Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [R. Fetzer et al., Phys. Rev. Lett. 95, 127801 (2005); O. Bäumchen et al., J. Phys.: Condens. Matter 24, 325102 (2012)] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, this change in boundary condition is affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare bidisperse mixed SAMs of alkyl silanes, with the composition as a control parameter. We find that all the mixed SAMs investigated show an enhanced slip boundary condition as compared to the single-component SAMs. The slip boundary condition is accessed using optical and atomic force microscopy, and we describe these observations in the context of X-ray reflectivity measurements. The slip length, varying over nearly two orders of magnitude, of identical polymer melts on chemically similar SAMs is found to correlate with the density of exposed alkyl chains. Our results demonstrate the importance of a well characterized solid/liquid pair, down to the angstrom level, when discussing the friction between a liquid and a solid.
We cannot guarantee that the composition of the resulting SAMs is identical to the relative silane compositions of the precursors, yet a comparison to molecular dynamics simulations provided later (see Figure 4) suggests that the electron density profile of the experimental ϕ = 0.50 monolayer is compatible with the one prepared in the simulations by randomly placing OTS or DTS molecules on silicon oxide binding sites with equal probability.
The quoted cubic dependence on contact angle of the dewetting velocity is strictly valid for small angles. Although the contact angles are not necessarily small in our experiments, the expected velocity is always increasing with the equilibrium angle, which is in opposition to the trend observed here.