On two-dimensionalization of three-dimensional turbulence in shell models

Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model, the signatures of so-called two-dimensionalization effect of three-dimensional incompressible, homogeneous, isotropic fully developed unforced turbulence have been studied and reproduced. Within the framework of shell models we have obtained the following results: (i) progressive steepening of the energy spectrum with increased strength of the rotation, and, (ii) depletion in the energy flux of the forward forward cascade, sometimes leading to an inverse cascade. The presence of extended self-similarity and self-similar PDFs for longitudinal velocity differences are also presented for the rotating 3D turbulence case

Reaction Front in an A+B -> C Reaction-Subdiffusion Process

We study the reaction front for the process A+B -> C in which the reagents move subdiffusively. Our theoretical description is based on a fractional reaction-subdiffusion equation in which both the motion and the reaction terms are affected by the subdiffusive character of the process. We design numerical simulations to check our theoretical results, describing the simulations in some detail because the rules necessarily differ in important respects from those used in diffusive processes. Comparisons between theory and simulations are on the whole favorable, with the most difficult quantities to capture being those that involve very small numbers of particles. In particular, we analyze the total number of product particles, the width of the depletion zone, the production profile of product and its width, as well as the reactant concentrations at the center of the reaction zone, all as a function of time. We also analyze the shape of the product profile as a function of time, in particular its unusual behavior at the center of the reaction zone

Monitoring a reaction at submillisecond resolution in picoliter volumes

Contains fulltext : 92092.pdf (publisher's version ) (Open Access)7 p

Optical blocking of microfluidic droplets through laser-induced thermocapillarity

International audienceThe localized heating produced by a tightly focused infrared laser leads to surface tension gradients at the interface of microfluidic drops, resulting in a net force on the drop whose origin and magnitude are the focus of this paper. First, by colocalization of the surfactant micelles with a fluorescent dye, we demonstrate that the heating alters their spatial distribution, driving the interface out of thermodynamic equilibrium. This soluto-capillary effect opposes and overcomes the purely thermal dependence of the surface tension, leading to anomalous Marangoni flows. This sets the interface into motion and creates recirculation rolls outside and inside the drop, which we measure using time-resolved micro-Particle Image Velocimetry. Second, the net force produced on the drop is measured to be in the range of a few hundred nN by using an original microfluidic design. This micro-dynanometer further shows that the magnitude of the heating, which is determined by the laser power and its absorption in the water, sets the magnitude of the net force on the drop. On the other hand, the dynamics of the force generation is determined by the time scale for heating which is independently measured to be t? = 4 ms. This time scale sets the maximum velocity that the drops can have and still be blocked, by requiring that the interface pass the laser spot in a time longer than t?. The maximum velocity is measured at Umax = 0.7 mm/s for our geometric conditions. Finally, a simple model is derived that describes the blocking force in a confined geometry as the result of the viscous stresses produced between the drop and the lateral walls. © 2009 IEEE

Microscopic airway reopening through cascades of plugs ruptures

International audienceThe inner surface of lung airways is covered by a thin layer of mucus whose thickness is usually about 2 or 3 % of the total radius of the duct. However certain diseases like asthma, chronic bronchitis or allergies can induce a hypersecretion of mucus, leading to the formation of liquid plugs which occlude the airways. These plugs can considerably alter the distribution of air during the breathing cycle. It is therefore fundamental to understand the propagation of air in the presence of such plugs and in particular airway reopening. Some studies have been performed on real lungs but there was no visualization of the airways, and only information at the entrance was reported. The purpose of this experimental work is to create a synthetic network, reproducing only the main features of the lung airways, to visualize and understand the physics of airway reopening. The human lung is made of about 24 generations with diameters ranging from about 2 cm for the trachea to 100 µm for the smallest ones. As a consequence, the physics is very different for the first and the last generations. The present work focuses on the last micrometric generations for which inertia and gravity can be neglected (small Reynolds and Bond numbers). For this purpose a binary network made of PDMS was designed and fabricated. It is composed of 6 generations with a width of 700 µm for the first generation and a width ratio of 0.8 between the branches of successive generations. A random initial distribution of plugs is inserted inside this network by using syringe pumps and finally some air is introduced inside the airways. The reopening of the network takes place through a series of cascades of plugs ruptures. A single cascade can be explained by a simple model, based on the flow resistance of the plugs and the liquid deposited on the walls. The correlation between successive cascades is extracted from a careful analysis of the data. This study improves considerably our understanding of cascades of plug ruptures, which might be valuable to enhance the treatment of such diseases. Copyright © 2009 by ASME