Stability of Thin Wetting Films on Chemically Nanostructured Surfaces

The morphology and stability of thin volatile wetting films on model chemically patterned surfaces composed of periodic arrays of alternating completely and partially wettable nanostripes are investigated. The equilibrium film morphology is recorded as a function of undersaturation using noncontact atomic force microscopy. Films spanning the entire pattern are found to be stable only for thicknesses in excess of a critical value, $h_c$, whereas thinner films spontaneously dewet the partially wettable regions of the substrate. The critical thickness $h_c$ increases linearly with the width of the partially wettable stripes in good agreement with an interface displacement model derived from microscopic density functional theory. These results provide detailed insights into the dewetting of thin films driven by competing intermolecular forces.

Figure 1

(a) Friction-mode AFM image showing an array of carboxylic stripes (bright, completely wettable) patterned on an OTS substrate (dark, partially wettable). (b) Time-resolved noncontact AFM image of an octane film wetting the pattern shown in (a). (c) Cross-sectional profiles of the film for $\Delta T=20(1)$, 110 (2), 113 (3), and 163 mK (4). See the main text for further details.

Figure 2

(a) Enlarged view of the measured (hollow red dots) and calculated (solid line) morphology of the film surface just before rupture. The shaded areas indicate the position of the partially wettable stripes. For all stripes $W=230\mathrm{nm}$, whereas the inhomogeneity of the gap size is modeled by using $G_0=47$, $G_1=45$, $G_2=42$, and $G_3=43\mathrm{nm}$. (b) Full view of the calculated (black lines) and measured film profiles before (hollow red dots) and after (hollow blue squares) complete dewetting.

Figure 3

Measured (symbols) and calculated (solid and dashed lines) critical film thicknesses versus the axially averaged gap size. Different symbols correspond to different values of $W$: 230 nm (open circle), 270 nm (open diamond), 300 nm, (left-pointing triangle) 400 nm (upward triangle), and 580 nm (open square). The inset shows the effective interface potentials of octane on homogeneous OTS (red) and OTSox (blue) substrates, denoted by “partial wetting” and “complete wetting,” respectively.