Convective instabilities in two superposed horizontal liquid layers heated laterally.

This work is devoted to the theoretical study of the stability of two superposed horizontal liquid layers bounded by two solid planes and subjected to a horizontal temperature gradient. The liquids are supposed to be immiscible with a nondeformable interface. The forces acting on the system are buoyancy and interfacial tension. Four different flow patterns and temperature profiles are found for the basic state. A linear perturbative analysis with respect to two- and three-dimensional perturbations reveals the existence of three kinds of patterns. Depending on the relative height of both liquids several situations are predicted: either wave propa- gation from cold to the hot regions, or waves propagating in the opposite direction or still stationary longitu- dinal rolls. The behavior of three different pairs of liquids which have been used in experiments on bilayers under vertical gradient by other authors have been examined. The instability mechanisms are discussed and a qualitative interpretation of the different behaviors exhibited by the system is provided. In some configurations it is possible to find a codimension-two point created by the interaction of two Hopf modes with different frequencies and wave numbers. These results suggest to consider two liquid layers as an interesting prototype ? nard-Marangoni problem.

Effects of forage type on diversity in bacterial pellets isolated from liquid and solid phases of the rumen content in sheep and goats.

Two sheep and two goats, fitted with a ruminal cannula, received two diets composed of 30% concentrate and 70% of either alfalfa hay (AL) or grass hay (GR) as forage in a two-period crossover design. Solid and liquid phases of the rumen were sampled from each animal immediately before feeding and 4 h post-feeding. Pellets containing solid associated bacteria (SAB) and liquid associated bacteria (LAB) were isolated from the corresponding ruminal phase and composited by time to obtain 2 pellets per animal (one SAB and one LAB) before DNA extraction. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S ribosomal DNA was used to analyze bacterial diversity. A total of 78 and 77 bands were detected in the DGGE gel from sheep and goats samples, respectively. There were 18 bands only found in the pellets from sheep fed AL-fed sheep and 7 found exclusively in samples from sheep fed the GR diet. In goats, 21 bands were found only in animals fed the AL diet and 17 were found exclusively in GR-fed ones. In all animals, feeding AL diet tended (P < 0.10) to promote greater NB and SI in LAB and SAB pellets compared with the GR diet. The dendrogram generated by the cluster analysis showed that in both animal species all samples can be included in two major clusters. The four SAB pellets within each animal species clustered together and the four LAB pellets grouped in a different cluster. Moreover, SAB and LAB clusters contained two clear subclusters according to forage type. Results show that in all animals bacterial diversity was more markedly affected by the ruminal phase (solid vs. liquid) than by the type of forage in the diet.