Bubbles determine the amount of alcohol in Mezcal.

Mezcal is a traditional Mexican spirit, obtained from the distillation of fermented agave juices. Its preparation has been conducted for centuries in an artisanal manner. The method used to determine the correct alcohol content is of particular interest: a stream of the liquor is poured into a small vessel to induce surface bubbles. These bubbles, known as pearls by the Mezcal artisans, remain stable for tenths of seconds only if the alcohol content is close to 50%. For higher or lower alcohol content, the bubbles burst rapidly. The long bubble lifetime is the result of surfactant-induced surface tension changes. However, the precise mechanism and its relation to alcohol content remain unexplained. In this investigation, the extended lifetime of pearls was studied both experimentally and numerically. It was found that changes in surface tension, density, viscosity (resulting from mixing ethanol and water), and the presence of surfactants are all relevant to extend the bubble lifetime. The dimensionless bubble lifetime was found to reach its maximum value when the Bond number was close to unity, corresponding to 2 mm Mezcal bubbles. These findings show that the traditional empirical method does work. Beyond this, the understanding of the process provides physical insight to many other natural and industrial problems for which the stability of surface bubbles is of importance, such as bio-foams, froth floatation, and volcanic flows.

Reduction of compaction force in a confined bidisperse granular media.

Experiments and simulations of the compaction force in a confined binary granular mixture column were conducted. We measured the resistance force encountered by a piston pushing a vertical granular mixture in this confined arrangement. Granular mixtures with two different particle sizes were considered; the size ratio and the size fraction were both varied. An important decrease of the compaction force was found for volumetric fractions between 15% and 40% and size ratios larger than 3. By conducting some supplementary discrete element simulations, we found that the force chain network is fractured and redistributed when small particles are present. Hence we argue that the reduction of compaction force results from the redistribution of force within the granular column.

Note: design of a novel rotating magnetic field device.

A novel device to produce a rotating magnetic field was designed, constructed, and tested. The system consists of a Helmholtz coil pair which is mechanically coupled to a dc electric motor whose angular velocity is controlled. The coil pair generates a uniform magnetic field; the whole system is rotated maintaining the coils energized using brushes. The magnetic field strength is uniform (≈5.8 mT) for a workspace of about 100 mm along the rotation axis. The system remains free of undesirable high amplitude mechanical vibrations for rotation frequencies below 10 Hz. We verified the performance of the apparatus by conducting experiments with magnetic swimmers.

Study of the velocity and strain fields in the flow through prosthetic heart valves.

A comparative experimental study of the velocity field and the strain field produced down-stream of biological and mechanical artificial valves is presented. In order to determine the spatial and temporal distributions of these fields, a phase-locked stereoscopic particle image velocimetry (or 3D-PIV) technique was implemented. Emphasis was placed on the identification of the fundamental differences between the extensional and the shear components of the strain tensor. The analysis of the characteristic flows reveal that the strains in every direction may reach high values at different times during the cardiac cycle. It was found that elevated strain levels persist throughout the cardiac cycle as a result of all these contributions. Finally, it is suggested that the frequency with which the strain variations occur at particular instants and locations could be associated to the cumulative damage process of the blood constituents and should be taken into account in the overall assessment of existing valve types, as well as in future design efforts.

The micromechanical behavior of lyophilized glutaraldehyde-treated bovine pericardium under uniaxial tension.

The micromechanical behavior of lyophilized glutaraldehyde bovine pericardium undergoing uniaxial tension was studied by digital image correlation. The experiments were conducted simultaneously at macromechanical and micromechanical levels, to correlate the mechanical response of this biomaterial at different scales. From the experiments, displacement and force data were acquired; in addition, an image sequence of each sample surface was registered with a high-definition camera. With the images, it was possible to obtain the vector displacement field between pairs of images and then the in-plane strain was calculated. The secant and final moduli of this material were obtained at macromechanical and micromechanical levels. A good agreement between the micro and macro moduli was observed. This analysis is a useful alternative technique for studying this biomaterial when local properties are needed for medical applications.

Compaction force in a confined granular column.

Experiments to determine the force required to push a granular column confined within a cylinder were performed. The experimental apparatus was mounted on a material testing system machine in order to obtain force and displacement measurements simultaneously. Experiments were performed for two different sphere diameters, two different cylinder diameters and for a range of piston displacement velocities. The force necessary to displace the column increases rapidly with the column height, in accordance with Janssen's theory. More importantly, we found that this force also increases with the displacement velocity. This unexpected behavior is an indication of the transition to rate-dependent behavior in dense granular flows.