Gentle film trapping technique with application to drop entry measurements

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, L 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.