A Cascade of droplets in a mixture of molten glass

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28/10/2016

Researchers from CNRS, ESPCI Paris, and Saint-Gobain used a 3D imaging technique of X-ray micro-tomography to follow the evolution of a mixture of molten glass at high temperature. They shed new light on an original mechanism of liquids fragmentation, which might apply to texturing glasses at tiny scales. Their work was recently published in Physical Review Letters.

Vérification de l'alignement de l'expérience avec le faisceau de lumière synchrotron de la ligne ID 19 de l'ESRF.

At high temperature (around 1000°C) and in their liquid state, some glasses behave as a highly viscous vinaigrette. Above a critical temperature, they spontaneously separate in two non-miscible liquids (like oil and water) and form an interlacing of tiny domains: inter-connected filaments at nanometer to micrometer scale. Within time the domains tend to grow, like droplets do in an emulsion: they coarsen. This domain growth phenomenon has interested scientists for a long time due to its universal nature. Many studies thus showed that the coarsening laws remain unchanged, regardless of the liquids nature, and that the domain geometry is self-similar: after rescaling with a zoom in, pictures taken at different times seem identical. Scientists call it dynamic scale invariance.

In comparison, the elementary mechanisms responsible for coarsening have been less studied. In order to characterize them, researchers from the PMMH Laboratory (CNRS/ESPCI Paris), and from the UMR Surface du Verre et Interfaces (CNRS/Saint-Gobain) benefited from the last advances in synchrotron tomography at ESRF. Using a 3D imaging technique they were able to directly film inside the molten glass the breaking and resorption events of the viscous filaments occurring at micrometric scales.

A gauche : Tomographie X du mûrissement d’un verre borosilicate de baryum à 1130°C. Seule la phase minoritaire, riche en baryum et moins visqueuse est ici représentée. Le domaine percolant est représenté en nuances de vert, en fonction de la courbure locale : les zones de forte courbure sont plus claires ; les domaines isolés sont en nuances de violet. En haut, un événement local de pincement capillaire entraîne la réorganisation des domaines. En bas, une vision plus large permet d’apprécier l’augmentation de la taille caractéristique et, dans cette expérience, de la fragmentation progressive. A droite : L'échantillon de verre est placé dans un petit creuset en alumine, et s'apprête à être introduit dans le four, le rayonnement rougeâtre est dû la température du four, de l'ordre de 1300°C

These observations confirm a theoretical scenario proposed by the American physicist E.D. Siggia almost 40 years ago: the resorption of each broken filament contributes to thickening the filaments which were connected to it. More surprisingly, the tracking with 3D imaging brought to light an unexpected phenomenon: when one of the liquids is more viscous than the other, filaments from the less viscous fluid are the only ones to break. This topological symmetry breaking has a major impact on the evolution of the mixture morphology. The more viscous liquid remains in a continuous network of interconnected filaments whereas the less viscous gradually fragments into a cascade of droplets.

With fast cooling it is still possible to freeze such structure into the solid state. The two liquids then form two tightly interlaced glasses. Tuning duration and temperature of the thermal treatment is a way to texture glass at small scale, by playing both with the size and morphology of the domains.

Associated publication

Topological Symmetry Breaking in Viscous Coarsening, David Bouttes, Emmanuelle Gouillart, and Damien Vandembroucq, Phys. Rev. Lett. 117, 145702
DOI:http://dx.doi.org/10.1103/PhysRevLett.117.145702

Researcher contact:

Damien Vandembroucq <damienvdb (arobase) pmmh.espci.fr>





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