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Title: Images of the floating world: Drainage patterns in thinning soap films

Description: Biological, industrial, and food-related foams often contain polymeric additives, which modify the rheological and drainage behavior within individual soap film lamellae. There are, however, few controlled studies of entrainment and drainage in polymeric soap films [1,2] which thin not by gravity but by capillary and disjoining pressures. In this study, aqueous soap films containing SDS surfactant (sodium dodecyl sulfate) above the critical micellar concentration and with small amounts of 1 M mol. wt. PEO polymer (polyethylene oxide) were entrained at constant speed on a frame consisting of three polyester fibers (18 mm OD) (gray lines). Interference colors observed with white light illumination were used to reconstruct the interfacial shape for later comparison with theoretical predictions. Film thicknesses ranged from tens of nanometers (black film) to several microns. Detailed quantitative studies were obtained with a modified optical setup [3,4]. Draining films of pure SDS generated fingering patterns (marginal regeneration process) along all the film edges. These evolve from Marangoni instabilities [5–7] caused by a surfactant surplus in the meniscus region (bottom edge). After some time, numerous holes containing black films (thinner than a quarter wavelength of visible light) grow to produce droplets (white) which decorate the black film circumference and fall toward the visible film border (light gray), a process resembling falling snow.
The presence of surfactant–polymer complexes as exist with SDS/PEO [1,2] modifies the entrainment and drainage [3,4,8] dynamics. The thickness fluctuations due to Marangoni instabilities, which create striking patterns resembling lava plumes, are observed to flow much more slowly due to the polymer-enhanced viscosity of the film. Above this border, there propagates from left to right a dendritic instability, which advances as a fractal front into the black film. The region behind the advancing front likely represents a collapsed state8 in which the less mobile polymer is denuded of surfactant micelles, which collect along the serpentine boundary. This boundary finally undergoes a transition from fractal to nonfractal shape (e), as numerous droplets (yellow) decorate the shrinking periphery. The video clip [9] ends with a closeup of the instability at the interface between the black and visible films.

Credits: S. Berg, E. A. Adelizzi, and S. M. Troian

References: [1] S. Lionti-Addad and J. M. diMeglio, "Stabilization of aqueous foam by hydrosoluble polymers. 1. Sodium dodecyl sulfate-poly(ethylene oxide) system," Langmuir 8, 324 (1992).
[2] S. Lionti-Addad and J. M. diMeglio, "Stabilization of aqueous foam by hydrosoluble polymers. 2. Role of polymer/surfactant interactions," Langmuir 10, 773 (1994).
[3] E. A. Adelizzi and S. M. Troian, "Interfacial slip in entrained soap films containing associated hydrosoluble polymer," Langmuir (in press).
[4] S. Berg, E. A. Adelizzi, and S. M. Troian, "Experimental study of entrainment and drainage flows in microscale soap films," Langmuir (submitted).
[5] V.A. Nierstrasz and G. Frens, "Marginal regeneration in thin vertical liquid films," J. Colloid Interface Sci. 207, 209 (1998).
[6] B. J. Fischer and S. M. Troian, "Thinning and disturbance growth in liquid films mobilized by continuous surfactant delivery," Phys. Fluids 15, 3837 (2003).
[7] J. M. Davis, B. J. Fischer, and S. M. Troian, in Interfacial Fluid Dynamics and Transport Properties, edited by R. Narayanan and D. Schwabe, Lecture Notes in Physics, Vol. 628 (Springer-Verlag, Berlin, 2003), p. 79.
[8] S. Berg and S. M. Troian, "Layering transitions and squeeze-out patterns in nanometer polymeric soap films," Nature (London) (submitted).
[9] Includes musical selection: Johann Sebastian Bach, "Air on a G String," from Orchestral Suite No. 3 in D Major (BWV1068), Franz Liszt Academy Chamber Orchestra, Sensual Classics compact disc (Teldec Classics International GmbH, Hamburg, Germany, 1992).

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Contributed By: Tausif Billah

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