Glassy interfacial dynamics of Ni nanoparticles: part I Colored noise, dynamic heterogeneity and collective atomic motion

Abstract

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

Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films

“…A similar relation between the scale of cooperative motion, the interfacial mobility scale and the 'coloured noise' exponent describing particle displacement fluctuations was found in the glassy interfacial zone of Ni nanoparticles 50,51 ; however, this suggested relation has not been confirmed in ordinary glassforming liquids. Tanaka and coworkers 52 have also found a relationship between the interfacial mobility scale as we have defined it and a correlation length associated with clusters having high local order and a class of particles that is highly immobile.…”

Communication: Towards first principles theory of relaxation in supercooled liquids formulated in terms of cooperative motion

“…There are also implications of the strings for the fast dynamics of cooled liquids for understanding the origin of colored noise in the transport properties of cooled liquids. 28 Interestingly, the present derivation proceeds in the opposite order of the prior analyses that demonstrate the similarities between the mass distributions of strings and selfassembling particles. We begin here with the knowledge that the observed dynamical strings display mass distributions corresponding to equilibrium self-assembling particles.…”

A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films

“…Our recent measurements 57 suggest a direct relation between the string length and the interfacial mobility scale near the polymer-air-boundary of supported films and offer a promising method for estimating the string length. Moreover, recent work 116 also suggests that noise measurements might be effective for estimating average string length, and we plan to pursue this possibility in the future.…”