Steady state dynamic dependence between local mobility and non-affine fluctuations in two-dimensional aggregates

T Das and MM Bandi, JOURNAL OF PHYSICS-CONDENSED MATTER, 32, 214004 (2020).

DOI: 10.1088/1361-648X/ab6e94

Motivated by qualitative experimental observations in collective behavior of self-propelled camphor particles at air-water interfaces, we study a generic aggregate forming system in two dimensions using canonical ensemble constant temperature molecular dynamics simulation. The aggregates form due to the competition between short-range attraction and long-range repulsion of pair-wise interactions as a generic proxy for the specific case of short-range capillary attraction competing with long-range Marangoni-assisted repulsion in camphor boat systems. Choosing the appropriate set of interaction parameters, we focus on characterising the local dynamics in two specific limiting morphologies, viz. compact and string-like aggregates. We focus on the temporal evolution of the mobility of an individual particle and the dynamic change in its nearest neighbourhood, measured in terms of the Debye-Waller factor ((u) over bar (2)(i)) and the non-affine parameter (chi), respectively (both defined in the text), and their interrelation over several lengths of observation time tau(w). The distribution for both measures are found to follow the relation: P(x; tau(w)) similar to tau(w) (gamma)x for the measured quantity x. The exponent gamma is equal to two and one respectively, for the compact and string-like morphologies following the respective ideal fractal dimension of these aggregates. A functional dependence between these two observables is determined from a detailed statistical analysis of their joint and conditional distributions. The results obtained can readily be used and verified by experiments on aggregate forming systems more generic than the specific camphor boat system that motivated us, such as globular proteins, nanoparticle self-assembly etc. Further, the insights gained from this study might be useful to understand the evolution of collective dynamics in diverse glass-forming systems.

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