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Register a new userGas diffusion among bubbles and the DCS risk
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Added by
Oswaldo Monteiro Del Cima
Publication date
2017
fields
Physics
and research's language is
English
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We present some experimental and simulation results that reproduces the Ostwald ripening (gas diffusion among bubbles) for air bubbles in a liquid fluid. Concerning the experiment, there it is measured the time evolution of bubbles mean radius, number of bubbles and radius size distribution. One of the main results shows that, while the number of bubbles decreases in time the bubbles mean radius increases, hence, it follows that the smaller bubbles disappear whereas the -- potentially dangerous for the diver -- larger bubbles grow up. Consequently, this effect suggests a possible contribution of the Ostwald ripening to the decompression sickness, and if so, it should be pursued its implementation to the Reduced Gradient Bubble Model (RGBM) so as to build up dive tables and computer programs for further diving tests.
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The Ostwald ripening phenomenon for gas bubbles in a liquid consists mainly in gas transfer from smaller bubbles to larger bubbles. An experiment was carried out in which the Ostwald ripening for air bubbles, in a liquid fluid with some rheological parameters of the human blood, is reproduced. There it has been measured time evolution of bubbles mean radius, number of bubbles and radius size distribution, where the initial bubbles radii normalized distribution behaves like a Tsallis ($q$-Weibull) distribution. One of the main results shows that, while the number of bubbles decreases in time the bubbles mean radius increases, therefore smaller bubbles disappear whereas the, potentially dangerous for the diver, larger bubbles grow up. Consequently, it is presumed that such a bubble broadening effect could contribute, even minimally, to decompression illness: decompression sickness and arterial gas embolism. This conjecture is reinforced by the preliminary results of Ostwald broadening to RGBM (Reduced Gradient Bubble Model) decompression schedules for a closed circuit rebreather (CCR) dive to 420fsw (128m) with 21/79 Heliox gas mixture.
The prediction of the lifetime of surface bubbles necessitates a better understanding of the thinning dynamics of the bubble cap. In 1959, Mysel textit{et al.} cite{mysels1959soap}, proposed that textit{marginal regeneration} i.e. the rise of patches, thinner than the film should be taken into account to describe the film drainage. Nevertheless, an accurate description of these buoyant patches and of their dynamics as well as a quantification of their contribution to the thinning dynamics is still lacking. In this paper, we visualize the patches, and show that their rising velocities and sizes are in good agreement with models respectively based on the balance of gravitational and surface viscous forces and on a Rayleigh-Taylor like instability cite{Seiwert2017,Shabalina2019}. Our results suggest that, in an environment saturated in humidity, the drainage induced by their dynamics correctly describes the film drainage at the apex of the bubble within the experimental error bars. We conclude that the film thinning of soap bubbles is indeed controlled, to a large extent, by textit{marginal regeneration} in the absence of evaporation.
We study analytically the joint dispersion of Gaussian patches of salt and colloids in linear flows, and how salt gradients accelerate or delay colloid spreading by diffusiophoretic effects. Because these flows have constant gradients in space, the problem can be solved almost entirely for any set of parameters, leading to predictions of how the mixing time and the Batchelor scale are modified by diffusiophoresis. We observe that the evolution of global concentrations, defined as the inverse of the patches areas, are very similar to those obtained experimentally in chaotic advection. They are quantitatively explained by examining the area dilatation factor, in which diffusive and diffusiophoretic effects are shown to be additive and appear as the divergence of a diffusive contribution or of a drift velocity. An analysis based on compressibility is developed in the salt-attracting case, for which colloids are first compressed before dispersion, to predict the maximal colloid concentration as a function of the parameters. This maximum is found not to depend on the flow stretching rate nor on its topology (strain or shear flow), but only on the characteristics of salt and colloids (diffusion coefficients and diffusiophoretic constant) and the initial size of the patches.
We perform molecular dynamics simulations to understand the translational and rotational diffusion of Janus nanoparticles at the interface between two immiscible fluids. Considering spherical particles with different affinity to fluid phases, both their dynamics as well as the fluid structure around them are evaluated as a function of particle size, amphiphilicity, fluid density, and interfacial tension. We show that as the particle amphiphilicity increases due to enhanced wetting of each side with its favorite fluid, the rotational thermal motion decreases. Moreover, the in-plane diffusion of nanoparticles at the interface becomes slower for more amphiphilic particles, mainly due to formation of a denser adsorption layer. The particles induce an ordered structure in the surrounding fluid that becomes more pronounced for highly amphiphilic nanoparticles, leading to increased resistance against nanoparticle motion. A similar phenomenon is observed for homogeneous particles diffusing in bulk upon increasing their wettability. Our findings can provide fundamental insight into the dynamics of drugs and protein molecules with anisotropic surface properties at biological interfaces including cell membranes.
The present article experimentally and theoretically probes the evaporation kinetics of sessile saline droplets. Observations reveal that presence of solvated ions leads to modulated evaporation kinetics, which is further a function of surface wettability. On hydrophilic surfaces, increasing salt concentration leads to enhanced evaporation rates, whereas on superhydrophobic surfaces, it first enhances and reduces with concentration. Also, the nature and extents of the evaporation regimes constant contact angle or constant contact radius are dependent on the salt concentration. The reduced evaporation on superhydrophobic surfaces has been explained based on observed via microscopy crystal nucleation behaviour within the droplet. Purely diffusion driven evaporation models are noted to be unable to predict the modulated evaporation rates. Further, the changes in the surface tension and static contact angles due to solvated salts also cannot explain the improved evaporation behaviour. Internal advection is observed using PIV to be generated within the droplet and is dependent on the salt concentration. The advection dynamics has been used to explain and quantify the improved evaporation behaviour by appealing to the concept of interfacial shear modified Stefan flows around the evaporating droplet. The analysis leads to accurate predictions of the evaporation rates. Further, another scaling analysis has been proposed to show that the thermal and solutal Marangoni advection within the system leads to the advection behaviour. The analysis also shows that the dominant mode is the solutal advection and the theory predicts the internal circulation velocities with good accuracy. The findings may be of importance to microfluidic thermal and species transport systems.
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