Transport phenomena in physisorbed films: Investigations with optically excited surface plasmons

Abstract

Experiments were performed on thl: latcral transpoti kinetics of wetting layers. Optically excited s~irface plasinon~ were used to study both the spreading behaviour of silicon oil and the equilibration dynamics of ethanol. The precursor film profiles of silicon oil wcrc found to agree well with the theory of viscous flow of nonvolatile 11quids. For ethanol, evaporation effects and VISCOUS transport could be observcd ~epai-alely. Apparently, the viccous transport is suppressed by almost two orders of magnitude. Possible explanation< are discussed. A lot of work has been dedicated to the transport kinetics of viscour, nonvolatile liquids on surfaces 11 - 51, i.e., the spreading of liquid drops. For systems in which evaporation plays a significant role, experiments become more difficult, since it may be necessary to separate different transport mechanisms in the analysis. Consequently, systematic measurements of lateral transport in wetting layers of volatile liquids have not been performed due to the lack of suitable experimental techniques. Fn the preFent work, the transport dynamics of thin films of both poly-dimethyl-siloxane (PDMS), a viscous fIuid which i~ only slightly volatile [3, 83, and ethanol is studied using optically excited surface plasmons. In this technique, one exploits the fact that the phase velocity of the surface plasmons propagating on the metallic substrate dependr very sensitively on the thickness of the adsorbed layer. By attenuated total reflectance, Furface plasmons can be resonantly excited by a laser beam at a certain angle of incidence corresponding to the phase veIocity of the surface plasmons and thus to the adsorbed layer thickness [6, 71. For the studies of the spreading behaviour of PDMS, we performed so-called vertical rise experiments. At time 1 = 0, the vertical substrate was put in a trough containing some PDMS 181, which wets metal surfaces completely [I, 31. We then observed the wetting layer of PDMS creeping up the substrate