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Matthieu Roché1, Zhenzhen Li1, Ian M. Griffiths1, 2, Arnaud Saint-Jalmes3, Howard A. Stone1
1Department of Mechanical and Aerospace Engineering,
Princeton University Princeton, New Jersey
2Oxford Centre for Collaborative Applied Mathematics,
University of Oxford
Oxford, United KIngdom
3Institut de Physique de Rennes
An oil-in-water emulsion spreads on the surface of pure water
Spreading of liquids on solid or liquid surfaces is of interest in many applications such as surface coating and to help neonates who suffer from respiratory distress syndrome. The addition of surface-active molecules, such as soap, in the liquid to be spread influences dramatically the spreading behavior. Indeed, these molecules induce a strong flow (known as a Marangoni flow) on the expanding surface of the spreading liquid because of their inhomogeneous concentration.
In the case pictured here, an olive-oil-in-water emulsion containing soap is constantly supplied through a steel needle on the surface of a layer of ultra-pure water. A front of oil droplets spreads until it reaches a characteristic position that depends on the chemical affinity between water and soap molecules. Oil droplets move with speeds on the order of 0.5 m/s across the transparent area (black), which has a diameter of approximately 60 mm. When droplets reach the white oil-droplet-rich external zone, they slow down dramatically and induce spectacular vortices similar to what is seen in turbulent soap films. These vortices exist as long as emulsion is flowing. Once the flow stops, the transparent area collapses to cover the surfactant-poor zone in the middle.
This experiment shows that a flow induced by an inhomogeneous distribution of surface active molecules can transport objects. This image also provides evidence that such a transport is efficient only on a characteristic length scale.
Reporters may freely use this image. Credit: Matthieu Roché, Zhenzhen Li, Ian M. Griffiths, Arnaud Saint-Jalmes and Howard A. Stone (2010).