Tracer transport in attractive and repulsive supercooled liquids and glasses

Abstract

The transport of small penetrants through disordered materials with glassy dynamics is encountered in applications ranging from drug delivery to chemical separations. Nonetheless, understanding the influence of the matrix structure and fluctuations on penetrant motions remains a persistent challenge. Here, we use event-driven molecular dynamics to investigate the transport of small, hard-sphere tracers embedded in matrices of square-well particles. Short-range attractions between matrix particles give rise to reentrant dynamics in the supercooled regime, in which the liquid’s relaxation time increases dramatically upon heating or cooling. Heating results in a “repulsive” supercooled liquid where relaxations are frustrated by steric interactions between particles, whereas cooling produces an “attractive” liquid in which relaxations are hindered by long-lived interparticle bonds. Further cooling or heating, or compression, of the supercooled liquids results in the formation of distinct glasses. Our study reveals that tracer transport in these supercooled liquids and glasses is influenced by the matrix structure and dynamics. The relative importance of each factor varies between matrices and is examined in detail by analyzing particle mean-square displacements, caging behavior, and trajectories sampled from the isoconfigurational ensemble. We identify features of tracer dynamics that reveal the spatial and temporal heterogeneity of the matrices and show that matrix arrest is insufficient to localize tracers.

Hierarchical Structure and Dynamics of a Polymer/Nanoparticle Hybrid Displaying Attractive Polymer–Particle Interaction

“…In practice, the motion of particles in a polymer liquid often differs from the previously described ideal systems. The differences can be attributed to a range of different factors, for example, the attractive interactions between particles and polymer, 35,36 De Gennes narrowing 37 near the static structure factor peak for charge-stabilized particles, 38 the proximity of temperature to the polymer glass transition temperature, 39−41 and the effects from chain grafting to the particles for entropic stabilization. 41 Furthermore, for particles near the size of the tube diameter of entangled polymers, dilation of the entanglement tube has been observed.…”

Molecular and colloidal transport in bacterial cellulose biofilms

“…Entrapped particles within the structure form a Gaussian center and free particles form an exponential tail of the distribution, just as we see here. 26,57,58 . Diffusion of particles within actin filament solutions 59,60 , through cells 61 , and within colloidal suspensions 58,62 is also observed to be non-Gaussian due to the spatiotemporal heterogeneity of the environment, where the elasticity of the structure controls the behaviour of the sub-populations.…”

“…In random fiber networks, mesh size, alignment, and heterogeneity can lead to strong variations in microparticle transport rates [21][22][23] and localization 24,25 . Particle dynamics are coupled to any spatial heterogeneity of the matrix 26 , for example affecting drug transport in cancer tissue 21,27 as well as movement of soft particle medical delivery vehicles [28][29][30] . Complex biological network structures can vary significantly 31 , but diffusive transport can be measured in the matrix via molecular or colloidal tracer studies [32][33][34][35][36] .…”

Molecular and Colloidal Transport in Bacterial Cellulose Hydrogels

“…In random fiber networks, mesh size, alignment, and heterogeneity can lead to strong variations in microparticle transport rates − and localization. , Particle dynamics are coupled to any spatial heterogeneity of the matrix, for example, affecting drug transport in cancer tissue , as well as the movement of soft particle medical delivery vehicles. − Complex biological network structures can vary significantly, but diffusive transport can be measured in the matrix via molecular or colloidal tracer studies. − Diffusive transport in EPS biofilm networks has been studied to measure how rheology, , diffusive permeability, , and restructuring modulate transport in the matrix, , but cellulose biofilms have not been characterized in the same way. In this study, we use optical microscopy to track molecular and particulate tracers to characterize mobility within anisotropic, hierarchically structured bacterial cellulose films and show the diversity of natural structures and properties found there.…”