The Cooperative Nature of Molecular Motion in Soft Matter: Finding Order in Disorder

Sharon C. Glotzer, National Institute of Standards and Technology

Soft materials and complex fluids exhibit novel properties that depend upon the degree and type of order present. Computer simulations have provided a wealth of information on these systems that elucidates phenomena, guides experimental investigation, and leads directly to new theories for soft matter. Molecular dynamics (MD) computer simulation in particular provides a unique window into the complex microscopic processes and ordering phenomena that control the properties and behavior of soft matter. As one example, we describe how MD simulations have uncovered the cooperative processes that give rise to novel ordering in liquids as they form glasses.

In dense atomic, molecular and polymeric fluids at low temperature, and in colloidal suspensions at high density, viscosity increases by many orders of magnitude over a narrow temperature or density range, and classical relations between transport coefficients break down. Our MD simulations have shown that underlying these phenomena are dynamics that become increasingly cooperative [1] and spatially heterogeneous [2] over a characteristic length scale which grows rapidly as the glass transition is approached, despite the fact that density and composition correlations remain short-ranged [3-8].

  MD simulation of a dense model glass-forming liquid showing the fastest 5% of particles in a given time frame when the liquid is (a) far from the glass transition and (b) closer to the glass transition.  In both cases, fast particles that are immediately next to each other, and are thus contained within the same "cluster", are shown by the same color.

One of the most striking predictions of our simulations is the emergence of a highly mobile subset of particles that move in "strings" and form ramified clusters which grow on cooling [3,4].  These ordered structures have now been directly observed in confocal microscopy experiments on colloidal liquids which form glasses at high density [9-12]. Based on the simulations, we have developed a statistical mechanical framework that allows us to quantify cooperative motion [5-8] and relate it to the decoupling of structural relaxation and particle transport [8].

In this talk, we describe this novel type of order that emerges in disordered systems, and demonstrate the unique role of simulation as a tool to identify the relevant quantities to measure. We outline open questions, and discuss the potential relevance of this work to applications ranging from information technology, food processing and sporting goods to nanotechnology. Through this example, we illustrate challenges and future directions in computational materials science of soft matter.

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[7] P. Ball, Nature 399, 207 (1999).
[8] S.C. Glotzer, V. Novikov and T.B. Schroeder, J. Chem. Phys. 112, 509 (2000).
[9] W. Kegel and A. van Blaaderen, Science 287, 5451 (2000).
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[11] P. Weiss, Science News, ``In glass, fast crowds boogie to brittle end,'' January 29, 2000.
[12] S. C. Glotzer, Physics World, 13, 22, 2000.