The dynamics of micrometer-size polystyrene latex particles confined in thinning foam films was investigated by microscopic interferometric observation. The behavior of the entrapped particles depends on the mobility of the film surfaces, the particle concentration, hydrophobicity, and rate of film formation. When the films were stabilized by sodium dodecyl sulfate, no entrapment of particles between the surfaces was possible. When protein was used as a stabilizer, a limited number of particles were caught inside the film area due to the decreased mobility of the interfaces. In this case, extraordinary long-ranged (>100 µm) capillary attraction leads to two-dimensional (2D) particle aggregation. A major change occurs when the microspheres are partially hydrophobized by the presence of cationic surfactant. After the foam films are opened and closed a few times, a layer of particles simultaneously adsorbed to the two interfaces is formed, which sterically inhibits any further film opening and thinning. The particles within this layer show an excellent 2D hexagonal ordering. The experimental data are relevant to the dynamics of defects in coating films, Pickering emulsions, and particle assembly into 2D arrays.
The thinning and the lifetime of foam and emulsion Ðlms formed in a model experimental cell have been investigated. The foam Ðlms were stabilised by either sodium dodecyl sulfate or sodium dodecyl polyoxyethylene-2 sulfate. The emulsion Ðlms contained either Tween 20 or Span 20. The time of hydrodynamic drainage of the Ðlms increased linearly as the logarithm of the surfactant concentration. This linear dependence was valid whatever the type of Ðlm or surfactant and not only below the critical micelle concentration (c.m.c.) but also much above this concentration threshold. The experimental results are relevant to the hydrodynamic basis of foam and emulsion stabilisation. They are compared with the earlier hydrodynamic theories of Ðlm drainage. A reasonable, but not excellent, agreement between the experimental data and the theory could be achieved in the region below the c.m.c. of the surfactant. The data about the complex system above the c.m.c. still remain unexplained by an adequate theory. The investigation provides some guidelines for choosing the optimal type and concentration of surfactant in colloid systems of practical importance.The stability of foams and emulsions is of high importance for many practical systems in the food, paint, oil, pharmaceutical and other industries. The breakdown of these colloid systems occurs mainly because of the rupture of the thin liquid Ðlms which form between the bubbles or droplets when these are pressed against each other either in collisions or by buoyancy.1,2 Therefore, the behaviour and the stability of these thin liquid Ðlms is a problem of major experimental and theoretical importance.1,2 It has been established, in principle, that one can distinguish between two types of foam and emulsion Ðlm stability, thermodynamic and kinetic.1,3 The thermodynamic stability is associated with the repulsive intermolecular interactions that act between the Ðlm surfaces and oppose their approach.1 The kinetic stability arises from the hydrodynamic friction in the Ðlm, that delays its thinning. Although the kinetic stabilisation is transient, it has been recognised1 that, in many cases, the hydrodynamic forces alone, or through enhancing the thermodynamic factors are the key to the stability of the foam or emulsion systems.There are many theoretical and experimental investigations dedicated to the hydrodynamics of thinning of foam and emulsion Ðlms4h7 (for a detailed review of the recent advances in this Ðeld see also ref. 1). The majority of these studies, however, deal with the case of Ðxed surfactant concentration (and usually below the c.m.c.). Of major practical importance is investigation of the e †ect of the surfactant concentration on the stability of thin Ðlms, foams and emulsions. The quantity of surface-active agent, that is introduced in practical foams and emulsions, is usually quite large and orders of magnitude higher than the c.m.c. Finding ways of reducing this quantity could be advantageous from both Ðnancial and environmental view points.Our study is dedicated t...
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