Structural glass transition of supercooled liquids

7 problems

• What sets the fragility of a glass-forming liquid?

  Open

  Some liquids, such as molten silica, slow down smoothly as they approach the glass point, while others, such as many organic liquids and polymers, slow down abruptly; this steepness is called fragility. Why liquids differ so much is not known.

• Does a growing static length control relaxation near $T_g$?

  Open

  Near the glass point, molecules rearrange in cooperating groups whose size grows on cooling. It is unknown whether a growing structural length that ordinary structure measurements cannot see controls how slow the liquid becomes.

• Microscopic nature of the Johari-Goldstein secondary relaxation

  Contested

  Below the glass point most molecules are frozen, yet glasses of even rigid molecules still show a faster, smaller relaxation, the Johari-Goldstein $\beta$ process. Which molecular motions produce it, and whether it is the precursor of the main relaxation, is disputed.

• Does an ideal glass transition occur at finite temperature in 3D?

  Contested

  If one extrapolates the entropy of a cooling liquid, it seems to drop to that of the crystal at a finite temperature, the Kauzmann temperature, where an ideal glass would form. Whether this really happens, or the extrapolation fails, is unknown.

• What causes the super-Arrhenius slowdown of supercooled liquids?

  Contested

  As a glass-forming liquid is cooled, the time its molecules take to rearrange grows by about 14 powers of ten over a narrow temperature range, much faster than the simple Arrhenius law (a fixed energy barrier) predicts. Which microscopic mechanism produces this growth is unknown.

• Why diffusion decouples from viscosity near the glass transition

  Partially resolved

  In ordinary liquids, how fast a molecule wanders is inversely proportional to the viscosity, the Stokes-Einstein relation. Near the glass point molecules wander much faster than this rule predicts, and a quantitative explanation is missing.

• How close can vapor-deposited glasses approach the ideal glass?

  Open

  Glasses grown slowly from vapor onto a cooled surface can be as stable as ordinary glasses aged for thousands of years or more. How far this route can go toward the hypothetical ideal glass, and what limits it, is unknown.