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.

Influence of Soluble Surfactants and Deformation on the Dynamics of Centered Bubbles in Cylindrical Microchannels.

In this study, we investigate, using direct numerical simulation, the motion of a small bubble in a horizontal microchannel filled with a liquid containing surfactants. In particular, we study the combined effect of surfactants and bubble deformability on the bubble shape, bubble-liquid relative velocity, velocity field in the liquid, liquid velocity on the gas-liquid interface, and surfactant distribution on the interface. The level-set method is used to capture the gas-liquid interface. The surfactant transport equation on the gas-liquid interface is solved in an Eulerian framework and is coupled to an equation describing the transport of surfactants inside the liquid phase. The Marangoni stress, induced by surfactant concentration gradients, is computed using the continuum surface force model. The simulation results give insights into the complexity of the coupling of the different phenomena controlling the dynamics of the studied system. For instance, the results show that for values of the capillary number much smaller than unity, that is, for spherical bubbles, the bubble velocity decreases as the bubble diameter increases. Moreover, surfactants tend to decrease significantly the bubble velocity, when compared with a bubble with a clean surface. Indeed, they accumulate at a convergent stagnation point/circle on the bubble surface and deplete at a divergent stagnation point/circle. As a consequence, the velocity of the liquid adjacent to the bubble is reduced in between the convergent and divergent stagnation points/circles because of Marangoni stresses. It is shown that regarding the bubble-liquid relative velocity, the bubble behaves as a rigid sphere when the Langmuir number is larger than unity, at least for the range of parameters explored in this study. For values of the capillary number of the order of unity, the bubble can take a "bullet shape". In this case, the bubble velocity increases as the bubble diameter increases. This increase of the bubble-liquid relative velocity is linked to a drastic change in the liquid flow structure near the bubble. Surfactants are swept to the rear of the bubble and have less influence on the bubble dynamics than for spherical bubbles. Finally, it is shown that increasing the amount of surfactants adsorbing to the surface eventually leads to the bursting of the bubble.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels.

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca4/3 depending on the distance from the nose of the bubble, where Ca = µUb/γ is the flow capillary number based on the bubble velocity Ub, liquid viscosity µ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance Lw between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, Lw always scales proportional to Caα, where 1.7 < α < 2 for the range of 10-5 < Ca < 10-2. Moreover, Lw increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of Lw provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

Gas dissolution in antibubble dynamics.

Antibubbles are ephemeral objects. Their lifetime is driven by the slow drainage of the air shell from the bottom to the top of the antibubble under the action of hydrostatic pressure. We show in this paper that this argument is only valid if the water used to make the surfactant mixture is saturated in air. Otherwise, two paths are used by the air, which conduct to the thinning and the eventual collapse of the air shell: the drainage from the bottom to the top of the antibubble and the dissolution of the air into the liquid. Using degassed water dramatically shortens the lifetime of the antibubbles, as observed experimentally and rationalised by time-dependent simulations. Consequently, the antibubble lifetime is not only correlated with physical and chemical properties of the air-liquid interface but also with the gas content of the liquid. We also show that pure gas dissolution does not depend on the antibubble radius, a behaviour that allows to rationalise unexplained experimental data found in literature.

Chronic progressive leukoencephalopathy in adult celiac disease.

Progressive leukoencephalopathy developed in a patient with adult celiac disease. Neurologic abnormalities appeared 4 years after the gastrointestinal manifestations despite a gluten-free diet and replacement of vitamins. Brain MRI showed marked confluent white matter abnormalities, and stereotactic brain biopsy revealed chronic leukoencephalopathy. Treatment with I.V. steroids and immunoglobulins did not stop disease progression. Celiac disease should be considered in the differential diagnosis of the leukoencephalopathies.

Time sequence of histamine release and formation in rat endotoxic shock.

Increased histamine release and formation (induced histamine) are two hypotheses considered in the pathogenesis of endotoxic shock development. To prove both hypotheses a sequence of four randomized controlled studies in rats was performed. Histamine release was measured indirectly as a decrease in tissue-histamine contents (lung, liver, spleen, stomach); histamine formation was estimated directly as an increase in histidine decarboxylase (HDC) activity in the same organs. Changes in contents and enzymatic activities were determined 4 and 8 h after shock induction; in addition, at the time of death, the activity of HDC was measured in heart, kidney, and small intestine. 4 h after shock induction, there was a significant decrease in the tissue-histamine content as measured only in the liver, with the same trend in lung and spleen. 8 h after endotoxin application, however, histamine concentration increased in the liver (significantly p < .05) and lung compared to the NaCl control group. The manifestation of changes in HDC activity in various organs was selective (i.e., not all organs showed alterations), not uniform (decreased as well as increased activities were measured), and time-dependent (no increase in HDC activity in animals dying at > 20 h). At 4 and 8 h, only the liver showed a strong increase in HDC activity which can explain the increase in histamine content. In lung, spleen, and stomach, a significant decrease occurred. The results on histamine release and formation let us conclude that histamine is involved in the pathogenesis of endotoxic shock development.