Particle-Size-Exclusion Clogging Regimes in Porous Media.

From observations of the progressive deposition of noncolloidal particles by geometrical exclusion effects inside a 3D model porous medium, we get a complete dynamic view of particle deposits over a full range of regimes from transport over a long distance to clogging and caking. We show that clogging essentially occurs in the form of an accumulation of elements in pore size clusters, which ultimately constitute regions avoided by the flow. The clusters are dispersed in the medium, and their concentration (number per volume) decreases with the distance from the entrance; caking is associated with the final stage of this effect (for a critical cluster concentration at the entrance). A simple probabilistic model, taking into account the impact of clogging on particle transport, allows us to quantitatively predict all these trends up to a large cluster concentration, based on a single parameter: the clogging probability, which is a function of the confinement ratio. This opens the route towards a unification of the different fields of particle transport, clogging, caking, and filtration.

Relationship between ultrastructure and biomechanical properties of the knee meniscus.

The purpose of this study was to determine the biomechanical properties of the knee meniscus and to relate them to its ultrastructure. The knee joint menisci are semicircular, fibrocartilaginous structures interposed between the femoral and tibial condyles. For a long time, they were considered to be embryologic vestiges. This study describes the response of the knee joint meniscus to circumferential, radial and axial compressive forces. The results show an anisotropic response of the knee joint meniscus to unconfined compression. The Young's modulus increased approximately twofold between vertical and circumferential or radial directions with a 10 mm/min-compression rate. This response is probably a direct consequence of the orientation of collagen fibres.

Dynamic stiffness and damping of porcine muscle specimens.

The aim of this study was to quantify the mechanical properties of the muscles of the buttock, using dynamic compression (5-->30 Hz). Tests were conducted in vitro on porcine muscles, using a lever arm device, which applied a dynamic load onto cylindrical samples. A two-parameter viscoelastic model allowed the calculation of stiffness and damping of the samples with respect to frequency. The average stiffness curve showed a monotonous increase (5 Hz: 8.5 kN/m-->30 Hz: 347 kN/m). Concerning damping, between 5 and 20 Hz, values were typically inferior to 300 Ns/m, which then increased till 30 Hz (556 Ns/m). The lever arm device may be used to evaluate dynamic properties of other biological tissues also.