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Horsch MA, Iacovella CR, Zhang ZL, Glotzer SC
Self-organization of nanoscopic building blocks into ordered assemblies
Mat. Res. Soc. Symp. Proc. Vol. 818, 2004
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Glotzer SC, Horsch MA, Lamm MH, Iacovella CR, Zhang Z-L
Teaching Computational Materials Science for Nanoscale Science and Engineering
Mat. Res. Soc. Symp. Proc. Vol. 827E, 2004
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Horsch MA, Zhang ZL, Glotzer SC
Tethered Nanoparticles: A New Class of “Macromolecule” for Bio-inspired Materials
Mat. Res. Soc. Symp. Proc. Vol. EXS-1, 2004
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Vogel M, Glotzer SC
Temperature dependence of spatially heterogeneous dynamics in a model of viscous silica
PHYSICAL REVIEW E 70 (6): Art. No. 061504 Part 1, DEC 2004
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Molecular dynamics simulations are performed to study spatially heterogeneous dynamics in a model of viscous silica above and below the critical temperature of the mode coupling theory, T-MCT. Specifically, we follow the evolution of the dynamic heterogeneity as the temperature dependence of the transport coefficients shows a crossover from non-Arrhenius to Arrhenius behavior when the melt is cooled. It is demonstrated that, on intermediate time scales, a small fraction of oxygen and silicon atoms are more mobile than expected from a Gaussian approximation. These highly mobile particles form transient clusters larger than that resulting from random statistics, indicating that the dynamics are spatially heterogeneous. An analysis of the clusters reveals that the mean cluster size is maximum at times intermediate between ballistic and diffusive motion, and the maximum size increases with decreasing temperature. In particular, the growth of the clusters continues into the crossover to Arrhenius behavior for the transport coefficients. These findings imply that the structural relaxation in silica cannot be understood as a statistical bond breaking process. Although the mean cluster sizes for silica are at the lower end of the spectrum of values reported in the literature for other model liquids, we find that spatially heterogeneous dynamics in models of strong and fragile glass formers are similar on a qualitative level. However, unlike the results for fragile liquids, we show that correlated particle motion along quasi-one-dimensional, stringlike paths is of little importance for the structural relaxation in this model of silica, suggesting that stringlike motion is suppressed by the presence of a network structure. To study transient clusters of localized particles, we calculate a generalized susceptibility corresponding to the self-part of a four-point time dependent density correlation function. We find that this generalized susceptibility is maximum on the time scale of the structural relaxation, where a strong increase of the peak height indicates a growing length of spatial correlations between localized particles upon cooling. Characterizing the local structural environments of the most mobile and the most immobile particles, respectively, we show that high particle mobility is facilitated by, but not limited to, the vicinity of defects in the network structure.
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Lacevic N, Glotzer SC
Dynamical heterogeneity and jamming in glass-forming liquids
JOURNAL OF PHYSICAL CHEMISTRY B 108 (51): 19623-19633 DEC 23 2004
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The relationship between spatially heterogeneous dynamics (SHD) and jamming is studied in a glass-forming binary Lennard-Jones system via molecular dynamics simulations. It has been suggested [Phys. Rev. Lett. 2001, 86, 111](1) that the probability distribution of interparticle forces P(F) develops a peak at the glass transition temperature T-g and that the large force inhomogeneities, responsible for structural arrest in granular materials, are related to dynamical heterogeneities in supercooled liquids that form glasses. It has been further suggested that "force chains" present in granular materials may exist in supercooled liquids and may provide an order parameter for the glass transition. Our goal is to investigate the extent to which the forces experienced by particles in a glass-forming liquid are related to SHD and compare these forces to those observed in granular materials and other glass-forming systems. Our results are summarized as follows. We calculate P(F) for positive (repulsive) instantaneous forces and find no peak in P(F) at any temperature in our system, even below T-g. We also find that particles that have been localized for a long time are less likely to experience high relative force and that mobile particles experience higher relative forces at shorter time scales, indicating a correlation between pairwise forces and particle mobility. We construct force chains based on the magnitude of pairwise positive instantaneous forces. We find that force chains constructed in this manner are composed of both localized and mobile particles; therefore there is no one-to-one correspondence between force chains as defined here and locally mobile or immobile regions of the liquid. We also find that force chains do not play the same role as force chains in granular materials but may indicate a difference in the evolution of the local environment of particles with different mobility. We also discuss a possible relationship between force chains found here and the development of stringlike motion found in this and other glass-forming liquids [Phys. Rev. Lett. 1998, 80, 2338; J. Chem. Phys. 2004, 120, 4415].
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Glotzer SC, Solomon MJ, Kotov NA
Self-assembly: From nanoscale to microscale colloids
AICHE JOURNAL 50 (12): 2978-2985 DEC 2004
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Horsch MA, Zhang ZL, Iacovella CR, Glotzer SC
Hydrodynamics and microphase ordering in block copolymers: Are hydrodynamics required for ordered phases with periodicity in more than one dimension?
JOURNAL OF CHEMICAL PHYSICS 121 (22): 11455-11462 DEC 8 2004
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We use Brownian dynamics (BD), molecular dynamics, and dissipative particle dynamics to study the phase behavior of diblock copolymer melts and to determine if hydrodynamics is required in the formation of phases with greater than one-dimensional periodicity. We present a phase diagram for diblock copolymers predicted by BD and provide a relationship between the inverse dimensionless temperature epsilon/k(B)T and the Flory-Huggins chi parameter, allowing for a quantitative comparison between methods and to mean field predictions. Our results concerning phase behavior are in good qualitative agreement with the theoretical predictions of Matsen and Bates [M. W. Matsen and F. S. Bates, Macromolecules 29, 1091 (1996)]; however, fluctuation effects arising from finite polymer lengths substantially alter the phase boundaries. Our results pertaining to the hydrodynamics are in contrast to earlier work by Groot [R. D. Groot, T. J. Madden, and D. J. Tildesley, J. Chem. Phys. 110, 9739 (1999); D. Frenkel and B. Smit, Understanding Molecular Simulation, 2nd ed. (Academic, New York, 2001)]. In particular, we obtain the hexagonal ordered cylinder phase with BD, a method that does not include hydrodynamics.
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Glotzer SC
Some assembly required
SCIENCE 306 (5695): 419-420 OCT 15 2004
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Materials science has traditionally involved the study of the self-assembly of atoms or molecules into bulk thermodynamic phases. In her Perspective, Glotzer predicts that this field will be revolutionized by new materials made from larger, more complex building blocks designed to assemble in specific ways. These building blocks allow potentially much greater control over a material's properties, enabling the design rather than the selection of materials for specific uses.
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Albano F, Lacevic N, Falk ML, Glotzer SC
Relating metallic glass mechanical properties to liquid structure
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 375: 671-674 Sp. Iss. SI, JUL 15 2004
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Molecular dynamics simulations indicate that the mechanical properties of a model metallic glass depend critically on the degree of supercooling attained in the liquid prior to vitrification. A well-characterized binary glass-forming system roughly analogous to ZrTi was quenched instantaneously from a series of liquid temperatures. These temperatures ranged from approximately twice that of the onset of supercooled liquid behaviour down to just above the mode-coupling temperature (T-MCT). The low-temperature mechanical properties of these glass samples is studied in simple shear at constant strain rate while the density of the system is held constant. All the glasses exhibit an initial linear response at small strain and approached a steady-state flow regime at large strain. However, glasses obtained by quenching from supercooled liquids just above T-MCT, which we will refer to as T-MCT glasses, exhibit higher shear strength and modulus than glasses obtained by quenching from higher temperature liquids. The T-MCT glasses also exhibit pronounced shear softening. The change in pressure during shear is markedly different in these glasses, and indicates that the T-MCT glasses retain aspects of the quenched-in structure out to as much as 25 train. The changes in pressure also indicate significant structural changes occur even during nearly linear stress-strain response.
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Zhang ZL, Glotzer SC
Self-assembly of patchy particles
NANO LETTERS 4 (8): 1407-1413 AUG 2004
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Molecular simulations are performed to study the self-assembly of particles with discrete, attractive interaction sites - "patches" - at prescribed locations on the particle surface. Chains, sheets, rings, icosahedra, square pyramids, tetrahedra, and twisted and staircase structures are obtained through suitable design of the surface pattern of patches'. Our simulations predict that the spontaneous formation of two-dimensional sheets and icosahedra occurs via a first-order transition while the formation of chains occurs via a continuous disorder-to-order transition as in equilibrium polymerization. Our results show how precise arrangements of patches combined with patch "recognition" or selectivity may be used to control the relative position of particles and the overall structure of particle assemblies. In this context, patchy particles represent a new class of building block for the fabrication of precise structures.
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Chen T, Lamm MH, Glotzer SC
Biomolecule-directed assembly of nanoscale building blocks studied via lattice Monte Carlo simulation
JOURNAL OF CHEMICAL PHYSICS 121 (8): 3919-3929 AUG 22 2004
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We perform lattice Monte Carlo simulations to study the self-assembly of functionalized inorganic nanoscale building blocks using recognitive biomolecule linkers. We develop a minimal coarse-grained lattice model for the nanoscale building block (NBB) and the recognitive linkers. Using this model, we explore the influence of the size ratio of linker length to NBB diameter on the assembly process and the structural properties of the resulting aggregates, including the spatial distribution of NBBs and aggregate topology. We find the constant-kernel Smoluchowski theory of diffusion-limited cluster-cluster aggregation describes the aggregation kinetics for certain size ratios.
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Vogel M, Glotzer SC
Spatially heterogeneous dynamics and dynamic facilitation in a model of viscous silica
PHYSICAL REVIEW LETTERS 92 (25): Art. No. 255901 JUN 25 2004
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We perform molecular dynamics simulations to study the structural relaxation dynamics of a model of viscous silica, the prototype of a strong glass former. We find that the melt dynamics are spatially heterogeneous regardless of whether the bulk relaxation is non-Arrhenius or Arrhenius, and cannot be understood as a statistical bondbreaking process. Further, we show that stringlike motion is suppressed by the covalent bondings, yet high particle mobility propagates continuously, supporting the concept of dynamic facilitation emphasized in recent theoretical work.
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Gebremichael Y, Vogel M, Glotzer SC
Formation of transient clusters on nanoscopic length scales in a simulated one-component supercooled liquid
MOLECULAR SIMULATION 30 (5): 281-287 APR 2004
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Dense liquids above their glass transition exhibit spatially heterogeneous dynamics in which regions within the liquid exhibit enhanced or diminished mobility relative to the average on some time scale. Substantial evidence suggests that these regions are confined to nanoscopic sizes at temperatures close to the mode coupling temperature T MCT , and reach well beyond the characteristic length scale over which the two-point static structure of the liquid is correlated. In this paper, we investigate the formation of clusters of mobile particles in a dynamically heterogeneous model liquid. We find that clusters are formed as a result of mobility propagation that begins from distributed locations confined within a nanoscopic local structure. This mobility is facilitated through the development of quasi-one dimensional string-like rearrangements within the nanoscopic region.
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Aichele M, Gebremichael Y, Starr FW, Baschnagel J, Glotzer SC
Polymer-specific effects of bulk relaxation and stringlike correlated motion in the dynamics of a supercooled polymer melt (vol 119, pg 5290, 2003)
JOURNAL OF CHEMICAL PHYSICS 120 (14): 6798-6798 APR 8 2004
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Particle rearrangements during transitions between local minima of the potential energy landscape of a binary Lennard-Jones liquid
Vogel M, Doliwa B, Heuer A, Glotzer SC
JOURNAL OF CHEMICAL PHYSICS 120 (9): 4404-4414 MAR 1 2004
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The potential energy landscape (PEL) of binary Lennard-Jones (BLJ) mixtures exhibits local minima, or inherent structures (IS), which are organized into metabasins (MBs). We study the particle rearrangements related to transitions between both successive IS and successive MB for a small 80:20 BLJ system near the mode-coupling temperature T-MCT. The analysis includes the displacements of individual particles, the localization of the rearrangements, and the relevance of string-like motion. We find that the particle rearrangements during IS and MB transitions do not change significantly at T-MCT. In particular, an onset of single particle hopping on the length scale of the interparticle distance is not observed. Further, it is demonstrated that IS and MB dynamics are spatially heterogeneous and facilitated by string-like motion. To investigate the mechanism of string-like motion, we follow the particle rearrangements during suitable sequences of IS transitions. We find that most strings observed after a series of transitions do not move coherently during a single transition, but subunits of different sizes are active at different times. Several findings suggest that, though string-like motion is of comparable relevance when the system explores a MB and when it moves from one MB to another, the occurrence of a successful string enables the system to exit a MB. Moreover, we show that the particle rearrangements during two consecutive MB transitions are basically uncorrelated. In particular, different groups of particles are highly mobile. We further find the positions of strings during successive MB transitions weakly but positively correlated, supporting the idea of dynamic facilitation. Finally, the relation between the features of the potential energy landscape and the relaxation processes in supercooled liquids is discussed.
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Gebremichael Y, Vogel M, Glotzer SC
Particle dynamics and the development of string-like motion in a simulated monoatomic supercooled liquid
JOURNAL OF CHEMICAL PHYSICS 120 (9): 4415-4427 MAR 1 2004
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The microscopic details of local particle dynamics is studied in a glass-forming one component supercooled liquid modeled by a Dzugutov potential developed for simple metallic glass formers. Our main goal is to investigate particle motion in the supercooled liquid state, and to ascertain the extent to which this motion is cooperative and occurring in quasi-one-dimesional, string-like paths. To this end we investigate in detail the mechanism by which particles move along these paths. In particular, we show that the degree of coherence-that is, simultaneous motion by consecutive particles along a string-depends on the length of the string. For short strings, the motion is highly coherent. For longer strings, the motion is highly coherent only within shorter segments of the string, which we call "microstrings." Very large strings may contain several microstrings within which particles move simultaneously, but individual microstrings within a given string are temporally uncorrelated with each other. We discuss possible underlying mechanism for this complex dynamical behavior, and examine our results in the context of recent work by Garrahan and Chandler [Phys. Rev. Lett. 89, 035704 (2002)] in which dynamic facilitation plays a central role in the glass transition.