Journal of Colloid and Interface Science 247, 125–131 (2002) doi:10.1006/jcis.2001.8124, available online at http://www.idealibrary.com on Monitoring Entering and Spreading of Emulsion Droplets at an Expanding Air/Water Interface: A Novel Technique N. E. Hotrum, ∗, † T. van Vliet, ∗ M. A. Cohen Stuart,† and G. A. van Aken ∗, ‡ , 1 ∗ Wageningen Centre for Food Sciences (WCFS), Laboratory of Food Physics, c/o Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands; †Laboratory of Physical Chemistry and Colloid Science, Wageningen University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands; and ‡NIZO Food Research, P.O. Box 20, 6710 BA Ede, The Netherlands Received July 16, 2001; accepted November 24, 2001 The entering and spreading of emulsion droplets at quiescent and expanding air/water interfaces was studied using a new apparatus consisting of a modified Langmuir trough in which the air/water interface can be continuously expanded by means of rollers in the place of traditional barriers. When sodium caseinate and whey pro- tein isolate-stabilized emulsion droplets were injected under the sur- face of sodium caseinate and whey protein isolate solutions, respec- tively, it appeared that the droplets entered the air/water interface only if the air/water surface pressure did not exceed a threshold value of ∼15 mN/m. This condition was satisfied either under qui- escent conditions for low protein concentrations or by continuous expansion of the interface at higher protein concentrations. Accord- ing to equilibrium thermodynamics, entering of the droplets and the formation of lenses should occur for all the systems investigated, but this was not observed. At surface pressures higher than ∼15 mN/m, immersed emulsion droplets were metastable. This is probably due to a kinetic barrier caused by the formation of a thin water film bounded by protein adsorption layers between the emulsion droplet and the air/water interface. C  2002 Elsevier Science (USA) Key Words: interfacial tension; sodium caseinate; whey protein isolate; emulsion; spreading coefficient; entering coefficient; thin film; air/water interface; surface pressure; whipping cream. INTRODUCTION Foam stability is strongly influenced by the presence of emul- sion droplets. Spreading of emulsion droplets at the air/water interface is known to cause bubble collapse (1), which may be desirable (e.g., for antifoaming agents) or unwanted (e.g., the collapse of beer foam). Emulsion droplets are also known to stabilize foams, for example by accumulation in the plateau bor- ders within a foam (2) or by adsorption to the bubble surface, as is the case in whipped cream (3, 4). In a good whipped cream, spreading of liquid fat is reduced by the presence of crystalline fat in the fat globules which helps to prevent oil from flowing out of the droplets when they adhere to the bubble surface (5). 1 To whom correspondence should be addressed. Fax: +31 (0) 317 483 669. E-mail: aken@nizo.nl. Many researchers have postulated that interfacial tension (5, 6) and the properties of the adsorbed protein layers at both the air bubble and the emulsion droplet surfaces (7–9) are main fac- tors influencing the interaction between emulsion droplets and the air/water interface during the whipping of cream. However, very little work has been reported which quantifies this inter- action. Sirks (10) reported that spreading of liquid fat on the air/water interface was not impeded by a preexisting protein film provided the surface pressure (not specified) caused by the film was low enough. Schokker et al. (11) found 13 mN/m to be the limiting surface pressure for the spreading of oil-in-water emulsion droplets at quiescent air/milk protein interfaces. King (12) observed that fat globules could enter the milk/air interface if the interface was disturbed by touching it with a platinum loop. Before contact, the milk surface was free from fat globules, sug- gesting the existence of an energetic and/or kinetic barrier to fat globule insertion. Thermodynamically, three conformations may arise for an oil droplet at the air/water interface. Robinson and Woods (13) derived an entering coefficient, E , which predicts whether a droplet will enter the air/water interface or remain submerged in the water phase. E is given by E = γ WA + γ OW − γ OA , [1] where γ is interfacial tension and the subscripts W, A, and O refer to water, air, and oil, respectively. Entering of an oil droplet occurs when E > 0. When entered, a droplet may either form a lens or spread out into a film covering the air/water interface. The tendency of an emulsion droplet to spread at an air/water interface is predicted by the spreading coefficient, S, defined by Harkins and Feldman (14) as S = γ WA − γ OW − γ OA . [2] Thus the three conformations can be predicted based on the balance of interfacial tensions for the initial system (i.e., be- fore the oil droplet contacts the air/water interface) these are E < 0, S < 0 < E , and S > 0, respectively. E and S can also 125 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.