Shreyas Mandre

Associate Professor
Mathematics Institute, University of Warwick

Flowing soap films

Measuring the Marangoni elasticity of soap films.
Measuring the Marangoni elasticity and surface tension of soap films.

Ever wondered why you can blow bubbles from soap solution in water, but not from water itself or from a solution of sugar or salt?

It is because soap is composed of a chemical compound known as a surfactant, which alters the surface tension of the air-water interface depending on the local soap concentration. The presence of this compound imparts the interface with an elasticity, the tendency of an object to regain its original shape, which in this case is called the Marangoni elasticity after the Italian physicist Carlo Marangoni.

In recent investigations, we presented ways to understand the influence of Marangoni elasticity in flowing soap films. ldoo Kim, a postdoc in my group, measured the Marangoni wave speed (analogous to the sound speed in a compressible gas) by establishing oblique shocks in the soap film and measuring their angle to the flow. Aakash Sane, an Sc. M students in my group, and Ildoo Kim also measured the surface tension of the soap film, which is an equally important quantity to know if the soap film deforms out of plane. Neither the Marangoni elasticity nor the surface tension were previously measured in situ. The basic principles we used are illustrated in the image on the left.

For our other work on surfactants, checkout this and this paper.


Sane, Mandre and Kim. Surface tension of flowing soap films. J. Fluid Mech. 841, R2 (2018).
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Abstract: The surface tension of flowing soap films is measured with respect to the film thickness and the concentration of soap solution. We perform this measurement by measuring the curvature of the nylon wires that bound the soap film channel and use the measured curvature to parametrize the relation between the surface tension and the tension of the wire. We find that the surface tension of our soap films increases when the film is relatively thin or is made of soap solution of low concentration; otherwise, it approaches an asymptotic value of 30 mN/m. ... (read more)

Fluid Mechanics Surfactants

Kim and Mandre. Marangoni elasticity of flowing soap films. Phys. Rev. Fluids 2, 082001(R) (2017).
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Abstract: We measure the Marangoni elasticity of a flowing soap film to be 22 mN/m irrespective of its width, thickness, flow speed, or the bulk soap concentration. We perform this measurement by generating an oblique shock in the soap film and measuring the shock angle, flow speed, and thickness. We postulate that the elasticity is constant because the film surface is crowded with soap molecules. Our method allows nondestructive measurement of flowing soap film elasticity and the value 22 mN/m is likely applicable to other similarly constructed flowing soap films. ... (read more)

Fluid Mechanics Surfactants

Mandre. Axisymmetric spreading of surfactant from a point source. J. Fluid Mech. 832, 777-792 (2017).
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Abstract: Guided by computation, we theoretically calculate the steady flow driven by the Marangoni stress due to a surfactant introduced on a fluid interface at a constant rate. Two separate extreme cases, where the surfactant dynamics is dominated by the adsorbed phase or the dissolved phase, are considered. We focus on the case where the size of the surfactant source is much smaller than the size of the fluid domain, and the resulting Marangoni stress overwhelms the viscous forces so that the flow is strongest in a boundary layer close to the interface. ... (read more)

Fluid Mechanics Surfactants

Bandi, Akella, Singh, Singh and Mandre. Hydrodynamic signatures of stationary Marangoni-driven surfactant transport. Phys. Rev. Lett. 119, 264501 (2017).
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Abstract: We experimentally study steady Marangoni-driven surfactant transport on the interface of a deep water layer. Using hydrodynamic measurements, and without using any knowledge of the surfactant physicochemical properties, we show that sodium dodecyl sulphate and Tergitol 15-S-9 introduced in low concentrations result in a flow driven by adsorbed surfactant. At higher surfactant concentration, the flow is dominated by the dissolved surfactant. Using camphoric acid, whose properties are a priori unknown, we demonstrate this method’s efficacy by showing its spreading is adsorption dominated. ... (read more)

Fluid Mechanics Surfactants