Mechanism of fast surface self-diffusion of an organic glass

Zhu et al. [L. Zhu, C. W. Brian, S. F. Swallen, P. T. Straus, M. D. Ediger, and L. Yu, Phys. Rev. Lett. 106, 256103 (2011)] measured the surface self-diffusion for an organic glass former, indomethacin, and found surface diffusion is more than 10(6) times faster than bulk diffusion at temperatures around T-g. With the help of dielectric relaxation and differential scanning calorimetry measurements on bulk indomethacin, and analysis of the data using the coupling model, we provide a quantitative explanation. We find the bulk alpha-relaxation time is longer than the primitive relaxation time also by about six orders of magnitude in a range of temperature above and below the bulk T-g. Thus, the cause of the fast surface diffusion is the nearly vanishing of intermolecular coupling of relaxation and diffusion at the surface. The results of related experimental studies of enhanced relaxation and diffusion at the surface of other glass formers also have been analyzed and quantitatively explained. Our predictions on surface diffusion from the coupling model are compared with that given by the random first order transition theory.