Supercooled dynamics of grain boundary particles in two-dimensional colloidal crystals

Determining local geometrical features of grain boundaries from microscopy

Grain boundaries are solid-solid interfaces whose dynamics is driven by their local curvature. As they are fluctuating interfaces and have a width comparable to the lattice spacing of the surrounding grains, the determination of their local geometrical characteristics is difficult. Here we present a method to determine the local normal direction, tangent plane and curvature of grain boundaries from microscopy images using point sampled surface analysis techniques. We apply our algorithm to study the boundary of a shrinking grain in a two-dimensional colloidal polycrystalline material. Our method is easily generalized to three dimensions, which makes it applicable to the wide range of interfaces encountered in soft matter.

String-like cooperative motion in homogeneous melting

Despite the fundamental nature and practical importance of melting, there is still no generally accepted theory of this ubiquitous phenomenon. Even the earliest simulations of melting of hard discs by Alder and Wainwright indicated the active role of collective atomic motion in melting and here we utilize molecular dynamics simulation to determine whether these correlated motions are similar to those found in recent studies of glass-forming (GF) liquids and other condensed, strongly interacting, particle systems. We indeed find string-like collective atomic motion in our simulations of "superheated" Ni crystals, but other observations indicate significant differences from GF liquids. For example, we observe neither stretched exponential structural relaxation, nor any decoupling phenomenon, while we do find a boson peak, findings that have strong implications for understanding the physical origin of these universal properties of GF liquids. Our simulations also provide a novel view of "homogeneous" melting in which a small concentration of interstitial defects exerts a powerful effect on the crystal stability through their initiation and propagation of collective atomic motion. These relatively rare point defects are found to propagate down the strings like solitons, driving the collective motion. Crystal integrity remains preserved when the permutational atomic motions take the form of ring-like atomic exchanges, but a topological transition occurs at higher temperatures where the rings open to form linear chains similar in geometrical form and length distribution to the strings of GF liquids. The local symmetry breaking effect of the open strings apparently destabilizes the local lattice structure and precipitates crystal melting. The crystal defects are thus not static entities under dynamic conditions, such as elevated temperatures or material loading, but rather are active agents exhibiting a rich nonlinear dynamics that is not addressed in conventional "static" defect melting models.

Glassy Interfacial Dynamics of Ni Nanoparticles: Part I Colored Noise, Dynamic Heterogeneity and Collective Atomic Motion

Most condensed materials exhibit a significant fraction of atoms, molecules or particles that are strongly interacting with each other, while being configured geometrically at any instant of time in an 'amorphous' state having a relatively uniform density. Recently, both simulations and experiments have revealed that the dynamics of diverse condensed amorphous materials is generally characterized by significant heterogeneity in the local mobility and by progressively increasing collective motion upon cooling that takes the form of string-like collective particle rearrangements. The direct experimental observation of this type of collective motion, which has been directly linked to the growing relaxation times of glass-forming materials, and its quantification under different thermodynamic conditions, has so far been restricted to colloidal and driven granular fluids. The present work addresses the fundamental problem of how to determine the scale of this type of collective motion in materials composed of molecules or atoms. The basic premise of our work is that large scale dynamic particle clustering in amorphous materials must give rise to large fluctuations in particle mobility so that transport properties, especially those related to particle mobility, should naturally exhibit noise related to the cooperative motion scale. In our initial exploratory study seeking a relationship of this kind, we find 1/fα or 'colored noise', in both potential energy and particle displacements fluctuations of the atoms within the glassy interfacial layer of Ni nanoparticles (NPs). A direct relation between the particle displacement (mobility) noise exponent α and the average polymerization index of the string-like collective motion L is observed for a range of NP sizes, temperatures and for surface doping of the NPs with other metal atoms (Ag, Au, Pt) to change of fragility of the glassy interfacial layer at the surface of the Ni NPs. We also introduce a successful analytic model to understand this relationship between α and L.

Glassy Interfacial Dynamics of Ni Nanoparticles: Part II Discrete Breathers as an Explanation of Two-Level Energy Fluctuations

Recent studies of the dynamics of diverse condensed amorphous materials have indicated significant heterogeneity in the local mobility and a progressive increase in collective particle motion upon cooling that takes the form of string-like particle rearrangements. In a previous paper (Part I), we examined the possibility that fluctuations in potential energy E and particle mobility μ associated with this 'dynamic heterogeneity' might offer information about the scale of collective motion in glassy materials based on molecular dynamics simulations of the glassy interfacial region of Ni nanoparticles (NPs) at elevated temperatures. We found that the noise exponent associated with fluctuations in the Debye-Waller factor, a mobility related quantity, was directly proportional to the scale of collective motion L under a broad range of conditions, but the noise exponent associated with E(t) fluctuations was seemingly unrelated to L. In the present work, we focus on this unanticipated difference between potential energy and mobility fluctuations by examining these quantities at an atomic scale. We find that the string atoms exhibit a jump-like motion between two well-separated bands of energy states and the rate at which these jumps occur seems to be consistent with the phenomenology of the 'slow-beta' relaxation process of glass-forming liquids. Concurrently with these local E(t) jumps, we also find 'quake-like' particle displacements having a power-law distribution in magnitude so that particle displacement fluctuations within the strings are strikingly different from local E(t) fluctuations. An analysis of these E(t) fluctuations suggests that we are dealing with 'discrete breather' excitations in which large energy fluctuations develop in arrays of non-linear oscillators by virtue of large anharmonicity in the interparticle interactions and discreteness effects associated with particle packing. We quantify string collective motions on a fast caging times scale (picoseconds) and explore the significance of these collective motions for understanding the Boson peak of glass-forming materials.