A novel method for determination of the three-phase contact angle at the surface of a micrometer-sized particle (latex sphere, oil droplet, or biological cell) is described. The particle is entrapped within a liquid film of equilibrium thickness smaller than the particle diameter. Thus a liquid meniscus (a layer of uneven thickness) is formed around the particle. When observed in reflected monochromatic light, this meniscus appears as an interference pattern of concentric bright and dark fringes. From the radii of the interference fringes, one can restore the meniscus shape by using the solution of the Laplace equation of capillarity. In this way the three-phase contact angle of the particle and the capillary pressure can be determined. We demonstrate the applicability of our method to latex spheres from several batch samples (between 1 and 7 µm in diameter) and to oil droplets, stabilized by adsorbed protein layer. The numerical procedures used for contact angle determination are described, and illustrative results are presented and discussed.

In the preceding paper of this series we studied the effect of several oils of different chemical structure on the foaming properties of sodium dodecylbenzenesulfonate solutions. A straightforward correlation was found between the foam stability and the so-called “entry barrier”, which prevents the emergence of pre-emulsified oil drops on the solution surface. In the present article we perform a systematic experimental study of the entry barriers for several oils by means of the recently developed film trapping technique. The latter consists of trapping oil drops in wetting films on a solid substrate, followed by a controlled increase of the capillary pressure of the meniscus that compresses the drops against the substrate. At a certain critical capillary pressure, P C CR, the asymmetric oil−water−air films rupture and the drops enter the water−air interface. This event is observed microscopically, and P C CR is determined as a function of various parameters (type of oil, surfactant concentration, drop size, and others). The entry barrier increases with the surfactant concentration, especially in the range where the surfactant micelles are expected to stabilize the asymmetric films. The results obtained with a series of alkanes (from octane to hexadecane) show that the entry barrier increases with the alkane chain length. Furthermore, it is shown that the presence of a spread oil (even as an ultrathin, molecular layer) on the surface of the foam film might lead to a significant change of the magnitude of the entry barrier. For decane and dodecane, the layer of spread oil reduces the entry barrier, whereas for hexadecane the effect is the opposite. As far as we know, such a role of oil spreading in the antifoaming action of oils has not been reported so far. Since the stability of thin liquid films is usually discussed in the literature in terms of the disjoining pressure, we estimate from the experimental data the critical disjoining pressure, ΠAS CR, at which the asymmetric oil−water−air film ruptures and the drop entry occurs. The estimates show that the curvature of the asymmetric film is very important in the overall consideration of the mechanical equilibrium in the system and there is a big difference between the numerical values of P C CR and ΠAS CR, unlike the case of planar films where P C CR = ΠAS CR. Additionally, we find that P C CR is a weak function of the oil drop size and of the asymmetric film radius, while ΠAS CR scales as (film radius)-1 for all of the studied systems. These results are discussed with respect to the possible mechanisms of film rupture. Concerning the foam stability, P C CR is a more convenient quantity for description of the entry barriers, because its magnitude correlates with the foam height, whereas the magnitude of ΠAS CR does not.

The film trapping technique, FTT, allows one to investigate the interactions between colloidal particles and a fluid interface that presses them against a flat solid substrate. The method was already applied for measuring the contact angles of micrometer sized latex spheres (Hadjiiski, A.; Dimova, R.; Denkov, N. D.; Ivanov, I. B.; Borwankar, R. Langmuir 1996, 12, 6665.), and the interaction of white blood cells with adsorption layers of antibodies (Patrick, S. M.; An, H.; Harris, M. B.; Ivanov, I. B.; Braunshtein, N. S.; Leonard, E. F. Ann. Biomed. Eng. 1997, 25, 1072. Ivanov, I. B.; Hadjiiski, A.; Denkov, N. D.; Gurkov, T. D.; Kralchevsky, P. A.; Koyasu, S. Biophys. J. 1998, 75, 545.). A new modification of the equipment is now proposed (FTT-gentle), which significantly increases the accessible range of capillary pressures exerted to particles, starting from a virtually zero value. This is particularly important for studying the highly deformable particles (e.g., biological cells) or the oil drops, which easily coalesce with the interface, like those used to promote a foam collapse (so-called antifoams). The basic principles of operation with FTT-gentle, illustrated with experiments and theoretical calculations of the shape of the interface around the trapped particle and the position of the three-phase contact line at the particle surface, are described. The feasibility of FTT-gentle is demonstrated by measuring the drop entry barriers (the critical capillary pressure inducing the coalescence of drops with the air−water interface) of several antifoams. The results show that the drop entry barrier strongly depends on the used surfactant and its concentration, and can be significantly decreased by addition of hydrophobized solid particles in the oil drops.