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An experimental study of the antifoam behaviour of mixtures of a hydrocarbon oil and hydrophobic particles

1994; Elsevier BV; Volume: 85; Issue: 2-3 Linguagem: Inglês

10.1016/0927-7757(93)02678-8

1873-4359

Peter R. Garrett, J.A. Davis, Henry M. Rendall,

Fluid Dynamics and Heat Transfer

An experimental study of the synergistic antifoam behaviour of hydrophobic particle—hydrocarbon oil mixtures is presented here. The system for study has been selected to minimise specific effects in order to highlight the phenomenology of the antifoam process. Thus, liquid paraffin and a commercial sodium alkyl benzenesulphonate were chosen as oil and surfactant respectively, because this oil exhibited a negative spreading coefficient at the air—water surface of the solution of that surfactant under all circumstances investigated. These circumstances included situations where the air—water surface was rapidly expanded to represent the conditions prevailing during foam generation. In the main, finely divided silica, hydrophobised with trimethylsilane, was used as the particulate component. Again, this material had no tendency to spread at air—water surfaces and had no effect on the spreading behaviour of the liquid paraffin. The study was also extended to include examples of intrinsically hydrophobic organic particles such as calcium stearyl acid phosphate. Contact angles, electrophoretic mobilities, electron microscopy and observations of emulsion behaviour all suggested that these oil—particle mixtures form composite entities where the particles tend to adhere to the oil—water surface with a contact angle (measured through the aqueous phase) θow§>π/2. The particles also exhibited a finite contact angle θow at the air—water surface so that θaq<π/2. Weak antifoam effects associated with the particles alone probably concerned dewetting and bridging in foam films to form holes in a manner analogous to that found previously with polytetrafluoroethylene particles. Preliminary observations indicated that the presence of particles adhering to the oil—water surface facilitates the emergence of oil droplets into the air—water surface by rupturing unsymmetrical oil—water—air films. The observations reported here are consistent with an antifoam mechanism where oil droplets form mechanically unstable bridging lenses in foam films. Formation of such configurations requires rupture of oil—water—air films by particles. The contact angle requirement for rupture of such films is less severe than that required for symmetrical air—water—air films. Therefore particles may promote the emergence of oil droplets into air—water surfaces to form lenses without themselves exhibiting significant antifoam effects.