First normal stress difference and crystallization in a dense sheared granular fluid

The first normal stress difference (${\mathcal N}_1$) and the microstructure in a dense sheared granular fluid of smooth inelastic hard-disks are probed using event-driven simulations. While the anisotropy in the second moment of fluctuation velocity, which is a Burnett-order effect, is known to be the progenitor of normal stress differences in {\it dilute} granular fluids, we show here that the collisional anisotropies are responsible for the normal stress behaviour in the {\it dense} limit. As in the elastic hard-sphere fluids, ${\mathcal N}_1$ remains {\it positive} (if the stress is defined in the {\it compressive} sense) for dilute and moderately dense flows, but becomes {\it negative} above a critical density, depending on the restitution coefficient. This sign-reversal of ${\mathcal N}_1$ occurs due to the {\it microstructural} reorganization of the particles, which can be correlated with a preferred value of the {\it average} collision angle $\theta_{av}=\pi/4 \pm \pi/2$ in the direction opposing the shear. We also report on the shear-induced {\it crystal}-formation, signalling the onset of fluid-solid coexistence in dense granular fluids. Different approaches to take into account the normal stress differences are discussed in the framework of the relaxation-type rheological models.Comment: 21 pages, 13 figure

Experimental approaches to study the nutritional value of food ingredients for dogs and cats

This review covers methods that have been applied to study the nutrient value or quality of specific ingredients fed to dogs, cats and comparable species (i.e. foxes, minks, rats, etc.). Typically, the nutritional value or utilization of a specific ingredient is measured by total tract digestibility and has been expanded through the measurement of total nutrient balance (i.e. nitrogen or energy). However, to better understand digestion it is necessary to obtain a more accurate measurement of nutrients entering and leaving the small intestine. Accurate measurement of small intestinal digestion is crucial in dogs and cats because nutrient digestion and absorption occurs primarily in the small intestine. Measuring small intestinal digestibility requires access to digesta leaving the small intestine and can be obtained by placing a cannula at the terminal ileum. This approach also necessitates the use of markers (e.g. chromic oxide) to monitor flow of digesta. Specifically, this approach has been used for the direct measurement of intestinal digestion of carbohydrates and amino acids. It also permits a separate measurement of large intestinal digestion which is particularly useful for the study of fiber fermentation. Passage of foods through the gastrointestinal tract is also an important component of utilization and these methods are reviewed