Deposition of oil drops on a glass substrate in relation to the process ofwashing

The attachment of emulsion drops to glass substrates is investigated in rel ation to the redeposition of oil drops in the process of washing. It turns out that the drops of a surfactant-stabilized oil-in-water emulsion cannot be attached to an immersed glass plate simply by the buoyancy force. Howeve r, the same drops can be deposited on the plate when the latter is pulled o ut of the emulsion, i.e., when the drops are pressed against the substrate by a receding meniscus, We measured the amount of the oily deposit as a fun ction of the pH, ionic strength, and composition of an amphoteric-anionic s urfactant mixture. The enhanced oil deposition at low pH correlates with th e domain in which the emulsion drops and the solid substrate bear opposite electric charges. This was established by zeta-potential measurements with oil drops and glass particles. The anionic surfactant brings negative surfa ce charge to the oil droplets and suppresses the oil deposition on the nega tively charged glass. With the increase of the fraction of the amphoteric s urfactant in the mixture, the zeta-potential is converted from negative to positive, and the oil deposition grows almost linearly with the potential. In general, the deposition of oil drops by a receding meniscus is governed by an interplay of electrostatic and hydrodynamic factors. (C) 2000 Academi c Press.