Molecular motion in a spreading precursor film

Spreading of a polymer drop on a solid substrate was monitored with molecular resolution. Three characteristic rates, i.e., the spreading rate of the precursor film D-spread=(3.9+/-0.2)x10(3) nm(2)/s, the flow-induced diffusion rate of molecules within the film D-induced=1.3+/-0.1 nm(2)/s, and the thermal diffusion coefficient of single molecules D(therm)less than or equal to0.10+/-0.03 nm(2)/s, were independently measured. Since D-spread>D-induced, the plug flow of polymer chains was identified as the main mass-transport mechanism of spreading with an insignificant contribution from the molecular diffusion.