RESUMO
We explain the rotating polygon instability on a swirling fluid surface [G. H. Vatistas, J. Fluid Mech. 217, 241 (1990) and Jansson et al., Phys. Rev. Lett. 96, 174502 (2006)] in terms of resonant interactions between gravity waves on the outer part of the surface and centrifugal waves on the inner part. Our model is based on potential flow theory, linearized around a potential vortex flow with a free surface for which we show that unstable resonant states appear. Limiting our attention to the lowest order mode of each type of wave and their interaction, we obtain an analytically soluble model, which, together with estimates of the circulation based on angular momentum balance, reproduces the main features of the experimental phase diagram. The generality of our arguments implies that the instability should not be limited to flows with a rotating bottom (implying singular behavior near the corners), and indeed we show that we can obtain the polygons transiently by violently stirring liquid nitrogen in a hot container.
RESUMO
In this paper the influence of the microscopic characteristics of a random medium on non polarized, incoherent steady light transport (ISLT) is investigated. After close examination of current diffusion models, the source term in those models is modified, allowing a complete modeling of experimental and simulated radial dependance of backscattered and transmitted intensities for media thicknesses larger than the transport length. The new model only presents an additional source with respect to the elementary point source model. Thanks to more than 200 Monte-Carlo simulations, this parameter is correlated to the backscattering part of the Mie phase function. Incoherent Steady Light Transport measurements on two industrial emulsions at various volume fractions validate experimentally this correlation. This establishes a complete link between the microscopic characteristic of the random medium (size, optical indexes and volume fraction) and its macroscopic description in terms of diffusion and source parameters, opening new potential applications of the ISLT technique to, for example, the evaluation of the particles interaction potential in concentrated suspensions.
RESUMO
We describe in this letter a new rheo-optical apparatus able to measure the photon transport length l* in an evolving random medium. First, the appropriate solution of the diffusion equation for oil in water, stable, micron-sized emulsions is compared successfully with the measured spatial distribution of incoherent backscattered light given by the new apparatus. Further validation is provided using stable samples of varying sizes and volume fractions (1% to 64%) and comparing measurements with Mie-Percus-Yevick calculations based on independent small-angle light scattering (SALS) measurements. As a typical example application of the system, the emulsification of a different oil in water system is studied in situ and dynamically. The continuous temporal measurements show the decrease of the average size versus time, in excellent quantitative agreement with independent SALS measurements. This evidences that this system is able to probe continuously and nonintrusively the microscopic and macroscopic flow-induced organization of suspensions.
RESUMO
One case of pneumomediastinum due to subcutaneous emphysema occurring after using a high speed air turbine drill during dental extraction was described, as only a few cases have been published in the literature. Air may have entered the soft tissues directly by being forced down the gingival crevice. The air may then have passed by way of the fascial planes to enter deeper structures and the mediastinum. The consequences of this condition after general anaesthesia were discussed, more common aetiologies being tracheal or oesophageal rupture, bronchial rupture or pneumothorax.
