“Intermittent molecular dynamics in disordered porous media:
models versus experiments”
PHENIX lab, Sorbonne Université- CNRS, Paris, France
Disordered porous materials and colloidal suspensions are examples of interfacial systems where an internal surface partitions and fills the space in a complex way. Slurry and clay suspensions, soil and muds, fibers, textiles, coating, membranes for nanofiltration (blue energy), cement and concrete, plaster and oil reservoirs are some few examples of these complex interfacial materials. Most of these systems are saturated by a fluid. The level of saturation and the specific properties of this confined fluid strongly influence fluid transport, rheological and poromechanical characteristics of these materials. Various implications in catalyst, nanofiltration, life cycle and durability of various systems can be considered.
The fluid confinement in these restricted geometries involves on the same foot a proximal (or adsorption) layer and a distal region characterized by a bulky confinement in the pore network. The molecular trajectory can be described as an alternate succession of a surface adsorption step followed by an excursion in the confined bulk, generating a new relocation on the surface. As first approximation, the diffusive transport process can be approximated as an intermittent random walk sensitive to the interaction of the vicinal fluid with the interface and to the nature of the bulk confinement.
In the first part of this talk a theoretical analysis of this intermittent random walk dynamics is presented . Two groups of interfacial systems are analyzed: first, suspensions of colloidal particles or macro porous systems exhibiting a relatively weak bulk confinement but variable surface interactions. second, nanometer scale pore networks where excursion of the confined fluid is restricted. When available, Brownian or molecular dynamics simulations are presented. Finally, the possibility to probe this intermittent dynamic using NMR relaxometry is outlined and direct comparison between experimental results and intermittent random walk models discussed .
In the second part of this presentation, we propose some possible ways to upscale the fluid dynamics allowing decoupling the scale of the so-called “elementary pore” (which involves adsorption and relocation dynamics) and the long-range exploration, sensitive to the porous network structure. More specifically, in the exploration of a multiscale pore network, we analyze the possibility to quantify the exchange between different length scales using first passage statistics. This approach will be confronted to 2D NMR experiments.
Molecular intermittent dynamics of interfacial water: probing adsorption and bulk confinement, Levitz, P; Bonnaud, PA; Cazade, PA; Pellenq, RJM; Coasne, B. Soft Matter Volume: 9 Issue: 36 Pages: 8654−8663 (2013).
 Interfacial and Intermittent Dynamics of Water in Colloidal Systems as Probed by Fast Field-cycling Relaxometry. P. Levitz Field-cycling NMR Relaxometry; New ! Developments in NMR Series, Chap 12, editor R Kimmich, RSC, (2018)
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