Hydrostatic instabilities in floating zone cristal growth process

If only Fluid Mechanics aspects are considered, the configuration appearing in the floating zone technique for crystal growth can be modelled as a mass of liquid spanning between two solid rods. Besides, if now the influence of temperature gradients and heat flow are not considered, the simplest fluid model consists of an isothermal liquid mass of constant properties (density and surface tension) held by capillary forces between two solid disks placed a distance L apart: the so called liquid bridge. As it is well known, if both supporting disks were parallel, coaxial and of the same diameter, 2R, the volume of liquid, V, were equal to that of a cylinder of the same L and R (V=KR~L) and no body forces were acting on the liquid column, the fluid configuration (under these conditions of cylindrical shape) will become unstable when the distance between the disks equals the length of the circumference of the supporting disks (L=2KR, the so-called Rayleigh stability limit). One should be aware that the Rayleigh stability limit can be dramatically modified when the geometry differs from the above described cylinder (due to having non-coaxial disks, different diameter disks, liquid volume different from the cylindrical one, etc) or when other external effects like accelerations either axial or lateral are considered. In this paper the stability limits of liquid bridges considering different types of perturbations are reviewed.

A regular perturbation approach to surface tension driven flows

Surface tension induced convection in a liquid bridge held between two parallel, coaxial, solid disks is considered. The surface tension gradient is produced by a small temperature gradient parallel Co the undisturbed surface. The study is performed by using a mathematical regular perturbation approach based on a small parameter, e, which measures the deviation of the imposed temperature field from its mean value. The first order velocity field is given by a Stokes-type problem (viscous terms are dominant) with relatively simple boundary conditions. The first order temperature field is that imposed from the end disks on a liquid bridge immersed in a non-conductive fluid. Radiative effects are supposed to be negligible. The second order temperature field, which accounts for convective effects, is split into three components, one due to the bulk motion, and the other two to the distortion of the free surface. The relative importance of these components in terms of the heat transfer to or from the end disks is assessed

Effects of high hydrostatic pressure on rheological and termal properties of chickpea Cicer arietinum L. flour slurry and heat-induced paste.

Thermorheological changes in high hydrostatic pressure (HHP)-treated chickpea flour (CF) slurries were studied as a function of pressure level (0.1, 150, 300, 400, and 600 MPa) and slurry concentration (1:5, 1:4, 1:3, and 1:2 flour-to-water ratios). HHP-treated slurries were subsequently analyzed for changes in properties produced by heating, under both isothermal and non-isothermal processes. Elasticity (G′) of pressurized slurry increased with pressure applied and concentration. Conversely, heat-induced CF paste gradually transformed from solid-like behavior to liquid-like behavior as a function of moisture content and pressure level. The G′ and enthalpy of the CF paste decreased with increasing pressure level in proportion with the extent of HHP-induced starch gelatinization. At 25 °C and 15 min, HHP treatment at 450 and 600 MPa was sufficient to complete gelatinization of CF slurry at the lowest concentration (1:5), while more concentrated slurries would require higher pressures and temperature during treatment or longer holding times. Industrial relevance Demand for chickpea gel has increased considerably in the health and food industries because of its many beneficial effects. However, its use is affected by its very difficult handling. Judicious application of high hydrostatic pressure (HHP) at appropriate levels, adopted as a pre-processing instrument in combination with heating processes, is presented as an innovative technology to produce a remarkable decrease in thermo-hardening of heat-induced chickpea flour paste, permitting the development of new chickpea-based products with desirable handling properties and sensory attributes.

Damage mechanisms of 3D woven hybrid composites in tension. Testing, inspection and simulation

Delamination reduces the strenght of the composites, mainly in compression. Several methods exist to overcome this problem, but they are either not feasible for large scale production or too expensive. 3D composites are a promising solution.

Desarrollo de un procedimiento para la observación y caracterización mecánica de células sobre sustratos opacos mediante microscopía de fuerzas atómicas : aplicación al estudio de linfocitos

Las células en los tejidos biológicos están continuamente sometidas a estímulos físicos tales como la presión hidrostática y esfuerzos de tracción, compresión o cortante, entre otros. La importancia de los estímulos mecánicos en el comportamiento de las células se ha reconocido recientemente al comprobarse cómo la naturaleza de estas fuerzas puede cambiar en patologías tales como las enfermedades vasculares o el cáncer. En respuesta a estos cambios, las células reaccionan modificando desde su forma o aspecto hasta su ciclo celular. Consecuentemente, el interés por el comportamiento mecánico de las células ha experimentado un auge creciente que ha requerido el desarrollo de varias técnicas de caracterización. En este contexto, se puede afirmar que una de las técnicas que ha irrumpido con más fuerza en esta nueva área, situada entre el mundo biológico y el físico, es la microscopía de fuerza atómica. En esta Tesis se ha abordado el estudio mediante microscopía de fuerza atómica de linfocitos de ratón que constituyen un linaje celular especialmente difícil de caracterizar mediante esta técnica por su tamaño y naturaleza no adherente. Los linfocitos, como actores fundamentales del sistema inmune, tienen gran importancia en la determinación de la respuesta que un organismo desencadena ante la presencia de un biomaterial. Bajo esta premisa, y como condición previa a la caracterización de los linfocitos, ha sido necesario el desarrollo de una metodología robusta y de amplia aplicabilidad que permita el estudio de células sobre biomateriales. Finalmente y con el objetivo de correlacionar el comportamiento mecánico de los linfocitos con alguna característica fisiológica relevante, se ha analizado la hipótesis de que el comportamiento mecánico pueda ser utilizado como marcador de la edad biológica. Consecuentemente se ha abordado el estudio del comportamiento mecánico de los linfocitos clasificados por grupos de edad, de manera que se han obtenido los primeros resultados que indican cómo puede manifestarse el proceso de inmunosenescencia -depresión del sistema inmune relacionada con el envejecimiento- en el comportamiento mecánico de las células del sistema inmune. Cells within tissues are continuously exposed to physical forces including hydrostatic pressure, shear stress, and compression and tension forces. The relevance of these mechanical stimuli has recently been recognised by different works in which significant changes were observed in these forces when they were measued in individuals affected by cardiovasvular diseases or cancer. Cells may alter their orientation, shape, internal constitution, contract, migrate, adhere, modify the synthesis and degradation of extracellular constituents, or even their life cycle in response to perturbations in their mechanical environment. As a consequence of this, the attention in cell mechanical behavior has undergone a significant thrust and novel techniques have been developed. In this context, atomic force microscopy has become a basic tool for the progress of this field. In this Thesis, the mechanical behavior of living murine T-lymphocytes was assessed by atomic force microscopy. Lymphocytes play a main role in the immune system of the individual and, consequently, in the immune response triggered by the presence of a biomaterial. The observation and characterization of the lymphocytes required the development of a robust experimental procedure that allowed overcoming the difficulties related to the analysis of this cell lineage, in particular their relatively large size and non-adherent character. These procedures could be easily transferred to other non-adherent cell lineages. Finally, to check the viability of developed method, we study the lymphocyte mechanical behavior as a function of the murine ageing. The obtained data represent a first step in the knowledge about how mechanical stimuli can affect the age-dependent decrease in immunological competence, i.e., the immunosenescence.

Surface tension and microgravity

The behaviour of confined liquids on board an orbiting spacecraft is mainly driven by surface tension phenomena, which cause an apparently anomalous response of the liquid when compared with the behaviour that can be observed on an Earth laboratory provided that the amount of liquid is high enough. The reason is that in an orbiting spacecraft the different inertial forces acting on the bulk of the liquid are almost zero, causing thus capillary forces to be the dominant ones. Of course, since gravity forces are proportional to the liquid volume, whereas surface tension forces are proportional to the liquid surface, there are situations on Earth where capillarity can be the dominant effect, as it happens when very small volume liquid samples are considered. However, work with small size samples may require the use of sophisticated optical devices. Leaving aside the neutral buoyancy technique, a way of handling large liquid interfaces is by using drop towers, where the sample falls subjected to the action of Earth's gravity. This approach is suitable when the characteristic time of the problem under consideration is much smaller than the drop time. In this work the transformation of an out-of-use chimney into a drop tower is presented. Because of the miniaturization, hardiness and low cost of current electronic devices, a drop tower can be used as an inexpensive tool for undergraduate students to experimentally analyse a large variety of surface tension driven phenomena.

Processing of grapes by high hydrostatic pressure. Influence on wine quality

The aim of this work is to evaluate the influence of S. pombe and T. delbrueckii species on the sensory quality of red wine when used in sequential and mixed fermentations with S. cerevisiae.

Mechanical behavior and failure micromechanisms of hybrid 3D woven composites in tension

The deformation and failure micromechanisms of a hybrid 3D woven composite were studied in tension. Plain and open-hole composite coupons were tested in tension until failure in the fill and warp directions, as well as fiber tows extracted from the dry fabric and impregnated with the matrix. The macroscopic evolution of damage in the composite coupons was assessed by means of periodic unloading–reloading (to obtain the elastic modulus and the residual strain), whereas the microscopic mechanism were established by means of X-ray computed microtomography. To this end, specimens were periodically removed from the mechanical testing machine and infiltrated with ZnI-containing liquid to assess the main damage modes as a function of the applied strain. The experimental observations and the predictions of an isostrain model were used to understand the key factors controlling the elastic modulus, strength and notch sensitivity of hybrid 3D woven composites in tension. It was found that the full contribution of the glass fibers to the composite strength was not employed, due to the premature fracture of the carbon fibers, but their presence increased the fracture strain and the energy dissipated during fracture. Thus, hybridization of the 3D woven composite led to a notch-insensitive behavior as demonstrated by open-hole tests

Disjoining pressure and the film-height-dependent surface tension of thin liquid films: New insight from capillary wave fluctuations

In this paper we review simulation and experimental studies of thermal capillary wave fluctuations as an ideal means for probing the underlying disjoining pressure and surface tensions, and more generally, fine details of the Interfacial Hamiltonian Model. We discuss recent simulation results that reveal a film-height-dependent surface tension not accounted for in the classical Interfacial Hamiltonian Model. We show how this observation may be explained bottom-up from sound principles of statistical thermodynamics and discuss some of its implications