Dewetting nucleated

The dependence on film thickness is attributed to the dewetting nucleation, which occurs in the 2.5-4.5 nm thickness range via the formation of randomly distributed droplets rather than the formation of holes. When the initial film thickness exceeds 4.5 nm, dewetting is trigged via nucleation of holes instead of droplets, and for film thickness above 10 nm, dewetting develops slowly via hole nucleation at defects. The different dewetting processes observed for different initial film thicknesses can be explained in terms of the variation of disjoining pressure and the inability of the polymer to spread on its own monolayer. 

The unstable situation caused when a spread him begins to dewet the surface has been studied [32, 33]. IDewetting generally proceeds from hole formation or retraction of the him edge [32] and hole formation can be a nucleation process or spinodal decomposition [34]. Brochart-Wyart and de Gennes provide a nice... 

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... 

Stange, T. G., Evans, D. F. and Hendrickson, W. A. (1997) Nucleation and growth of defects leading to dewetting of thin polymer films. Langmuir, 13, 4459-4465. 

Dewetting driven by van der Waals forces has been observed in many instances [25]. It is characterized by a wave pattern, as opposed to heterogeneously nucleated film break-up caused by imperfections in the film, leading to the formation of isolated holes that cause the dewetting of the film [26, 27],... 

Fig. 17 Results of a typical dewetting experiment for high molecular weight PS film. A 55-nm thick film = 4,840 kg/mol) was dewetted at 130°C on a silicon wafer coated with an adsrahed PDMS layer (M =139 kg/mol). The growth of a nucleated hole is compared to dewetting liom a straight edge, established by breaking the silicon wafer into two parts...

We anticipate that the entanglement density in the dry spin-coated films is lower than in an equilibrated bulk system. Such a departure fi om equilibrium most likely generates residual stresses. As one consequence, we tentatively attribute the observed hole nucleation, which initiated the dewetting process of holes, to the presence of such residual stresses within the film [165]. In this way, rupture can be related to the spin-coating process used to prepare thin polymer films. 

F. 25 Growth of typical dewetting holes in a 45-nm thick PS film was followed by optical microscopy, (a) The four holes marked 1-4 were nucleated after an incubation time of 20, 900, 1,440, and 3,180 s, respectively, after the film was brought to 125°C. (b) The radius of each hole (1-4 are shown limn top to bottom, respectively) is plotted as a function of the time the film was incubated at 125°C. (c) The ordinate is rescaled to the time that each hole had grown since it was nucleated, rather than the time elapsed since the film reached 125°C... 

Dewetting Surface Forces Isotropic, random structures (well-defined mean length scale) From tens of micrometers down to 100 tun Occur in molten ultrathin films or films with low elastic modulus. Nucleation can destabilize thicker films [125-131]... 

In the course of time (see Fig. 2.2a-d) these nucleated holes grew in size and eventually reached the periphery of the patch where dewetting stopped. The dewetted material form the patch was collected in a rim surrounding the dewetted patch. Thus, after dewetting reached the periphery of the patch and stopped, patches were enclosed by a circular rim, as can be clearly seen in Fig. 2.2e. The shape of the holes formed in such thin films was circular with both a diameter and a distribution on the substrate identical to the pattern of the imprinted domains. Some tiny droplets within the dewetted region resulted from the coalescence of rims of multiple holes nucleated within this area. Therefore, dewetting allowed to transform a quasi-two... 

A reduction in nucleation density of dewetted holes was also fotmd for such polystyrene films on homogeneous substrates [5]. Thus, a threshold film thickness hi can be determined for which the nucleation density of dewetted holes is identical to the number density of imprinted non-wettable domains. Obviously, h[Pg.31]

In accordance with Fig. 2.5, when increasing the film thickness h further, the number Vnoies of nucleated holes per unit area decreased further (see Fig. 2.8). Thus, the average distance between holes increased and the droplet deposition process could be followed over longer distances, i.e. the number of deposited droplets within a dewetted area increased. Interestingly, the width of the rim was found to be almost constant in time because the fluid added in the course of progressing dewetting was compensated by the continuous detachement of micro-droplets. 

Fig. 6.6 Schemes of two possible mechanisms whereby surface chemistry templates can guide the phase separation in polymer blend films. In both cases, the substrate is predominately one stnface chemistry—light green areas—with a second surface chemistry covering a minority of the substrate—black areas. In (a)-(e), the film vertically phase separates as induced by the majority surface chemistry, and then the minority surface chemistry induces dewetting of the lower polymer. In (f)-(j), the minority surface chemistry nucleates phase separation at precise locations before spontaneous nucleation can occur [6, 14]...

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. 1.24. Dewetting experiment (a) small droplet deposited (b) large droplet deposited (c) spreading of the large droplet, using a pipette, to make a metastable film, fixed around its outer edge to the wetting ring (d) nucleation and growth of a dry zone...

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