Seminar Gulliver, Paddy Royall (University of Bristol)

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8 octobre 11:30 » 12:30 — Bibliothèque PCT - F3.04

Revealing the Nature of Solidification : Fivefold Symmetry at the Nanoscale

That fivefold symmetry should play a crucial role in the non-equilibrium behaviour of condensed matter was proposed by Sir Charles Frank in the 1950s. Six decades later, the basic mechanism of the solidification of liquids remains unexplained, either in the case that the material crystallises, or that it forms an amorphous solid, a glass. We will explore the implications of fivefold symmetry in the solidification of liquids and discuss two recent developments.

Crystallisation is among the most common everyday physical phenomena. Yet in the prinicple material in which quantitative comparison has been made between experiment and theory (hard spheres) predictions of crystal nucleation rates are up to 20 orders of magnitude slower than measurements, the second worst prediction in physics. This discrepancy casts doubt upon the theoretical methods concerned (importance sampling) which is important not only for crystallisation, because these methods are used to tackle a very wide range of problems, such as drug uptake in cells and chemical reaction pathways. While particle-resolved studies reveal in intimate detail local mechanisms of structural change, they can only access rather fast crystallisation, where there is no discrepancy. However gravitational sedimentation may influence the local structure of the colloidal liquid, which we show can drastically alter crystallisation. We further image nano-particles with STED nanoscopy, whose timescales are orders of magnitude faster than the larger colloids : thus by rescaling time, we access new dynamical regimes where the massive discrepancy is found.

The nature of amorphous solids (glasses) is not understood : the possibility of a phase transition to a thermodynamically stable ideal glass is a contentious and challenging issue. Mutually exclusive theories give equally good descriptions of experimental data . Again, rescaling time enables us to approach this putative transition closer than other techniques, revealing exceptional levels of fivefold symmetry and a tantalising drop in configurational entropy.

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