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How does whipped cream destabilise ?


Whipped cream and foamed bitumen are both examples of foamed emulsions. They are unstable, as both foam and emulsion evolve in time. A group of researchers from the LPS has characterised the coupled evolution of foamed emulsions. The work has been published in the journal Soft Matter.

Foamed emulsions are made by mixing foam and emulsion (or by injecting gas bubbles into an emulsion). They are mixtures of gas bubbles and oil drops in water. The interfaces between gas and water or oil and water are stabilised using surfactants, without which it is not even possible to make foams or emulsions. Despite the protection of surfactants, foams and emulsions are not stable in time. Because oil and gas are lighter than water, bubbles and drops will rise in water. This phenomenon is called creaming (the fat drops rise up in milk and are skimmed to obtain cream) or, once the bubbles and drops are tightly packed, drainage. After whipped cream is made, the slow evolution of the bubbles and drops starts, and continues until the complete collapse of the foam. To understand the evolution of such complex systems, researchers at the LPS have studied the destabilisation of model foamed emulsions.

Figure 1. Image of a foamed emulsion taken using a confocal microscope, the oil drops fluoresce in red and the bubbles are black.

The emulsions have been prepared with rapeseed oil and surfactant and, once foamed, are composed of three fluids : gas in the bubbles, oil in the drops and the aqueous phase with surfactant. The concentration of bubbles and drops changes in time. To follow the composition of the foam, two experimental methods are combined. The total liquid fraction is measured using image analysis of surface photographs (figure 1), and the fraction of oil in the aqueous phase is obtained from conductivity measurements. Their evolution in time is shown in Figure 2, where three regimes have been identified. Initially, the liquid drains quickly and homogeneously (T1), before stabilising to a period of slow drainage (T2). During this time the liquid fraction in the foam continues to decrease as liquid flows out of the foam. The drainage is strongly influenced by the concentration of oil in the emulsion, which changes the viscosity, and thus the ease with which the liquid can flow through the channels around the bubbles. As the foams become drier, the flow slows down almost to a halt due to the peculiar flow properties of emulsions. They are shear-thinning, and their viscosity decreases at higher flow rates. Therefore, as the flow slows down, the emulsions become more viscous until the drainage almost stops. In the third regime (T3) the drops no longer flow out with the liquid but cream and the concentration of oil increases. Once the emulsion drops pierce the foam films they accelerate the final foam collapse.

The identification of three phases of foamed emulsion destabilisation will help design more stable foamed emulsions. Returning to whipped cream, we understand why the use of fattier cream makes the foams more stable : drainage is slowed down and the final stages of destabilisation are delayed. Such oily foams are increasingly studied as they are found both during material processing and in final products in many materials, whether in cosmetics, foods, or construction. Understanding the destabilisation mechanisms of foamed emulsions will help optimise the use of such foams.

Figure 2. The evolution of the liquid fraction in the foam (black triangles) and evolution of the fraction of oil in the emulsion (red circles).

Reference

Foamed emulsion drainage : flow and trapping of drops
M. Schneider, Z. Zou, D. Langevin et A. Salonen
Soft Matter (2017)
doi : 10.1039/C7SM00506G

Contact

Anniina Salonen