Dewetting Mechanisms

It is important to clarify the distinction among stable, metastable, and unstable films using an easily detectable parameter like the thickness of a thin polymer film, defined by the distance separating the interfaces of substrate/polymer and the air/polymer. The stability of a film depends on the effective interface potential qp h) as a function of h, defined as the excess free energy per unit area that is necessary to bring two interfaces (solid-liquid and liquid-gas interface) from infinity to the distance h. 

Considering mass conservation, surface fluctuation has a critical wave vector q, = 3a/h, where a is a molecular 

For q q, the fluctuations of the film surface are amplified exponentially with a fastest growing wave vector q  

As the amplified fluctuation touches the substrate, a hole forms in the film. This rupture mechanism is called spinodal dewetting [26,27,36]. 

---

The authors distinguish between three different cases for the dewetting mechanism (Figure 9). In thin films as well as in the bulk, a regular bilayer morphology is developed. Disordering into an isotropic melt occurs in two steps. A smectic mesophase is formed before the layered structure finally breaks up at elevated temperatures. This transition is characteristically effected by the interfaces in thin films on a flat substrate and, as a consequence, a peculiar self-dewetting is observed. 

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. 

The first example concerns deep, enclosed structures such as those presented in Fig. 15.15a,b. As illustrated in the previous example, the dewetting mechanism in such structures is modified... 

The discussion up to this point may give the impression that polymer thin Aims dewetting experiments are rather simple, as they involve a few easy steps like Aim preparation, heating/solvent vapor exposure and Anally, characterization of the evolved structures with an optical or atomic force microscope. However, extracting the exact information about the dewetting mechanism from a particular experiment... 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

Adicoff, Dynamic Mechanical Behavior of Highly Filled Polymers Dewetting Effect , Rept No NWC-TP-5486 (1971) 11) H. Yasu-... 

The above statements are adequate for liquid defoamers that are insoluble in the bulk. Experience has proven, however, that certain dispersed hydrophobic solids can greatly enhance the effectiveness of defoaming. A strong correlation between the effectiveness of a defoamer and the contact angle for silicone-treated silica in hydrocarbons has been established [300]. It is believed that the dewetting process of the hydrophobic silica causes the collapse of a foam by the direct mechanical shock occurring by this process. 

One approach which does not utilize a confining fluid has been developed by Saylak (83). This technique involves an optical system which continuously monitors the lateral strain in a uniaxial specimen. The specimen must be circular in cross section, and the volume change computation requires uniform dewetting throughout the sample. This method is not rate and temperature limited since no mechanical attachments or fluids are in contact with the sample. A schematic of the lateral strain device is shown in Figure 13. Surland and Givan (104) also describe an... 

Surface cracks migration products roughness accumulation of degradation products oxidizer concentration owing to moisture localized dewetting All surface and sub-surface changes that effect reflected radiation density Subjective observations traveling mechanisms required... 

---