Thursday, November 30, 2017

The Origin of Rain and Smells

Everyday questions like what really causes clouds and rain, what gives sparkling wines their distinctive aroma and why do tires generate so much smoke when they burn have answers that are intimately connected.

The University of Seville teacher Alfonso Gañán has developed a particularly exact model to show the origin of all these phenomena from a universal microscopic mechanism that occurs on the surface of liquids, independently of mere evaporation. His results have been published in an article in Physical Review Letters, the general Physics review Physical Review Letters, which receives the most citations in the world.

It deals with one of the most common phenomena since the liquid phase appeared in the universe: all liquid, especially when it is in continuous movement like in the sea, always contains gases in a greater or lesser concentration, depending on the pressure and temperature to which it is subjected. Almost always, these gases end up as more or less small bubbles on the surface of the liquid. When these bubbles explode, especially if they are microscopic, minuscule drops are expelled at great velocity, and these drops almost instantly travel notable distances from the surface of the liquid that they came from.

Credit:  University of Seville

These microscopic drops generate the seed of clouds (microscopic grains of salt that form the condensation nuclei of the drops of the clouds) on the surface of the sea, or they can spread all the flavours of a broth in the air independently of its volatility, or form smoke on burning liquids.

The size of these “ghost drops” and their speed are the principle factors that the model designed by Gañán explains and precisely determines, predicting perfectly the results of hundreds of exhaustive experiments carried out from the start of the 20th century until the present day. In accordance with this model, in function of the properties of a determined liquid, there exists a critical size of gas bubble which determines a remarkable singularity: the drop expelled becomes incredibly small, while its speed increases limitlessly at the same time as the size of the bubble shrinks and approaches this limit. Below this limit, no drops are expelled. Specifically, when this size is small enough (as in the case of small bubbles in water), the new model shows that the “ghost” micro-drops can reach supersonic speeds and reach truly meaningful heights.

This finally, and precisely, answers the questions at the beginning of this text. In the particular case of the sea, pollution and waste – which are especially concentrated on the surface, so lessening the surface tension, the principal origin of the problem – the model would explain a drastic decline in the production and size of these “cloud seeds”. If it could be checked, this fact would show us a new pernicious human effect on climate, due to its impact on precipitation.

Contacts and sources:
University of Seville

Citation:  Revision of Bubble Bursting: Universal Scaling Laws of Top Jet Drop Size and Speed Alfonso M. Gañán-Calvo Phys. Rev. Lett. 119, 204502 – Published 16 November 2017

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