Thin films stabilizing mechanisms

Foams are thermodynamically unstable. To understand how defoamers operate, the various mechanisms that enable foams to persist must first be examined. There are four main explanations for foam stabiUty (/) surface elasticity (2) viscous drainage retardation effects (J) reduced gas diffusion between bubbles and (4) other thin-film stabilization effects from the iateraction of the opposite surfaces of the films. 

Film studies in the past decade have revealed the existence of another film stability mechanism If the continuous phase contains not only a small amount of surface active substances but also a "sufficient amount" of "small particles", these particles can form layers inside the draining film (see Fig. 6, right)[9,22-32]. As a result, such films thin step-wise, by several step-transitions (also called stratification) when at a step transition a layer of small particles leaves the film. 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

An extended work, experimental as well as theoretical, on mechanisms in thin microscopic foam lamellas and macroscopic foams has been published by Krugljakov Exerowa (1990). In the theory of Exerowa Kashiev (1986) thin-film stability is explained in terms of lateral diffusion of vacancies in a lattice like adsorption layers. As an example, selected experimental results of Exerowa et al. (1983) are shown in Fig. 3.18. 

BiaxiaHy orieated PPS film is transpareat and nearly colorless. It has low permeability to water vapor, carbon dioxide, and oxygen. PPS film has a low coefficient of hygroscopic expansion and a low dissipation factor, making it a candidate material for information storage devices and for thin-film capacitors. Chemical and thermal stability of PPS film derives from inherent resia properties. PPS films exposed to tolueae or chloroform for 8 weeks retaia 75% of theh original streagth. The UL temperature iadex rating of PPS film is 160°C for mechanical appHcatioas and 180°C for electrical appHcations. Table 9 summarizes the properties of PPS film. 

In the second area, improvements to the thermal and mechanical stability of nanoporous materials from ordered block copolymers should be targeted. To expand the application base for these materials, high temperature stability is a key requirement. For example, in templating applications that require elevated processing temperatures in either thin films or monolithic materials... 

K. Kozco, A.D. Nikolov, D.T. Wasan, R.P Borwankar, and A. Gonsalves Layering of Sodium Caseinate Submicelles in Thin Liquid Films-A New Stability Mechanism... 

Spray pyrolysis of ethanolic solutions of Fe(acetylacetone)3 or FeCls between 370°C and 450°C onto a glass substrate are reported for the fabrication of a-Fe20s thin-film photoanodes [75]. Upon illumination by a 150 W Xe lamp samples consistently demonstrate photocurrents of 0.9 mAcm , IPCE values up to 15%, and robust mechanical stability with no signs of photocorrosion for the undoped samples. With simultaneous multiple doping of 1% A1 and 5% Ti, an IPCE of 25% can be reached at 400 nm. Zn doping is known to induce p-type character in Ee20s thin film electrodes [76]. 

Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

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