Factors affecting the stability of foamed concentrated emulsions

This paper attempts to quantify the stability of three-phase systems genera ted by aerating concentrated water-in-oil emulsions. In such materials, whi ch we call foamed emulsions, the continuous phase is itself a two-phase sys tem. In this work, we modify and extend the method originally proposed by I glesias et al. (Colloids and Surfaces A, 98 (1995) 167-174) to viscous thre e-phase foams. The modified method involves imparting a destabilising force to the sample to make the foam short-lived and measuring the change in hei ght as a function of decay time. The change of height during decay represen ts the rate at which gas is evolved from the foamed emulsion and is logarit hmic with time. The data treatment yields two values, the decay constant an d half-life, which are used as a means of measuring and comparing stability . Two distinct decay mechanisms (smooth decay and catastrophic collapse) op erate in foamed emulsions that are subjected to oscillations. For a given d ecay mechanism, the decay constant is an intrinsic property of the foamed e mulsion and is independent of the imposed oscillations. Experimental result s indicate that different bubble stabilising surfactants and emulsion morph ology significantly affect the foam stability, and that the stability is in versely related to the initial expansion. Examination of the gas-emulsion i nterface shows a segregation of droplets, with smaller droplets found prefe rentially at the gas-emulsion interface. (C) 1999 Elsevier Science B.V. All rights reserved.