Extensive molecular dynamics simulations are performed on a glass-forming Lennard-Jones mixture to determine the nature of the cooperative motions occurring in this model fragile liquid. We observe stringlike cooperative molecular motion ("strings") at temperatures well above the glass transition. The mean length of the strings increases upon cooling, and the string length distribution is found to be nearly exponential. [S0031-9007(98)05583-5] PACS numbers: 61.20.Lc, 61.43.Fs, 64.70.Pf The concept of cooperative molecular motion [1,2] is commonly invoked to rationalize dramatic changes in the transport properties of liquids as they are cooled toward their glass transition [3]. In the heuristic Adam and Gibbs model [1] of supercooled liquids, relaxation occurs through "cooperatively rearranging regions" which grow with decreasing temperature. A more rigorous treatment of collective motion in liquids by Zwanzig and Nossal emphasizes the occurrence of momentum density excitations whose lifetime grows as the temperature is lowered [4]. Modecoupling theory (MCT) [5] attributes the slowing down of particle motion at low temperatures to "backflow" collective particle motion which eventually causes a structural arrest of the liquid dynamics. However, there has been no direct experimental observation of these kinds of cooperative motion.Computer simulations offer advantages over experiments on real liquids for the investigation of collective particle motion. In molecular dynamics the position and velocity of all the particles are known at all times. As a consequence, correlation functions quantifying the motion of particular subsets of particles can be readily determined. Recent experiments [6,7] and simulations [8,9] have identified dynamical heterogeneity in supercooled liquids and spin glasses [10]. It is natural to suppose that cooperative motion might be associated with this dynamical heterogeneity. A well studied Lennard-Jones (LJ) system, recently introduced to study the dynamics of simple supercooled liquids, provides a particularly good model to test for cooperative motion, since its properties have been well characterized [11] and evidence for dynamical heterogeneity has already been identified for this model [9].In this Letter, we test whether cooperative molecular motion occurs in this model fragile glass-forming liquid. We find that molecular motion indeed becomes increasingly collective upon cooling. However, the regions involved in this motion are not compact, as usually supposed [1,12], but instead form stringlike structures. The average string length increases with decreasing temperature, and the string length distribution is nearly exponential.We performed extensive molecular dynamics simulations of a three dimensional binary mixture (80:20) of 8000 LJ particles where the interaction parameters [13] are chosen to prevent demixing and crystallization [11]. Ten temperatures T between 0.550 and 0.451 above the fitted mode-coupling temperature T c ഠ 0.431 [11] are studied. We emphasize that this temperatu...

It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.

In order to better understand the physical basis of the biological activity of nanoparticles (NPs) in nanomedicine applications and under conditions of environmental exposure, we performed an array of photophysical measurements to quantify the interaction of model gold NPs having a wide range of NP diameters with common blood proteins. In particular, absorbance, fluorescence quenching, circular dichroism, dynamic light scattering, and electron microscopy measurements were performed on surface-functionalized water-soluble gold NPs having a diameter range from 5 to 100 nm in the presence of common human blood proteins: albumin, fibrinogen, gamma-globulin, histone, and insulin. We find that the gold NPs strongly associate with these essential blood proteins where the binding constant, K, as well as the degree of cooperativity of particle--protein binding (Hill constant, n), depends on particle size and the native protein structure. We also find tentative evidence that the model proteins undergo conformational change upon association with the NPs and that the thickness of the adsorbed protein layer (bare NP diameter <50 nm) progressively increases with NP size, effects that have potential general importance for understanding NP aggregation in biological media and the interaction of NP with biological materials broadly.

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