Kinematic Model of Transient Shape-Induced Anisotropy in Dense Granular Flow

Nonspherical particles are ubiquitous in nature and industry, yet previous theoretical models of granular media are mostly limited to systems of spherical particles. The problem is that in systems of nonspherical anisotropic particles, dynamic particle alignment critically affects their mechanical response. To study the tendency of such particles to align, we propose a simple kinematic model that relates the flow to the evolution of particle alignment with respect to each other. The validity of the proposed model is supported by comparison with particle-based simulations for various particle shapes ranging from elongated rice-like (prolate) to flattened lentil-like (oblate) particles. The model shows good agreement with the simulations for both steady-state and transient responses, and advances the development of comprehensive constitutive models for shape-anisotropic particles.

Production of Macrophage Inhibitory Factor (MIF) by Primary Sertoli Cells; Its Possible Involvement in Migration of Spermatogonial Cells

Macrophage migration inhibitory factor (MIF) is a multifunctional molecule. MIF was originally identified as a T-cell-derived factor responsible for the inhibition of macrophage migration. In testicular tissue of adult rats, MIF is constitutively expressed by Leydig cells under physiological conditions. The aim of this study was to examine MIF levels in testicular homogenates from different aged mice, and the capacity of Sertoli cells to produce it. We also examined MIF involvement in spermatogonial cell migration. Similar levels of MIF protein were detected in testicular homogenates of mice of different ages (1-8-week-old). However, the RNA expression levels of MIF were high in 1-week-old mice and significantly decreased with age compared to 1-week-old mice. MIF was stained in Sertoli, Leydig cells, and developed germ cells in the seminiferous tubules. Isolated Sertoli cells from 1-week-old mice stained to MIF. Cultures of Sertoli cells from 1-week-old mice produced and expressed high levels of MIF which significantly decreased with age. MIF was localized in the cytoplasm and nucleus of Sertoli cell cultures isolated from 1-week-old mice; however, it was localized only in the cytoplasm and branches of cultures isolated from 8-week-old mice. MIFR was detected in GFRα1 and Sertoli cells. MIF could induce migration of spermatogonial cells, and this effect was synergistic with glial cell-line neurotrophic factor. Our results show, for the first time, the capacity of Sertoli cells to produce MIF under normal conditions and that MIFR expressed in GFRα1 and Sertoli cells. We also showed that MIF induced spermatogonial cell migration. J. Cell. Physiol. 232: 2869-2877, 2017. © 2016 Wiley Periodicals, Inc.

Eddy viscosity in dense granular flows

We present a seminal set of experiments on dense granular flows in the stadium shear geometry. The advantage of this geometry is that it produces steady shear flow over large deformations, in which the shear stress is constant. The striking result is that the velocity profiles exhibit an S shape, and are not linear as local constitutive laws would predict. We propose a model that suggests this is a result of wall perturbations which span through the system due to the nonlocal behavior of the material. The model is analogous to that of eddy viscosity in turbulent boundary layers, in which the distance to the wall is introduced to predict velocity profiles. Our findings appear pivotal in a number of experimental and practical situations involving dense granular flows next to a boundary. They could further be adapted to other similar materials such as dense suspensions, foams, or emulsions.

Thermal transients and convective particle motion in dense granular materials

The mechanism of dry granular convection within dense granular flows is mostly neglected by current analytical heat equations describing such materials, for example, in geophysical analyses of shear gouge layers of earthquake and landslide rupture planes. In dry granular materials, the common assumption is that conduction by contact overtakes any other mode of heat transfer. Conversely, we discover that transient correlated motion of heated grains can result in a convective heat flux normal to the shear direction up to 3-4 orders magnitude larger than by contact conduction. Such a thermal efficiency, much higher than that of water, is appealing and might be common to other microscopically structured fluids such as granular pastes, emulsions, and living cells.

Internal relaxation time in immersed particulate materials

We study the dynamics of the static-to-flow transition in a model material made of elastic particles immersed in a viscous fluid. The interaction between particle surfaces includes their viscous lubrication, a sharp repulsion when they get closer than a tuned steric length, and their elastic deflection induced by those two forces. We use soft dynamics to simulate the dynamics of this material when it experiences a step increase in the shear stress and a constant normal stress. We observe a long creep phase before a substantial flow eventually establishes. We measure the change in volume (dilatancy) and find that during the creep phase, it does not change significantly. We find that the typical creep time relies on an internal relaxation process, namely, the separation of two particles driven by the applied stress and resisted by the viscous friction. The present mechanism should be relevant for granular pastes, living cells, emulsions, and wet foams.

Force attractor in confined comminution of granular materials

We reveal a novel attractor in the space of contact forces that bounds the behavior of granular materials during confined comminution. The attractor is reached asymptotically as the porosity reduces and the grain size distribution attains an ultimate power law scaling. The ultimate distribution of the contact forces follows a clear log-normal distribution, distinctively different from previous observations in uncrushable systems. Supporting evidence comes both from comprehensive discrete element simulations and a theoretical Apollonian model.