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Frustrated solidification in microjets of undercooled liquid hydrogen mixtures

AutorFernández Sánchez, José María ; Kühnel, Matthias; Tejeda, Guzmán ; Moreno, Elena ; Kalinin, Anton; Grisenti, Robert E.; Montero, Salvador
Fecha de publicación2013
EditorCzech Academy of Sciences
CitaciónXXV International Symposium on Molecular Beams (ISMB 2013), June, 09 - 14, 2013, Prague, Czech Republic
ResumenNovel experiments on undercooled liquid microjets (filaments) of para-hydrogen (p-H2) and ortho-deuterium (o-D2) mixtures will be reported. These highly collimated filaments, less than 10 microns in diameter, are an ideal medium to produce undercooled liquid samples and to investigate the homogeneous solidification process, free from wall effects [1]. The filaments exit from cryogenic capillary nozzles into vacuum, to cool down fast by surface evaporation, with a temperature gradient across the jet due to their finite size radius and thermal conductivity. The filaments are monitored by laser shadowgraphy, and analyzed by means of high performance Raman spectroscopy [2], revealing their structure and temperature. The high spatial resolution of Raman spectroscopy allows observing in situ the structural changes of the liquid microjets, with a time resolution of ~10 ns. The filaments of pure p-H2 can be cooled down to 9 K (normal melting point at 13.8 K), before eventually solidifying at a crystal growth rate of ~33 cm/s [3]. Crystal growth rate in o-D2 is ~¿2 smaller, consistent with a collision-limited process [4]. Our new experiments also show that adding small amounts of o-D2 to the p-H2 sample leads to a dramatic slowing down in the crystallization kinetics of the mixture. Phase transformations in undercooled liquids, and especially the understanding of the exact interplay between crystallization and glass formation, still poses great challenges to both theory and experiments [5]. One possible framework to explain the origin of the slowing down of the dynamics in glass-forming liquids is offered by the concept of geometric frustration associated to the development of locally favoured structures in the undercooled liquid. Simulation studies have reported a correlation between specific structural features and slowed dynamics in a variety of model systems, but the experimental demonstration of such an intrinsic link has proven challenging so far. Here, that link is supported by our measurements on the slowing down in the crystallization kinetics of diluted mixtures of o-D2 in p-H2 in liquid microjets. Full path-integral Monte Carlo simulations show that the observed effect is a result of the (weak) zero-point contribution to the interaction potential between the particles. This favours the development in the undercooled liquid mixture of icosahedral local structures around the o-D2 solute molecules, and thus frustrates the p-H2 crystal growth. While our work strongly supports the view of an intrinsic link between local order and frustrated crystallization, it provides as well the first experimental evidence for the role played by quantum fluctuations during structural transformations in undercooled liquids. [1] R. E. Grisenti, R. A. Costa-Fraga, N. Petridis, R. Dorner, and J. Deppe, EuroPhys. Lett. 73, 540-546 (2006). [2] S. Montero, J. H. Morilla, G. Tejeda, and J. M. Fernandez, Eur. Phys. J. D 52, 31-34 (2009). [3] M. Kühnel, J. M. Fernández, G. Tejeda, A. Kalinin, S. Montero, and R. E. Grisenti, Phys. Rev. Lett. 106, 245301 (2011). [4] J. M. Fernández, M. Kühnel, G. Tejeda, A. Kalinin, R. E. Grisenti, and S. Montero, AIP Conf. Proc. 1501, 1296-1304 (2012). [5] M. D. Ediger and P. Harrowell, J. Chem. Phys. 137, 080901 (2012).
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