Process-Dependent Solute Transport in Porous Media

Solute transport under single-phase flow conditions in porous micromodels was studied using high-resolution optical imaging. Experiments examined loading (injection of ink-water solution into a clear water-filled micromodel) and unloading (injection of clear water into an ink-water filled micromodel). Statistically homogeneous and fine-coarse porous micromodels patterns were used. It is shown that the transport time scale during unloading is larger than that under loading, even in a micromodel with a homogeneous structure, so that larger values of the dispersion coefficient were obtained for transport during unloading. The difference between the dispersion values for unloading and loading cases decreased with an increase in the flow rate. This implies that diffusion is the key factor controlling the degree of difference between loading and unloading transport time scales, in the cases considered here. Moreover, the patterned heterogeneity micromodel, containing distinct sections of fine and coarse porous media, increased the difference between the transport time scales during loading and unloading processes. These results raise the question of whether this discrepancy in transport time scales for the same hydrodynamic conditions is observable at larger length and time scales.

Dynamics of hydraulic and contractile wave-mediated fluid transport during Drosophila oogenesis

From insects to mice, oocytes develop within cysts alongside nurse-like sister germ cells. Prior to fertilization, the nurse cells’ cytoplasmic contents are transported into the oocyte, which grows as its sister cells regress and die. Although critical for fertility, the biological and physical mechanisms underlying this transport process are poorly understood. Here, we combined live imaging of germline cysts, genetic perturbations, and mathematical modeling to investigate the dynamics and mechanisms that enable directional and complete cytoplasmic transport in Drosophila melanogaster egg chambers. We discovered that during ‘nurse cell (NC) dumping’, most cytoplasm is transported into the oocyte independently of changes in myosin-II contractility, with dynamics instead explained by an effective Young-Laplace’s law, suggesting hydraulic transport induced by baseline cell surface tension. A minimal flow network model inspired by the famous two-balloon experiment and genetic analysis of a myosin mutant correctly predicts the directionality of transport time scale, as well as its intercellular pattern. Long thought to trigger transport through ‘squeezing’, changes in actomyosin contractility are required only once cell volume is reduced by ∼75%, in the form of surface contractile waves that drive NC dumping to completion. Our work thus demonstrates how biological and physical mechanisms cooperate to enable a critical developmental process that, until now, was thought to be a mainly biochemically regulated phenomenon.

On the Tachocline Zone Location in the Sun, the Luminosity Transport time scale, the Rotational Inertia and their Time Variation in Standard Solar Evolution Models

Studies with GONG Standard Solar Evolution Models sampling the evolution of the sun from its ZAMS stage show the following. The location of the tachocline zone is nearly fixed as it is not affected by shell burning although it co-moves with the expansion of the sun up to the present age of 4.6 Gyr. The luminosity transport time scale of the sun is entirely dominated by photon diffusion and during the evolution has decreased from over 204000 years to 187000 years. The rotational inertia of the sun shows a small gradual increase from