmental Protection Agency.)

By studying how a layer of molecules grows into an ordered layer from the edge of a rectangular silicon wafer, engineers at North Carolina State University working with researchers from the National Institute of Standards and Technology (NIST) have established the time evolution of self-propagating self-assembly fronts. The team is the first to confirm the phenomenon in a real physical system.

The NC State researchers, Dr. Jan Genzer, professor of chemical and biomolecular engineering, and Dr. Kirill Efimenko, research assistant professor of chemical and biomolecular engineering, and NIST researchers, Dr. Jack Douglas, Dr. Daniel Fischer and Dr. Frederick Phelan, examined the spontaneous assembly of organosilane molecules into a monolayer film formed on an oxidized silicon surface.

They found that if a supply of the carbon-silicon-based molecule 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 where the ordering initiates, ultimately covering the surface at long times. By following this process using a high resolution synchrotron X-ray technique and using computer simulations, the NC State/NIST team established that the propagating wavefronts did not follow the constant width predicted by the classical mean-field theory (MFT) that is widely believed to govern reaction-diffusion and self-assembly processes. (A wavefront is the leading edge of a wave or line of points that have the same phase or stage in a process.) What actually occurred is described as a “power-law broadening in time” when an autocatalyst is present.

A paper describing the research appears in the 19 June 2007 issue of the Proceedings of the National Academy of Sciences (PNAS); the paper is also mentioned as a highlight on the journal cover.

“We found that simple diffusion is not going to explain our data; it will not govern the molecular processes involved in our experiment,” said Genzer. “So we began looking for the connection between diffusion and wavefront propagation, something the NIST team of theorist has been looking for some time.”

Propagating fronts are fueled by catalysts, which typically involve small chemical compounds that participate in the reaction and recover themselves after it. In the present case, the process may be governed by a very different type of autocatalyst, namely the system’s confinement. One of the benefits of the present set up is that the “broadening” of the front can be adjusted (and thus studied systematically) by varying the characteristics of the “building blocks”, the organosilane molecules, and the conditions under which the gradients are formed.

In addition to providing validation for simulation and theoretical predictions for precisely how these fronts should broaden, the researchers say that these results should be important to understanding self-organization in diverse other material processing and biological systems where similar fronts arise. Examples include (but are not limited to): material sciences, human health, social sciences, anthropology, and many others

“Processes in Nature follow just a few principles that apply to nanoscale, microscale and macroscale,” said Genzer. “Since pattern formation processes exist everywhere in the natural world, we believe the model can be used to verify dynamics in other (seemingly rather unrelated) phenomena such as the spread of disease, tumor growth, wound healing, the spread of epidemics, and the spread of languages across Europe.”

The research was funded by the National Science Foundation.

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Note to editors: An abstract of the paper follows.

“Propagating Waves of Self-Assembly in Organosilane Monolayers”

Authors: Jack F. Douglas, Daniel A. Fischer, Fredrick R. Phelan, Polymers and Ceramics Divisions, National Institute of Standards and Technology, Gaithersburg, Md.; Kirill Efimenko and Jan Genzer, Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh

Published June 19, 2007, in of the Proceedings of the National Academy of Sciences (PNAS),

Volume 104, No. 25, pp. 10324-10329.

Abstract:

Wavefronts associated with reaction–diffusion and self-assembly processes are ubiquitous in the natural world. For example, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes, in polymerization fronts involved in cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. Although it is often claimed that self-sustaining or autocatalytic front propagation is well described by mean-field ‘‘reaction–diffusion’’ or ‘‘phase field’’ ordering models, it has recently become appreciated from simulations and theoretical arguments

that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean-field theory (MFT) of this type of front propagation. The present work explores these fluctuation effects in a real physical system. In particular, we consider a high-resolution near-edge x-ray absorption fine structure spectroscopy (NEXAFS) study of the spontaneous frontal self-assembly of organosilane (OS) molecules into self-assembled monolayer (SAM) surface-energy gradients on oxidized silicon wafers. We find that these layers organize from the wafer edge as propagating wavefronts having well defined velocities. In accordance with two-dimensional simulations of this type of front propagation that take fluctuation effects into account, we find that the interfacial widths w(t) of these SAM self-assembly fronts exhibit a power-law broadening in time, w(t) t, rather than the constant width predicted by MFT. Moreover, the observed exponent values accord rather well with previous simulation and

theoretical estimates. These observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials-processing contexts.

Media contact:
Jennifer Weston, 919.515.3848, weston@ncsu.edu

Technical contact:
Dr. Jan Genzer, 919.515.2069, jan_genzer@ncsu.edu

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