Surface self-diffusion has been measured for an organic glass for the first time. The flattening of 1000 nm surface gratings of liquid indomethacin occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 10(6) times faster than bulk diffusion, indicating a highly mobile surface. Our data suggest that surface diffusion is the leading mechanism of surface evolution for organic glasses at micrometer to nanometer length scales.

Surface self-diffusion coefficients have been determined for the organic glass Nifedipine using the method of surface grating decay. The flattening of 1000 nm surface gratings occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 10(7) times faster than bulk diffusion, indicating a highly mobile surface. Nifedipine glasses have faster surface diffusion than the previously studied Indomethacin glasses, despite their similar bulk relaxation times. Both glasses exhibit fast surface crystal growth, and its rate scales with surface diffusivity. The observed rate of surface diffusion implies substantial surface rearrangement during the preparation of low-energy glasses by vapor deposition. The Random First Order Transition Theory and the Coupling Model successfully predict the large surface-enhancement of mobility and its increase on cooling, but disagree with the experimental observation of the faster surface diffusion of Nifedipine.

Surface self-diffusion coefficients have been measured for the model molecular glass o-terphenyl (OTP) through surface-grating decay driven by capillarity. The decay mechanism transitions from viscous flow at high temperatures to surface diffusion at low temperatures; for 1000 nm wavelength gratings, the transition occurs at Tg + 11 K. The surface diffusion of OTP is 10(8) times faster than bulk diffusion at Tg and even faster at lower temperatures because of its weaker temperature dependence. At Tg, OTP has approximately the same bulk diffusivity as the previously studied molecular liquid indomethacin, but its surface diffusion is 100 times faster. While the molecular glass-formers exhibit transitions from viscous flow to surface diffusion as the mechanism of capillarity-driven surface flattening, polystyrenes and silicates show no such transition under comparable conditions, suggesting slower surface diffusion on these materials and a general dependence of surface diffusion on intermolecular forces. The velocity of surface crystal growth on molecular glasses is proportional to surface diffusivity, indicating a common kinetic barrier for both processes for temperatures below Tg.

The effect of physical aging on surface diffusion has been determined for two organic glasses, Indomethacin and Nifedipine. The two systems exhibit similar aging kinetics typical of organic glasses. Surface diffusivity remains unchanged despite significant bulk aging that nearly equilibrates the systems and increases the bulk relaxation time by orders of magnitude. The finding is relevant for understanding the stability of amorphous materials and the formation of low-energy glasses by vapor deposition.