Dewetting holes

The occurrence and growth of dewetted holes, or dry patches, is followed using a video camera rigged up to a low-power microscope. Using a video recorder, it is possible to analyze up to 24 frames/second. 

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... 

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]

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 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. 

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],... 

This review has discussed the phase behavior of polymer blends and symmetric block copolymer melts in thin film geometry, considering mostly films confined between two symmetrical hard walls. Occasionally, also an antisymmetric boundary condition (i.e. one wall prefers component A while the other wall prefers component B) is studied. These boundary conditions sometimes approximate the physically most relevant case, namely a polymeric film on a solid substrate exposed to air or vacuum with a free, fiat surface (Fig. 1). The case where the film as a whole breaks up into droplets (Fig. 2) due to dewetting phenomena is not considered, however, nor did we deal with the formation of islands or holes or terraces in the case of ordered block copolymer films (Fig. 4b-d). 

A. Sharma and E. Ruckenstein, Dewetting of solids by the formation of holes in macroscopic liquid films, /. Colloid Interface Sci. 133, 358 (1989) G.l. Taylor and D.H. Michael, On making holes in a sheet of fluid, J. Fluid Mech. 58, 625 (1973). 

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