Stepped ellipsometric profiles and structural interaction

Surface-induced layering corresponds to oscillations of the disjoining pressure in the molecular range of thickness (see also chapter 1). Well-known saw-tooth force profiles are obtained with surface forces apparatus (SFA) setups. In the case of microdroplets, where the thickness is free to adjust, an unstable part in the disjoining pressure behaves just like the usual pressure at three dimensions a Maxwell construction leads to a horizontal part in n(f), corresponding to the coexistence of films with different thickness (see chapter 3). 

Stepped profiles result if the liquid is nonvolatile, not only at 3D, but also at 2D, where the number of neighbors is less. ° 

Then a thickness range where n(f) is constant corresponds to a steep part in the droplet profile. The exact profile of the steps edges is not accessible using ellipsometry, because of the poor spatial resolution. This is why Eqs. 5.2 and 5.8a, where the interface slope is assumed to be small, are enough for ellipsometric studies. Although the spatial resolution is poor, the high-thickness resolution provides information about interactions and sometimes about molecular configurations in the film. 

The thickness profiles of microdroplets of polydimethylsiloxane (PDMS) oligomers deposited on oxidized silicon wafers exhibit equal thickness steps, each 0.7 nm thick (Fig. 5.4). The number of visible steps, their lengths, and their growth d3mamics depend on the surface chemistry of the wafers. The common step thickness is the transverse size of the PDMS chain, which means that each step corresponds to a compact flat monolayer of chains, that is, a 2D nonvolatile liquid. Information on the disjoining pressure, and on the friction coefficients between successive layers, can be extracted. The d3mamics of adsorption of silanol ends when silanol-terminated PDMS (PDMS-OH) is deposited on the wafer can also be monitored in real time.  

With TK, the picture is somewhat different. Depending on the substrate cleaning, either all the steps have equal thickness, around 1 nm, which is the diameter of the molecule in a spherical conformation, or the first step is thinner (0.8 nm) and the next 

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