Slip Length in Deeply Supercooled Molecular Glass Formers

In polymer melts, the hydrodynamic boundary condition at the substrate interface can deviate from the normally assumed no-slip boundary condition such that fluid velocity at the substrate is non-zero in supported high-molecular weight polymer thin films. Depending on the substrate/polymer interactions, the measured slip lengths can be on the order of the film thickness or larger, greatly influencing film dynamics during dewetting. De Gennes’ original theory for slip in polymers predicts that the slip length should approach the monomer size in molecular liquids. We show that for deeply supercooled molecular glass formers near their T_g, in which chain dynamics cannot play a role in slip, the slip lengths can be surprisingly large and still on the order of the film thickness on a weakly interacting substrate. To better understand the origin of slip in these systems, we investigate the role of molecular shape and orientation near the substrate using a series of molecules that range from rod-like to nearly spherical. We also employ coarse-grained molecular dynamics simulations to gain insight into the origins of the observed slip.