Film on a Solid Substrate

In a total wetting regime S 0), a film is always stable. By contrast, when 5 0, a film will dewet below a critical thickness Cc, as can be observed in a Teflon pan or on a plastic sheet. When you place a small drop of water on one of these materials, it will contract into a spherical cap. When you place a larger drop, it will form a puddle flattened by gravity. If you use 

FIGURE 7.2. Free energy F e) as a function of the thickness e of a liquid film deposited on a non wet-table substrate. Maxwell s construction describes the coexistence between the dry solid (e = 0) and a puddle (e = Cc), and shows that a film of thickness e 6c lowers its energy by breaking up into two phases (dry solid and puddle), as indicated by the arrow. 

For thick films, F e) is the sum of the interfacial energy and the gravitational energy  

Equilibrium between the dry solid and the puddle can then be expressed 

Equation (7.2) does not apply to microscopic films because F(e —0) fails to merge with F(0) = 750- To remedy this problem, we must include in F e) the contribution P e) of the long-range forces discussed in chapter 4. The complete expression for F e) then reads 

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The force exerted by a thin fluid film on a solid substrate can be measured with nearly molecular precision in the SFA [27]. In the SFA a thin film is... 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a solid substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of solid materials (193—195). A wide variety of SAM-based supramolecular structures have been generated and used as functional components of materials systems in a wide range of technological applications ranging from nanolithography (196,197) to chemical sensing (198—201). 

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). 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

Figure 6-1. A sketch of the generic thin-film configuration for a liquid film on a solid substrate that is tilted to an angle a from the horizontal. The upper surface of the film is an interface.

W. W. Zhang and J. R. Lister, Similarity solutions for van der Waals rupture of a thin film on a solid substrate, Phys. Fluids 11, 2454-62 (1999). 

The concept for the rupture of a free liquid film or a thin liquid film on a solid substrate is used in some applications, for example in the flotation process. Scheludko et al. (1968) have published first contact angle measurements for liquid films. 

Figure 3. Schematic drawing illustrating formation of a thin film on a solid substrate.

R. Yerushalml-Rozen and J. Klein, Stabilization of nonwetting thin liquid films on a solid substrate by polymeric additives, Langmuir, 11, 2806... 

The main purpose of the SFA is to measure the forces exerted by a thin fluid film on a solid substrate with nearly molecular precision (143). In the SFA, a thin film is confined between the surfaces of two macroscopic cylinders arranged sucli that their axes are at a right angle (143). In an alternative setup, the fluid is confined between the surface of a macroscopic sphere and a planar substrate [144]. However, crosscd-cylinder and sphere-plane configurations can be mapped onto one another by differential-geometrical arguments [145]. The surface of each macroscopic object is covered by a thin mica sheet with a silver backing, which permits one to measure the separation h between the surfaces by optical interferometry [143]. 

In systems of particles at a thin liquid film on a solid substrate, the relevant disjoining pressure can be formally approximated by an effective gravitational field [36] (see Section 2.3.1.1.). 

The approach that we use in the preceding expression can also be utilized in the case of a concave meniscus in a capillary, or in the case of drops in partial wetting, that is, forming a finite contact angle, 0 with the equilibrium films on a solid substrate. In Figure 2.31, we qualitatively show the form of distribution of the excess forces/(jc), normal to the substrate for a meniscus in a flat capillary with a width of 2H (Figure 2.31a, curve 1, > 0) and for a drop (Figure 2.31b,... 

Films spread at liquid-liquid interfaces or on liquids other than water are discussed followed by the important effects of charged monolayers on water. Finally, the most technologically important application of Langmuir films, the Langmuir-Blodgett film deposited on a solid substrate, is reviewed. 

Four possible mechanisms for solid-state extraction (a) adsorption onto a solid substrate (b) absorption into a thin polymer or chemical film coated on a solid substrate (c) metal-ligand complexation in which the ligand is covalently bound to the solid substrate and (d) antibody-antigen binding in which the receptor is covalently bound to the solid substrate. 

When LiMn204 electrodes are deposited as thin films on a platinum substrate, either by electron-beam evaporation or radiofrequency (rf) sputtering, structures are sometimes formed that exhibit unusual electrochemical behavior [146, 147]. Such electrodes have been evaluated in solid-... 

For the investigation of polymer systems under spatial confinement, fluorescence microscopy is a powerful method providing valuable information with high sensitivity. A fluorescence microscopy technique with nanometric spatial resolution and nanosecond temporal resolution has been developed, and was used to study the structure and dynamics of polymer chains under spatial confinement a polymer chain in an ultra-thin film and a chain grafted on a solid substrate. Studies on the conformation of the single polymer chain in a thin film and the local segmental motion of the graft polymer chain are described herein. 

In many practical situations, the waveguiding layer consists of an optically dense media formed as a thin film with a thickness dp and refractive index nP. This film is usually applied on a solid substrate (typically glass) with refractive index ns less than the RI of the film. The macroscopically thick substrate creates a mechanically stable foundation for the above film, which has a thickness typically in the submicron range. In this case, the light can travel by TIR inside the film, in a zigzag manner (see Fig. 15.2a), creating the evanescent wave both in the aqueous cover solution with refractive index nc and in the substrate10. 

In contrast to solid dendrimers [71, 84, 88], molecular resolution was not achieved for a closed film of the carbosilane dendrimers. However, one could extract structural information about the molecular conformation from the film thickness and wetting edges on a solid substrate and from surface pressure/area... 

The monolayer can also be held at a constant surface pressure, which is enabled by a computer-controlled feedback system between the electrobalance and the motor responsible for the movements of the compressing barrier. This is useful when producing LB films, that is, when the monolayer is deposited on a solid substrate. 

Tanaka et al. have studied the surface molecular motions of PS films coated on a solid substrate by lateral force microscopy and revealed that the Tg at the surface was much lower than the corresponding bulk one [148]. Possible reasons for this included an excess free volume induced by localized chain ends, a reduced cooperativity for of-relaxation process, a reduced entanglement, and a unique chain conformation at the surface. For comparison, they examined surface relaxation behavior of high-density PMMA brushes. 

When some NC particles are surface treated to make them hydrophobic by, e.g., silylation and floated on the surface of water, a nanoparticle layer can be made on a solid substrate by a method similar to Langmuir-Blodgett film formation. Interparticle separation is well controlled by choosing an appropriate surface pressure prior to making LB film, as discussed next. 

Still another method used to produce PV cells is provided by thin-film technologies. Thin films are made by depositing semiconductor materials on a solid substrate such as glass or metal sheet. Among the wide variety of thin-film materials under development are amorphous silicon, polycrystalline silicon, copper indium diselenide, and cadmium telluride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film silicon alloys or other semiconductors tailored to respond to specific portions of the light spectrum. 

It is important to state the difference between particles and particulate films at the onset of this section. Particles are separate nanometer- to micron-sized colloids dispersed in solution. Physically interconnected colloidal metal particles constitute a particulate film which may be supported by a monolayer floating on an aqueous subphase or be deposited on a solid substrate. 

Amorphous Films. An amorphous film is generally prepared by solvent evaporation of a dry organic solution of Chi on a solid substrate surface. The vacuum sublimation technique, which is widely employed for most synthetic dyes, is not applicable to Chi due to possible thermal degradation of the pigment. 

Diffuse reflectance spectroscopy has been widely used to characterize the surface of solids as well as films and coatings present on a solid substrate (6). The application of this technique to the study of adsorbed species has been much more limited (26) and, thus far, has not involved the use of FT spectroscopy. 

Huang R (2005) Kinetic wrinkling of an elastic film on a viscoelastic substrate. J Mech Phys Solids 53 63-89... 

In the 1940s, it was demonstrated in the pioneering work of Zisman and coworkers [8] that the LB technique is not the only way to create an organized organic monolayer on a solid substrate. It was demonstrated that when a compatible substrate is exposed to a solution of an amphiphilic compound, the dissolved molecules form a self-assembled monolayer on the substrate surface. Such films maintain their structural integrity after they are removed from solution. The most common examples of such films are organosulfur films on gold substrates [9] and alkyltrichlorosilane films on silicon dioxide substrates [10]. Compared with the LB films, the self-assembled films are somewhat less ordered. On the other hand, these films are easier to prepare, since they do not require special instrumentation and can easily be deposited on both planar and non-planar substrates. Also, in many cases the amphiphilic molecules which make the self-assembled film are chemisorbed on the substrate. Such films are more stable when heated or exposed to solvents than are typical LB films, which are held to the substrate by non-covalent interactions. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

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