Dynamics of Glass Relaxation at Room Temperature

The problem of glass relaxation under ambient conditions has intrigued scientists and the general public for centuries, most notably in the legend of flowing cathedral glass windows. Here we report quantitative measurement of glass relaxation at room temperature. We find that Corning® Gorilla® Glass shows measurable and reproducible relaxation at room temperature. Remarkably, this relaxation follows a stretched exponential decay rather than simple exponential relaxation, and the value of the stretching exponent ($\beta =3/7$) follows a theoretical prediction made by Phillips for homogeneous glasses.

Figure 1

Schematic diagram of the experimental setup for measuring strain as a function of relaxation time. (a) Grid of scribe lines marked on both the reference plate and the sample. (b) Image of overlaid grid lines from the reference and the sample, used to calculate the $x$ and $y$ displacements.

Figure 2

Room temperature relaxation of two $1050\times 1050\times 0.7\mathrm{mm}$ sheets of Corning® Gorilla® Glass (code 2318) after being quenched from an initial 30-min heat treatment at $250°\mathrm{C}$. The symbols show the individual linear strain measurements over a total experimental time of 1.5 yr. Throughout the course of the experiment the glass sheets were stored in controlled temperature and humidity environments as described in the text. The solid blue line shows a fit of this strain data using a stretched exponential decay function with an exponent of $\beta =3/7$, which is characteristic of relaxation dominated by long-range pathways. The fitted value of the relaxation time is $\tau =27.6\mathrm{d}$. A separately fitted simple exponential decay curve is also shown for comparison.

Figure 3

Viscosity of Corning Gorilla Glass in the equilibrium (liquid) and nonequilibrium (glassy) regimes. Viscosity measurements were performed according to the procedures described in the text and in Refs. 14, 15. The liquid- and glassy-state viscosity data are fit using the MYEGA [14] and MAP [15] models, respectively. Extrapolation of the nonequilibrium glassy viscosity curve to room temperature gives an estimated viscosity of ${10}^{22.25}\mathrm{Pa}\mathrm{s}$, which corresponds to a structural relaxation time on the order of 19 000 yr. Thus, the room temperature relaxation shown in Fig. 2 is not a product of viscous flow.