Thin liquid films surface forces

The situation is still more complex in the presence of surfactants. Recently, a self-consistent electrostatic theory has been presented to predict disjoining pressure isotherms of aqueous thin-liquid films, surface tension, and potentials of air bubbles immersed in electrolyte solutions with nonionic surfactants [53], The proposed model combines specific adsorption of hydroxide ions at the interface with image charge and dispersion forces on ions in the diffuse double layer. These two additional ion interaction free energies are incorporated into the Boltzmann equation, and a simple model for the specific adsorption of the hydroxide ions is used for achieving the description of the ion distribution. Then, by combining this distribution with the Poisson equation for the electrostatic potential, an MPB nonlinear differential equation appears. 

The London-van der Waals dispersion forces have long been recognized as being important in thin liquid films. These forces have been calculated for different surfaces by pairwise smnmation of the individual dispersion... 

Surface force apparatus has been applied successfully over the past years for measuring normal surface forces as a function of surface gap or film thickness. The results reveal, for example, that the normal forces acting on confined liquid composed of linear-chain molecules exhibit a periodic oscillation between the attractive and repulsive interactions as one surface continuously approaches to another, which is schematically shown in Fig. 19. The period of the oscillation corresponds precisely to the thickness of a molecular chain, and the oscillation amplitude increases exponentially as the film thickness decreases. This oscillatory solvation force originates from the formation of the layering structure in thin liquid films and the change of the ordered structure with the film thickness. The result provides a convincing example that the SFA can be an effective experimental tool to detect fundamental interactions between the surfaces when the gap decreases to nanometre scale. 

Berman, A., and Israelachvili, J. N., "Surface Forces and Microrheology of Molecularly Thin Liquid Films, Handbook of Micro/Nanotribology, 2nd ed., B. Bhushan, Ed., CRC Press, Boca Raton, FL, 1999. 

Chu X L, Nikolov AD, Wasan DT (1995) Thin liquid film structime and stability The role of depletion and surface-induced structural forces. J Chem Phys 103 6653-6661... 

Equation (6.25) not only allows us to calculate the Hamaker constant, it also allows us to easily predict whether we can expect attraction or repulsion. An attractive van der Waals force corresponds to a positive sign of the Hamaker constant, repulsion corresponds to a negative Hamaker constant. Van der Waals forces between similar materials are always attractive. This can easily be deduced from the last equation for 1 = e2 and n = n2 the Hamaker constant is positive, which corresponds to an attractive force. If two different media interact across vacuum ( 3 = n3 = 1), or practically a gas, the van der Waals force is also attractive. Van der Waals forces between different materials across a condensed phase can be repulsive. Repulsive van der Waals forces occur, when medium 3 is more strongly attracted to medium 1 than medium 2. Repulsive forces were, for instance, measured for the interaction of silicon nitride with silicon oxide in diiodomethane [121]. Repulsive van der Waals forces can also occur across thin films on solid surfaces. In the case of thin liquid films on solid surfaces there is often a repulsive van der Waals force between the solid-liquid and the liquid-gas interface [122],... 

In the systems we have used for 3D MESA, the capillary forces act through thin liquid films that coat the faces of the objects [refs. 6, 15]. These films possess a high interfacial free energy in contact with water. When two surfaces... 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

Thin liquid films (especially foam films) stabilised with phospholipids, proteins, etc., prove to be very suitable in the study of surface forces, since they could model the interacting biological membranes in aqueous medium. 

The method of equilibrium foam film employs the experimental measurement of the equilibrium thickness and from the DVLO theory it is possible to determine (po and, respectively, the surface charge at the solution/air interface. This is a very valuable possibility since an equilibrium potential can be evaluated and all complications occurring at kinetic measurements, are avoided. The equilibrium values of (fo are important in the interpretation of electrostatic forces in thin liquid films, along with the other surface forces, acting in them. 

Microscopic foam films from amphiphilic ABA triblock copolymers have been used to assess steric interactions. Most of the work on copolymers [128,129] has been carried out with the Thin Liquid Film-Pressure Balance Technique (see Chapter 2, Section 2.1.8). Nevertheless, some intriguing results have been obtained with the dynamic method for surface force measurement [127]. 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

The CBF/NBF transition has already been considered in Section 3.4.1 with respect to the experimental n(/i) isotherms of disjoining pressure obtained with the Thin Liquid Film-Pressure Balance Technique. Theoretical concepts and comparison with the DLVO- and contemporary theories describing surface forces acting in this range of film thicknesses have also been discussed. 

Thus capillary force, holding the particle to the surface by a thin liquid film between the two, is directly proportional to the particle radius and the liquid surface tension. For water Yl = 72 dyn/cm, and for a 1 pm diameter particle = 4.5x10 dyn. Thus capillarity forces can be significantly higher than the van der Waal s forces. Also note that use of the liquids with lower Yl (e g-, ethylene glycol with Yl = 48 dyn/cm or n-hexane with Yl = 18.5 dyn/cm) will lower and thus lead to easier removal of the particle during cleaning. 

The rupture mechanisms of thin liquid films were considered by de Vries [15] and by Vrij and Overbeek [16]. It was assumed that thermal and mechanical disturbances (having a wavelike nature) cause film thickness fluctuations (in thin films), leading to the rupture or coalescence of bubbles at a critical thickness. Vrij and Overbeek [16] carried out a theoretical analysis of the hydrodynamic interfacial force balance, and expressed the critical thickness of rupture in terms of the attractive van der Waals interaction (characterised by the Hamaker constant A), the surface or interfacial tension y, and the disjoining pressure. The critical wavelength, for the perturbation to grow (assuming that the disjoining pressure just exceeds the... 

In conclusion, it should be noted that the width of the transition region between a thin liquid film and Plateau border is usually very small — below 1 pm. That is why the optical measurements of the meniscus profile give information about the thickness of the Plateau border in the region r > (Figure 5.16). Then, if the data are processed by means of the Laplace equation (Equation 5.101), one determines the contact angle, a, as discussed above. Despite that it is a purely macroscopic quantity, a characterizes the magnitude of the surface forces inside the thin liquid film, as implied by Equation 5.148. This has been pointed out by Derjaguin and Princen and Mason. ... 

Oscillatory structural forces appear in two cases (1) in thin films of pme solvent between two smooth solid surfaces (2) in thin liquid films containing colloidal particles (including macromolecules and surfactant micelles). In the first case, the oscillatory forces are called the solvation... 

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