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

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.

Marangoni-driven spreading along liquid-liquid interfaces

“…17,29 Historically, most theoretical and experimental studies of Marangoni-driven spreading have focused on the air-liquid interface. 1,2, 5,7,8,11,[13][14][15][16][17][18][19][20][21][22][23]25 Comparatively few studies address other configurations such as the spreading of a fluids on solid substrates 30,31 and porous substrates. 32 So far no systematic studies on the spreading kinetics along the interface between immiscible liquids have been reported in literature.…”

“…In order to determine the power n, radial positions of the leading edge r i are measured at times t i typically over at least 1 decade in t. 8,25 In most of the experimental studies reported in literature, t i are within the range of 10 −2 -10 3 s and normal consumer video cameras can be used to record experiments. There are large similarities between the leading edge of a spreading film and the Reynolds ridge, a sharp transition in surface levels h between clean and contaminated surface areas of flowing water.…”

“…11 In many studies, the deflection h͑r , t͒ of the interface at the leading edge is used to detect the position of the leading edge, r͑t͒. The distortion h͓r͑t͔͒ propagating along an airliquid interface can be visualized either by means of tracers ͑e.g., Teflon particles 8 ͒, which are dragged along with the leading edge, or by optical techniques such as laser shadowgraphy, 6 Schlieren interferometry, 24,46,47 and Moiré topography, 25,48 which image directly the distortion of the interface. Shadowgraphy, Schlieren interferometry and Moiré topography have the advantage that they do not require any tracers ͑which are often suspected of introducing contamination into the system altering the properties of the surface͒ and visualize several features following behind the leading edge as well.…”

“…2,3, 11 The phenomenon of fluid flow caused by surface tension gradients ͑Marangoni effect 12 ͒ has been known for several decades and has attracted substantial interest in industry and science. 1,2, 5,7,8,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Its comparatively high velocity makes Marangoni-driven spreading an interesting transport mechanism for technological processes such as detergency, antifoaming, printing, and coating. One example is the Langmuir-Blodgett technique where organic coatings are formed in a very controlled way by repeated dip coating of a substrate with mono molecular films that have been produced by the spreading of organic molecules on water.…”

Growing surfactant waves in thin liquid films driven by gravity

“…These forces can be generated by thermal [2,23], electrical [19], or chemical means [20]. Flows driven by Marangoni stresses have been found to yield interesting instabilities and new kinds of waves [2,5,14,21]. Chemical Marangoni stresses can be generated by the presence of surfactants (surface active agents), which typically decrease surface tension; nonuniform distributions of surfactant molecules creates such surface stresses [3,13,14].…”