Confined liquids can have properties that are poorly predicted from bulk parameters. We resolve with 0.5 nm resolution the nanoscale perturbations that interfaces cause on fluidity, in thin 3-methylpentane (3MP) films. The films of glassy 3MP are much less viscous at the vacuum-liquid interface and much more viscous at the 3MP-metal interface, compared to the bulk of the film. We find that the viscosity at the interfaces continuously returns to the bulk value over about a 3 nm distance. The amorphous 3MP films are constructed using molecular beam epitaxy on a Pt(111) substrate at low temperatures (<30 K). Ions are gently inserted at specific distances from the substrate with a 1 eV hydronium (D〈inf〉3〈/inf〉O〈sup〉+〈/sup〉) or Cs〈sup〉+〈/sup〉 ion beam. The voltage across the film, which is directly proportional to the position of the ions within the film, is monitored electrostatically as the film is heated at a rate of 0.2 K/s. Above the bulk glass transition temperature (T〈inf〉g〈/inf〉) of 3MP (77 K), the ions are expected to begin to move down through the film. However, ion movement is observed at temperatures as low as 50 K near the vacuum interface, well below the bulk T〈inf〉g〈/inf〉. The fitted kinetics predict that at 85 K, the glass is about 6 orders of magnitude less viscous near the free interface compared to that of the bulk.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry