Boiling bubbles

Every year, during the Fluid Dynamics conference organised by the American Physical Society, there is a competition to find the best film-demonstration of phenomena related to fluid mechanics. Alexis Duchesne (post-doc), Charles Dubois (doctoral student ARC) and Hervé Caps (professor) at GRASP (Group for Research and Applications in Statistical Physics) of the University of Liege, have won first prize in a competition involving more than 100 videos.

The video illustrates results obtained from the dynamics of gas bubbles produced by a boiling liquid. This is an everyday occurrence, for example, the steam bubbles produced at the bottom of a saucepan of boiling water.

While the phenomenon of bubble nucleation has been widely studied, the dynamics of these bubbles has not been the subject of particular attention. In the present case, the heat source is a metal wire that is heated by means of an electric current.

It was observed that if the temperature of the wire is less than the boiling temperature of the liquid (silicon oil in this case), nothing happens. Beyond this temperature, bubbles appear. Two surprising phenomena then occur: (i) despite the fact that the bubbles are lighter than the liquid, they do not rise to the surface but remain “stuck” to the wire; (ii) they move…and do so very quickly (almost 10cm/second).

When two bubbles meet, they can rebound off each other or coalesce. When several bubbles have merged, the resulting bubble can become large enough to escape the “attraction” of the wire and break away. By increasing the heat of the wire, clusters of bubbles form. These clusters are fixed and isolated bubbles rebound between them. By increasing the temperature of the wire even more, it is possible to create clusters of bubbles that are the same size as the wire while creating a surprisingly calm environment in this boiling liquid.

Finally, when the temperature of the wire is very high, we enter the realm of film boiling. The wire is so hot that it is entirely covered by a layer of steam. This layer is unstable and becomes deformed to form bubbles. In the case of a thicker wire, the bubbles produced are perfectly regular. When a thinner wire is used, this can result in large trapped bubbles all around the wire. The pattern created in this way is unstable and giant “cannibal” bubbles appear.


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