Thin liquid films, repulsive forces

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. 

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],... 

Figure 5.6 shows an example of a total interaction energy curve for a thin liquid film stabilized by the presence of ionic surfactant. It can be seen that either the attractive van der Waals forces or the repulsive electric double-layer forces can predominate at different film thicknesses. In the example shown, attractive forces dominate at large film thicknesses. As the thickness decreases the attraction increases but eventually the repulsive forces become significant so that a minimum in the curve may occur, this is called the secondary minimum and may be thought of as a thickness in which a meta-stable state exists, that of the common black film. As the... 

As already mentioned, if the van der Waals force (or other attractive force) is not predominant, first a dimple forms in the thinning liquid films. Usually the dimple exists for a short period of time initially it grows, but as a result of the swift outflow of liquid it decreases and eventually disappears. The resulting plane-parallel film thins at almost constant radius R. When the electrostatic repulsion is strong, a thicker primary film forms (see point 1 in Figure 5.13). From the viewpoint of conventional DLVO theory, this film must be metastable. Indeed, the experiments with microscopic foam films, stabilized with sodium octyl sulfate or sodium dodecyl sulfate in the presence of different amount of electrolyte, show that a black spot may suddenly form and a transition to... 

The topics of the early scientific work of Derjaguin and his collaborators was the evaluation of the term "disjoining pressure" as basic property of a thin liquid film. Derjaguin Obuchov (1936) and Derjaguin Kussakov (1939) have detected the growth of repulsive forces in such films as the film becomes thitmer. The classic thermodynamics of Gibbs was extended by the thermodynamic formulation of the disjoining pressure concept. 

Staring from the Gouy-Chapman theory the observed repulsion force in thin liquid films must be of electrostatic nature caused by the overlap of the corresponding diffuse electrical double... 

To understand the stability of films we must, just as in the case of particulate dispersions, seek sources of a repulsive potential which resists the thinning process. Such repulsive forces develop when the liquid is a solution of a surfactant which is adsorbed at the liquid/vapour interfaces they can arise from electrostatic,... 

Thin liquid films can be formed between two coUiding emulsion droplets or between the bubbles in foam. Formation of thin films accompanies the particle-particle and particle-wall interactions in colloids. From a mathematical viewpoint, a film is thin when its thickness is much smaller than its lateral dimension. From a physical viewpoint, a liquid film formed between two macroscopic phases is thin when the energy of interaction between the two phases across the film is not negligible. The specific forces causing the interactions in a thin liquid film are called surface forces. Repulsive surface forces stabilize thin films and dispersions, whereas attractive surface forces cause film rupture and coagulation. This section is devoted to the macroscopic (hydrostatic and thermodynamic) theory of thin films, while the molecular theory of surface forces is reviewed in Section 4.4. 

Several types of surface forces determine the interactions across thin liquid films. In addition to the universal van der Waals forces, the adsorbed ionic surfactants enhance the electrostatic (double-layer) repulsion. On the other hand, the adsorbed nonionics give rise to a steric repulsion due to the overlap of hydrophilic polymer brushes. The presence of surfactant micelles in the continuous phase gives rise to oscillatory structural forces, which can stabilize or destabilize the liquid films (and dispersions), depending on whether the micelle volume fraction is higher or lower. These and other surface forces, related to the surfactant properties, were considered in Sec. VI. 

The observed equilibrium thickness represents the film dimensions where the attractive and repulsive forces within the film are balanced. This parameter is very dependent upon the ionic composition of the solution as a major stabilizing force arizes from the ionic double layer interactions between any charged adsorbed layers confining the film. Increasing the ionic strength can reduce the repulsion between layers and at a critical concentration can induce a transition from the primary or common black film to a secondary or Newton black film. These latter films are very thin and contain little or no free interlamellar liquid. Such a transition is observed with SDS films in 0.5 M NaCl and results in a film that is only 5 nm thick. The drainage properties of these films follows that described above but the first black spot spreads instantly and almost explosively to occupy the whole film. This latter process occurs in the millisecond timescale. 

Electric double layers can be present at the gas/liquid interfaces between bubbles in foams. In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

In foam stability, gas bubbles and the liquid films between them, would be stabilized by the repulsive forces created when two charged interfaces approach each other and their electric double layers overlap. The repulsive energy VR for the double layers at each interface in the thin film is still given by Eq. (5.1) where H is the film thickness. Here also, for extremely thin films, such as the Newton black films, Bom repulsion becomes important as an additional repulsive force. 

Since equation (12.6) is symmetrical in 1 and 2, the same equation applies (perhaps surprisingly at first) to the free energy of a film of 1 in air [Figure 12.5(b)], This free energy becomes increasingly negative as 11 decreases (Figure 12 6) so that van der Waals forces, if not opposed by repulsive forces between the surfaces, will cause the film to thin and eventually burst. Repulsive forces arc absent in the case of pure liquids, so that they do not form stable films. ... 

The extremely low friction achieved on PLL(20)-g[2.9]-PEG(5) films in SFA experiments is not a unique property of this specific copolymer but has also been observed for other grafted poly(ethylene glycol) films as well as for other polymer-brush systems in a good solvent, such as grafted polystyrene in toluene. The osmotic pressure, which leads to strong repulsive forces as the polymer brushes are compressed, effectively prevents the direct contact of the solid surfaces. At comparable solid surface separations, thin films of water or aqueous salt solutions have been shown to retain a shear fluidity characteristic of the bulk liquid. In a control experiment, this was also observed in 10 mM HEPES buffer solution (data not shown). 

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