Experimental Characterization of Front Propagation by Marangoni Forces in Ultrathin Liquid Films
Italian physicist Carlo Marangoni published a treatise in 1865 which first described the spontaneous flow of a liquid film caused by gradients in surface t ension at a gas/liquid interface. Such gradients are established by variations in surface concentration of molecular species which lower the...
| Summary: | Italian physicist Carlo Marangoni published a treatise in 1865 which first described the
spontaneous flow of a liquid film caused by gradients in surface t ension at a gas/liquid
interface. Such gradients are established by variations in surface concentration of molecular
species which lower the surface tension in proportion to the interface concentration.
While the dynamics and power law behavior of the initial circular advancing front in surfactant
coated films has been studied for about two decades, there has been relatively little
work conducted on characterizing the fractal spreading patterns which develop behind this
front. In this thesis, we examine the dynamics of front propagation and subsequent fractal
evolution in ultrathin liquid films of glycerol driven to spread by concentration gradients
in Aerosol OT, an insoluble anionic surfactant. We vary only the initial deposition volume
and concentration while maintaining the initial liquid film thickness at 2.5 microns.
Contrary to theoretical analyses in the literature, our results reveal that the coefficient
and exponent characterizing the advance of both the first front as well as the following
unstable front is not a constant depending only on geometry (i.e. rectilinear vs axisymmetric
spreading) but varies with the initial surfactant concentration of the deposition
volume. Furthermore, the overall speed of the fronts is observed to increase with ambient
relative humidity, an effect we ascribe to a reduction in the viscosity of the glycerol film
clue to absorption of water from the ambient atmosphere. Only in cases of relatively high
humidity (higher than approximately 40% relative humidity) do we observe the formation
of fractal patterns in the unstable front. Measurements of the fractal dimension yield
values in the range 1.45-1.65. The largest values approximate those fractal dimensions
observed in systems undergoing diffusion driven aggregation or viscous fingering, neither
of which mechanism is pertinent to Marangoni flow. |
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