On new Marangoni effects and new interpretation of old Marangoni effects in physics of interfaces

Rouslan Krechetnikov, Control and Dynamical Systems, Caltech

While Marangoni effects seem to be well studied in interfacial physics beginning with the famous "wine tears" problem, recent experiments suggest some new ways these effects may exhibit themselves through new instabilities and nonlinear dynamics. As an example of a new instability, we discuss the displacement of a less viscous fluid by a more viscous one in a Hele-Shaw cell, the interface between which should be stable according to Saffman-Taylor criteria. However, an addition of a surface active substance (surfactant) introduces Marangoni stresses, which make the advancing interface unstable and lead to fingering.

As an illustration of a nonlinear dynamics induced by Marangoni effect, we discuss a self-agitated pendant drop, which results from a chemical reaction at the drop interface and which generates a number of nontrivial features: nonlinear auto-oscillations, conical drop shape,
tip-streaming and droplet trajectory splitting. Some of these features are analogous to the ones of Taylor cones problem, but the analysis revealed a drastically different underlying physics. While the above phenomena are new manifestations of Marangoni effects, an old phenomenon -- thickening of a film deposited by a substrate withrawal (Landau-Levich problem) -- for a long time attributed to Marangoni effects turned out to have a different cause, which has an intricate microscopic origin as the careful theoretical and experimental study demonstrated.

(Joint work with George "Bud" Homsy)