Events

Jeremy Lechman, Sandia National Labs, will give a talk entitled "Modeling Transport Heterogeneity in Particle-based Porous Media" as part of the Claude R. Hocott Graduate Seminar Series.

Abstract:

Not surprisingly, materials of interest to many engineering applications are often far from ideal and homogeneous. Material inhomogeneity and heterogeneity arise due to the presence of multiple constituent species and phases along with associated meso-structure as well as smaller scale material microstructure: poly-crystallinity, grain boundaries and interfaces, defects, impurities, etc. These present themselves on a hierarchy of scales. Hence, the resulting variability of properties and related dynamical/stochastic transport processes across length and time scales creates significant challenges to predictive modeling and experimental characterization for applications such as material failure (ductile or brittle) and initiation and ignition of energetic materials and beyond. Modeling and simulation promise cost effective insight to the questions of multi-scale material properties and behavior and their relationship to manufacturing processes. In this talk, we will present recent efforts to develop computational tools, techniques and workflows for meso-to-macroscale modeling of transport processes. In particular we will focus on efforts to predict bulk conductive (thermal, electrical, etc.) properties of particulate-based heterogeneous materials. We explore diffusive transport through packs of monodisperse spheres near the jamming transition using simulations based on a random walk technique. The time dependence of various diffusion properties is resolved over several orders of magnitude, and two well-separated regimes of Fickian diffusion (MSD ~ t) are observed. The first corresponds to diffusion on scales smaller than individual particles, while the latter is the expected large-scale and long-time bulk diffusion. The intermediate anomalous diffusion regime and the long-time value of the diffusion coefficient are both shown to be controlled by the contact between particles, which in turn depends on the proximity to the critical jamming transition of the particle packs. Scaling laws are established for both the time required to recover Fickian diffusion as well as the long-time diffusivity. The mean first passage time (MFPT) associated with the escape of random walkers between neighboring particles is shown to control both as the jamming limit is approached. The behavior is not unique to systems near the jamming transition, but holds more generally for particle packs with vanishingly small interparticle contacts.

Bio:

Jeremy Lechman is a staff member in the Reactive and Nanoscale Processes department at Sandia National Labs and a graduate of Colorado School of Mines in Golden, CO. His technical work includes coarse-grained and meso-scale computational mechanics (discrete element, finite element, et al.) with applications to multiphase, particulate, and composite materials. His current interests are in bridging micro to meso to macro scales, developing continuum governing equations and constitutive models for transport processes in reactive heterogeneous materials, and understanding the relationship between manufacturing processes and the resultant material properties.