Simulation of tracer particle diffusion in attractive and repulsive glassy matrices

Anomalous transport in confined systems is most commonly associated with particle sub-diffusion. Characterization of confined particle dynamics has been extended to matrices of different topologies, including ordered and... [ view full abstract ]

Anomalous transport in confined systems is most commonly associated with particle sub-diffusion. Characterization of confined particle dynamics has been extended to matrices of different topologies, including ordered and disordered particles; the effects of slow relaxations of the confining matrix, however, have received comparatively little attention. Physical systems in which this scenario arises include colloidal gels¹ and polymer nanocomposites,² which are used in a variety of technological applications. A matrix of particles exhibiting glassy dynamics provides a convenient model in which to explore the effect of slow structural relaxations on the dynamics of strongly confined tracer particles. In the limit of infinite tracer dilution, the motions of confined tracers minimally affect the matrix relaxation; hence characterization of tracer dynamics may provide insights into the structural relaxation processes of glassy matrices, whose underlying physical mechanisms remain the subject of vigorous debate.

We use event-driven molecular dynamics to scrutinize the diffusion of tracer particles confined within “attractive” and “repulsive” glassy matrices formed from a well-characterized, bi-disperse system of colloidal spheres with hard cores.³ Fixing the volume fraction of both the tracer particles and glassy matrix, the temperature is modulated to confine tracer particles within ‘repulsive’ high-temperature and ‘attractive’ low-temperature glassy matrices. These two broad types of glasses are dominated by different relaxation dynamics and mechanisms.⁴ Here, we show that these distinct relaxation processes strongly influence the dynamics and trajectories of the tracer particles. By varying the size of the tracer relative to that of a matrix particle, we investigate the role of the size asymmetry on the particle trajectories and dynamics. Because dispersing particles within slowly-relaxing matrices with varying interactions appear in settings ranging from the crowded cytoplasm inside cells to natural soils in the environment to artificial fiber nanocomposites, these results provide insight into the coupling between particle transport and matrix dynamics across a wide range of scientifically and technologically relevant processes.

• Babu, S., Gimel, J. C. & Nicolai, T. Tracer diffusion in colloidal gels. J. Phys. Chem. B 112, 743–748 (2008).
• Kalathi, J. T., Yamamoto, U., Schweizer, K. S., Grest, G. S. & Kumar, S. K. Nanoparticle diffusion in polymer nanocomposites. Phys. Rev. Lett. 112, 108301 (2014).
• Zaccarelli E. and Poon W., Colloidal glasses and gels: the interplay of bonding and caging, PNAS. 106, 36 (2009).
• K. N. Pham, et. al., Multiple glassy states in a simple model system. Science, 296,104–106, (2002).