Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Mechanisms of benthic prey capture in wrasses (Labridae)

Teleost fishes capture prey using ram, suction, and biting behaviors. The relative use of these behaviors in feeding on midwater prey is well studied, but few attempts have been made to determine how benthic prey are captured. This issue was addressed in the wrasses (Labridae), a trophically diverse lineage of marine reef fishes that feed extensively on prey that take refuge in the benthos. Most species possess strong jaws with stout conical teeth that appear well-suited to gripping prey. Mechanisms of prey capture were evaluated in five species encompassing a diversity of feeding ecologies: Choerodon anchorago (Bloch, 1791), Coris gaimard (Quoy and Gaimard, 1824), Hologymnosus doliatus (Lacepède, 1801), Novaculichthys taeniourus (Lacepède, 1801) and Oxycheilinus digrammus (Lacepède, 1801). Prey capture sequences were filmed with high-speed video at the Lizard Island Field Station (14°40primeS, 145°28primeE) during April and May 1998. Recordings were made of feeding on pieces of prawn suspended in the midwater and similar pieces of prawn held in a clip that was fixed to the substratum. Variation was quantified among species and between prey types for kinematic variables describing the magnitude and timing of jaw, hyoid, and head motion. Species differed in prey capture kinematics with mean values of most variables ranging between two and four-fold among species and angular velocity of the opening jaw differing seven-fold. The kinematics of attached prey feeding could be differentiated from that of midwater captures on the basis of faster angular velocities of the jaws and smaller movements of cranial structures which were of shorter duration. All five species used ram and suction in combination during the capture of midwater prey. Surprisingly, ram and suction also dominated feedings on attached prey, with only one species making greater use of biting than suction to remove attached prey. These data suggest an important role for suction in the capture of benthic prey by wrasses. Trade-offs in skull design associated with suction and biting may be particularly relevant to understanding the evolution of feeding mechanisms in this group

Strategies for ensuring good hydration in the elderly

Dehydration is a frequent etiology of morbidity and mortality in elderly people. It causes the hospitalization of many patients and its outcome may be fatal. Indeed, dehydration is often linked to infection, and if it is overlooked, mortality may be over 50%. Older individuals have been shown to have a higher risk of developing dehydration than younger adults. Modifications in water metabolism with aging and fluid imbalance in the frail elderly are the main factors to consider in the prevention of dehydration. Particularly, a decrease in the fat free mass, which is hydrated and contains 73% water, is observed in the elderly due to losses in muscular mass, total body water, and bone mass. Since water intake is mainly stimulated by thirst, and since the thirst sensation decreases with aging, risk factors for dehydration are those that lead to a loss of autonomy or a loss of cognitive function that limit the access to beverages. The prevention of dehydration must be multidisciplinary. Caregivers and health care professionals should be constantly aware of the risk factors and signs of dehydration in elderly patients. Strategies to maintain normal hydration should comprise practical approaches to induce the elderly to drink enough. This can be accomplished by frequent encouragement to drink, by offering a wide variety of beverages, by advising to drink often rather than large amounts, and by adaptation of the environment and medications as necessar

The forces exerted by aquatic suction feeders on their prey

Successful prey capture by aquatic suction feeders depends on the ability of the predator to generate a flow of water external to the mouth that overcomes any movements and forces that the prey uses to resist the suction flow. Elucidating the nature and magnitude of these forces is a key to understanding what limits suction feeding performance. We identify three potential forces produced by the suction flow field: drag, acceleration reaction and the fluid pressure gradient. Using a mathematical model parametrized with empirical data from feeding bluegill, Lepomis macrochirus, we explore the relative magnitude of these forces under three encounter scenarios with a 5 mm diameter, spherical prey: an immobile mid-water prey; a similar prey that executes an escape response; and a prey item that grips a substratum. Contrary to the almost exclusive emphasis on drag in the suction feeding literature, it made a minor contribution to the total forces in all three cases. In all three scenarios, the pressure gradient is the largest of the three forces. These results are important because previous researchers have emphasized drag and have not explicitly recognized a role for the pressure gradient force in suction feeding. The simulations suggest previously unrecognized mechanisms that suction feeders can use to enhance the forces that they exert, by increasing the steepness of the pressure gradient that the prey item is exposed to. This can be accomplished either by increasing the rate of increase in fluid velocity or by restricting the size of the mouth aperture, which creates a steeper spatial gradient in pressure

Hydration assessment techniques

Water in the human body is essential for metabolism, temperature regulation, and numerous other physiological processes that are consistent with good health. Accurate, precise, and reliable methods to assess body fluid compartments are needed. This review describes the hydration assessment techniques of isotope dilution, neutron activation analysis, bioelectrical impedance, body mass change, thirst, tracer appearance, hematologic indices, and urinary markers. It also provides guidance for selecting techniques that are appropriate for use with unique individuals and situations. © 2005 International Life Sciences Institute

Relative importance of growth and behaviour to elasmobranch suction-feeding performance over early ontogeny

Development of the ability to capture prey is crucial to predator survival. Trends in food-capture performance over early ontogeny were quantified for leopard sharks Triakis semifasciata and whitespotted bamboosharks Chiloscyllium plagiosum by measuring suction pressure and flow in front of the mouth during feeding. At any size, C. plagiosum produce greater subambient pressure and ingest more rounded water parcels. Maximum subambient pressure scaled with negative allometry in T. semifasciata and was accompanied by an increase in the time to reach maximum gape. Despite a similar trend in buccal expansion timing, maximum pressure in C. plagiosum scaled with isometry and was accompanied by an earlier onset of hyoid depression and a positive allometric increase in buccal reserve volume. Growth was the primary factor responsible for developmental trends in both species, with size-independent behavioural changes contributing little to overall performance variability. Ontogenetic dietary shifts are predicted for both species as a consequence of size-dependent changes in performance. Chiloscyllium plagiosum becomes anatomically and behaviourally canalized towards suction feeding, limiting the effective range of prey capture and possibly necessitating stalking. Triakis semifasciata, by contrast, retains the flexibility to employ both ram and suction and therefore captures more elusive prey with age