On a macroscopic scale the friction, shear and flow of (confined) fluids is usually reasonably well described with a non-slip condition or with an effective slip length. However, on a molecular scale the details of the flow (diffusion, motion) of molecules at interfaces are still poorly known. We present real-time investigations of the interfacial molecular flow as it occurs in the course of the solidification processes within thin films of long chain alkanes at SiO2/air interfaces. Upon solidification the alkanes form solid monolayer domains with a uniform height (e.g. 4.3nm for C30H62). This height can be thicker or thinner than the liquid film thickness (which is given by the sample preparation(1)). As a consequence the domain formation is accompanied by a lateral alkane transport towards or away from the domain solidification front. This alkane flow/diffusion is imaged in real-space and real-time with a newly developed interference-enhanced optical microscopy technique with a thickness resolution of about 0.5 nanometer (2). We study the thickness profiles in the liquid alkane film ahead of the solidification front for various different overall alkane coverages. The observed (pronounced) "depletion" zones reveal details on the lateral alkane transport for local coverages varying between less than 0.5 nm (this corresponds to less than a complete monolayer of alkanes lying completely flat at the interface!) up to approximately bulk-like films (substantially thicker than one coiled molecule).  There are indications that the flow (lateral transport) within the submonolayer film is faster than in thicker films.

(1) H. Riegler and R. Koehler, Nature Physics 754 (2007)

(2) R. Koehler, P. Lazar, and H. Riegler, Applied Physics Lett. 89, 241906 (2006)

Domain growth at non-equilibrium after rapidly cooling an all liquid, 1.6nm thick film. The depletion zone in front of the solid domains is due to the alkane flow towards the solid domains (darker = thinner film). The image area is 0.7mm x 0.3mm. The gray-scale analysis is taken from the selected area indicated by the white frame.