Angular Distribution of Diffusely Transmitted Light

The angular dependence of light diffusely transmitted through an opaque medium is shown to depend directly on the reflective nature of the sample boundary, independent of scattering anisotropy. Experimental data are presented for glass frits and for liquid samples, such as colloidal suspensions and aqueous foams, contained in glass cells and placed in either air, water, or glycerin baths. Results compare well with a simple theoretical prediction based on the diffusion approximation and also with random walk simulations. The importance of this work is not only in providing a simple quantitative explanation of a complex transport problem, but in establishing the proper treatment of boundary conditions for diffusion theory analyses of multiple light scattering experiments

Enhanced Drainage and Coarsening in Aqueous Foams

Experiments are presented elucidating how the evolution of foam microstructure by gas diffusion from high to low pressure bubbles can significantly speed up the rate of gravitational drainage, and vice versa. This includes detailed data on the liquid-fraction dependence of the coarsening rate, and on the liquid-fraction and the bubble-size profiles across a sample. These results can be described by a “coarsening equation” for the increase of bubble growth rate for drier foams. Spatial variation of the average bubble size and liquid fraction can also affect the growth and drainage rates

Influence of general convective motions on the exterior of isolated rotating bodies in equilibrium

The problem of describing isolated rotating bodies in equilibrium in General Relativity has so far been treated under the assumption of the circularity condition in the interior of the body. For a fluid without energy flux, this condition implies that the fluid flow moves only along the angular direction, i.e. there is no convection. Using this simplification, some recent studies have provided us with uniqueness and existence results for asymptotically flat vacuum exterior fields given the interior sources. Here, the generalisation of the problem to include general sources is studied. It is proven that the convective motions have no direct influence on the exterior field, and hence, that the aforementioned results on uniqueness and existence of exterior fields apply equally in the general case.Comment: 8 pages, LaTex, uses iopart style files. To appear in Class. Quatum Gra

Free Drainage of Aqueous Foams: Container Shape Effects on Capillarity and Vertical Gradients

The standard drainage equation applies only to foam columns of constant cross-sectional area. Here, we generalize to include the effects of arbitrary container shape and develop an exact solution for an exponential, Eiffel Tower , sample. This geometry largely eliminates vertical wetness gradients, and hence capillary effects, and should permit a clean test of dissipation mechanisms. Agreement with experiment is not achieved at late times, however, highlighting the importance of both boundary conditions and coarsening

Diffusing-Light Spectroscopies Beyond the Diffusion Limit: The Role of Ballistic Transport and Anisotropic Scattering

Diffuse transmission and diffusing-wave spectroscopy (DWS) can be used to probe the structure and dynamics of opaque materials such as colloids, foams, and sand. A crucial step is to model photon transport as a diffusion process. This approach is acceptable for optically thick samples, far into the limit of strong multiple scattering; however, it becomes increasingly inaccurate for thinner samples for several reasons. Here, we correct for two of these defects. By modeling photon propagation by a telegrapher equation with suitable boundary conditions, we can account for the ballistic transport of photons at finite speed between successive scattering events. By introducing a discontinuity in the photon concentration at the source point, and then averaging over a range of penetration depths, we can account for the fact that photons usually scatter anisotropically into the forward direction, rather than being completely randomized at each event. The accuracy of our approach is tested by comparison both with random walk computer simulations and with experiments on specially designed suspensions of polystyrene spheres. We find that our predictions extend the utility of diffuse transmission to slabs of all thicknesses and of DWS to slabs down to about two transport mean free paths

Instabilities in a Liquid-Fluidized Bed of Gas Bubbles

Gas bubbles in an aqueous foam can be unjammed, or fluidized, by introducing a forced flow of the continuous liquid phase at a sufficiently high rate. We observe that the resulting bubble dynamics are spatially inhomogeneous, exhibiting a sequence of instabilities vs increasing flow rate. First irregular swirls appear, then a single convective roll, and finally a series of stratified convection rolls each with a different average bubble size

The influence of shock pressure, pre-shock temperature, and host rock composition on the survival rate of endolithic microorganisms during impact ejection from Mars

Petrographic and biological analysis of shock recovery experiments confirms the possible life transport due to an impact from Mars to Earth

Forage Yield and Nutritive Value of 30 Cultivars of Maize for Silage in the Highland Valleys of Central Mexico

In Mexico, the selection of maize cultivars for forage has mainly been based on dry matter (DM) yields, not considering nutritional quality as an important evaluation parameter. The objective of this study was to assess forage yield and nutritive value of Highland and Subtropical maize cultivars for silage in the Highland Valleys of Central México

Biofilms in Endodontics—Current Status and Future Directions

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Molecular dynamics study of accelerated ion-induced shock waves in biological media

We present a molecular dynamics study of the effects of carbon- and iron-ion induced shock waves in DNA duplexes in liquid water. We use the CHARMM force field implemented within the MBN Explorer simulation package to optimize and equilibrate DNA duplexes in liquid water boxes of different sizes and shapes. The translational and vibrational degrees of freedom of water molecules are excited according to the energy deposited by the ions and the subsequent shock waves in liquid water are simulated. The pressure waves generated are studied and compared with an analytical hydrodynamics model which serves as a benchmark for evaluating the suitability of the simulation boxes. The energy deposition in the DNA backbone bonds is also monitored as an estimation of biological damage, something which is not possible with the analytical model