Journal of Fluid Mechanics



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Spreading characteristics of an insoluble surfactant film on a thin liquid layer: comparison between theory and experiment


ANNE D. DUSSAUD a1, OMAR K. MATAR a2 and SANDRA M. TROIAN a3
a1 Unilever Research and Development, 40 Merritt Blvd, Trumbull, CT 06611, USA anne.dussaud@unilever.com
a2 Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, UK
a3 Department of Chemical Engineering, Princeton University, Princeton, NJ 08544, USA

Article author query
dussaud ad   [Google Scholar] 
matar ok   [Google Scholar] 
troian sm   [Google Scholar] 
 

Abstract

We describe measurements of the surface slope and reconstruction of the interface shape during the spreading of an oleic acid film on the surface of a thin aqueous glycerol mixture. This experimental system closely mimics the behaviour of an insoluble surfactant film driven to spread on a thin viscous layer under the action of a tangential (Marangoni) surface stress. Refracted image Moiré topography is used to monitor the evolution of the surface slope over macroscopic distances, from which the time variant interface shape and advancing speed of the surfactant film are inferred. The interfacial profile exhibits a strong surface depression ahead of the surfactant source capped by an elevated rim at the surfactant leading edge. The surface slope and shape as well as the propagation characteristics of the advancing rim can be compared directly with theoretical predictions. The agreement is quite strong when the model allows for a small level of pre-existing surface contamination of the initial liquid layer. Comparison between theoretical and experimental profiles reveals the importance of the initial shear stress in determining the evolution in the film thickness and surfactant distribution. This initial stress appears to thin the underlying liquid support so drastically that the surfactant droplet behaves as a finite and not an infinite source, even though there is always an excess of surfactant present at the origin.

(Published Online November 18 2005)
(Received December 14 2004)
(Revised May 15 2005)



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