A direct quantitative measure of surface mobility in a glassy polymer
  Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature $T_{\textrm{g}}$. Above $T_{\textrm{g}}$ the entire film flows, while below $T_{\textrm{g}}$ only the near surface region responds to the excess interfacial energy. An analytical thin film model for flow limited to the free surface region shows excellent agreement with sub-$T_{\textrm{g}}$ data. The system transitions from whole film flow to surface localized flow over a narrow temperature region near the bulk $T_{\textrm{g}}$. The experiments and model provide a measure of surface mobility in a sample geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications. 
Yu Chai, Thomas Salez, Joshua D. McGraw, Michael Benzaquen, Kari   Dalnoki-Veress, Elie Rapha\"el, and James A. Forrest
cond-mat.soft cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.other physics.flu-dyn