Mechanical trapping of fine particles in a medium of mono‐sized randomly packed spheres

Mechanical trapping (or straining) of fine particles is a key mechanism in many filtration systems. For example, the performance of rapid sand filters depends in part on mechanical trapping of larger fine particles, while relying on adsorptive processes to trap very small fine particles and microbes. The ability to trap these particles is directly related to the construction of the packed bed used for filtration in this system. Thus, the ability to model the effect of the inner structure of the packed bed can lead to more efficient design for improved filtration. Because of its significant efficiency, gravitational sphere packing is employed in this work to simulate a bed of mono‐sized randomly packed spheres. The simulated bed provides a way to visualize the pore network and estimate the pore size distribution associated with the void space between particles. Furthermore, by subsequently introducing fine particles into the bed, we evaluate the mass‐rate of fine particles passing through and possibly saturating the packed bed. Results show that fine particles between 15% and 25% of the coarse particle size can be physically strained within the randomly packed bed. These results differ significantly from the results obtained assuming a periodically spaced bed. The technique therefore provides an efficient yet accurate alternative for understanding how fine particles pass through a coarse particulate medium. Copyright © 2014 John Wiley & Sons, Ltd.     

Why are the continents just so…?

Variations in gravitational potential energy contribute to the intraplate stress field thereby providing the means by which lithospheric density structure is communicated at the plate scale. In this light, the near equivalence in the gravitational potential energy of typical continental lithosphere with the mid‐ocean ridges is particularly intriguing. Assuming this equivalence is not simply a chance outcome of continental growth, it then probably involves long‐term modulation of the density configuration of the continents via stress regimes that are able to induce significant strains over geological time. Following this notion, this work explores the possibility that the emergence of a chemically, thermally and mechanically structured continental lithosphere reflects a set of thermally sensitive feedback mechanisms in response to Wilson cycle oscillatory forcing about an ambient stress state set by the mid‐ocean ridge system. Such a hypothesis requires the continents are weak enough to sustain long‐term (10⁸ years) strain rates of the order of ～10^?17 s^?1 as suggested by observations that continental lithosphere is almost everywhere critically stressed, by estimates of seismogenic strain rates in stable continental interiors such as Australia and by the low‐temperature thermochronological record of the continents that requires significant relief generation on the 10⁸ year time‐scale. Furthermore, this notion provides a mechanism that helps explain interpretations of recently published heat flow data that imply the distribution of heat‐producing elements within the continents may be tuned to produce a characteristic thermal regime at Moho depths.