Study shows glass surfaces retain memory

A study published today has revealed that glass surfaces can retain a memory of the direction of flow of the glass in its past liquid state.

This surprising conclusion is the result of a collaboration between scientists from the Optoelectronics Research Centre (ORC) at the University of Southampton and research units PMMH and SIMM (CNRS/ESPCI Paris), the Neurophotonics Lab (CNRS/Université Paris Descartes).

The structure of a vitreous material, such as a window pane, very closely resembles a snapshot of the liquid it was, before it cooled down to reach its solid state. Similarly, the surface of a glass remembers its past as a liquid interface. The ultra-low (sub-nanometer) roughness of a glass surface results from the freezing of the thermal fluctuations of the liquid interface. This ultra-smoothness corresponds to the lower bound predicted by equilibrium thermodynamics. Published in the journal Physical Review Letters, the study shows glass interfaces that have experienced high levels of mechanical stress in their past liquid state can become even smoother than this thermal equilibrium limit.

The team of scientists performed high precision Atomic Force Microscopy (AFM) measurements on the interfaces of hollow silica glass fibres produced in a standard fibre drawing process. The results show that the roughness of the glass surfaces obtained in such out-of-equilibrium conditions is about 40 per cent lower than the expected lower bound.

The typical roughness can be decreased down to 0.15 nanometres, equivalent to the interatomic distance in silica. Furthermore, surface fluctuations are reduced along the drawing direction. The solid glass surface therefore records in its structure the direction of the flow to which it was subject in the high temperature liquid state.

Beyond scientific interest, the ability of glass surfaces to retain memory is of technological interest to the development of hollow-core photonic band-gap optical fibres. In these fibres, light is guided within a central hollow core surrounded by a honeycomb-like microstructure. Reducing the roughness of glass interfaces could help reduce loss due to light scattering from glass interfaces, and may ultimately improve the data-carrying properties of this new class of optical fibres.

The study formed part of the European ModeGap (Multimode Capacity Enhancement with Photonic Band-Gap Fibres) project and has also benefitted from funding from the Région Ile-de-France (DIM OxyMore) and the UK Engineering and Physical Research Council.