Contact angles in relation to the effects of solid particles on foam stability
This thesis is concerned with the bridging-dewetting mechanism of foam breakdown by spherical hydrophobic particles. The role of contact angles (of solid with surfactant solution) in foam breakdown by particles has previously been investigated using particles of ill-defined shape. As contact angle requirements are shape dependent, conclusions have often been tentative. Further, it has been assumed that the contact angles measured on flat surfaces represent those occurring on particles of similar surface molecular structure. In the work to be described, we first establish, using a Langmuir trough technique, that contact angles of particles with surfactant solutions correspond closely to those obtained conventionally on flat plates of similar surface structure. The technique is suited to the investigation of very small monodisperse particles, which do not cause curving of the interface up to the point of contact with the particle. Spherical glass particles in the size range 40 to 53µm were then hydrophobised to different extents by controlled treatment with either octadecyl trichlorosilane or perfluorodecyl trichlorosilane. Flat glass plates were treated in a similar fashion so that contact angles (both static advancing and receding) could be measured with surfactant solutions using the sessile drop technique. The hydrophobic particles were incorporated into foams produced from aqueous solutions of CTAB, AOT, SDS and SPFO. It is found that the particles exhibiting advancing contact angles less than 70 to 80° have little effect on long term stability because they are swept from the foam lamellae by the flow of interstitial liquid during film drainage. For particles which give advancing contact angles lying between 80 and 95° , however, the long term foam stability is dramatically enhanced. These particles become attached to the film interfaces and are not easily swept from the foam structure. Foam stability is enhanced because particles accumulate in the interfaces of the Plateau border regions between foam films where they act as a physical barrier slowing the drainage of liquid from the foam lamellae. Then for particles which exhibit contact angles in excess of 95° the stability falls sharply and is accounted for by the bridging-dewetting mechanism. Similarly hydrophobised thin cylindrical glass rods were incorporated into single CTAB soap films and the effect of contact angle studied in terms of the change in film lifetime and the % film rupture. Thin rods with advancing contact angles below 90° had little effect on film lifetime but any angle above 90° resulted in virtually 100% film rupture. The organosilyl coating is susceptible to hydrolysis in aqueous surfactant solution and consequently, with time, silanised particles may loose their hydrophobicity and hence also their antifoaming ability. Through a comparative study of silane coatings prepared from a variety of silanising agents we find that tri-functional silanes produce coatings which retain their hydrophobicity more effectively than other agents. This is because the tri-functional silanes produce thick polymeric coatings.