Microscopic Film Dewetting

Below ec, there are two dewetting mechanisms (a) a macroscopic film is metastable and dewets by nucleation and growth of dry zones (b) a microscopic film is unstable and spontaneously breaks into a multitude of droplets. Capillary waves are amplified and this mechanism is called spinodal decomposition, by analogy with what happens in phase transitions. 

Fig. 1.28. Dewetting of microscopic films through the spinodal decomposition mechanism for a polystyrene film deposited on a silicon wafer (Reiter, G.) (a) spontaneous nucleation of the hole (b) final stage, in which the microdroplets arrange themselves into polygons. The length of the black bar is 10 lm. Film A has thickness 25 nm and film B has thickness 45 nm...

Experiments with liighly viscous PDMS on silanized silicon wafers have shown that, for thick films (e > 10 pm), the dewetting is of the classic type and obeys the law i oc <. On the contrary, for microscopic films (e < 1 pm), C. Redon found a dependence of the type R [Pg.168]

The technique used to study dewetting dynamics on materials consists of making a flat, smooth elastomer surface. A hquid puddle is deposited within a 50-mm-diameter ring of 0.1-mm-thick plasticized adhesive paper adhering to the substrate. The adhesive paper acts as a spacer. A microscope slide is drawn over the liquid to obtain a liquid film of ca. 0.1-mm thickness. At this thickness, the liquid film is unstable, being much less than the equilibrium value, of ca. 1.5 mm calculated from Eq. (29). Nucleation of dry patches... 

A prototypical form of the dependence of the interface potential, g, on temperature or strength of the attraction between liquid and substrate is sketched in Figure 10. In this figure we assume that Ap = 0, that is, liquid and vapor coexist. At weak attraction or low temperatures, T interface potential exhibits a sharp minimum at a very small film thickness, It. In the case that the liquid dewets from the substrate, h characterizes the residual amoimt of liquid on the dewetted substrate areas. It is a microscopic value, which can be vanishingly small for strongly first-order wetting transitions. The minimal value of the interface potential sets the contact angle. 

Redon C, Brzoka JB, Brochard-Wyart F. Dewetting and slippage of microscopic polymer films. Macromolecules 1994 27 468 71. 

Optical microscopy (OM), Olympus microscope in a reflection mode, was used to check the dewetting behavior of PDMS and PDMS ionomers on the silica surface. Imaging of as-cast Li-CPDMS and annealed films were performed with an atomic force microscope (AFM) MFP (Asylum Research, Inc.) in tapping mode. Silicon tips (tip curvature radius = 5-10 nm) with a spring constant of about 2.5 N/m were used. 

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