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Wetting and elasticity: 2 experimental illustrations

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1. Marangoni bursting Depositing a droplet of pure water on a bath of sunflower oil is generally a frustrating experience as it leads to a steady liquid lens. However, if water is replaced by a mixture of alcohol and water, a spectacular phenomenon is observed: the droplet spreads and then spontaneously fragments into a myriad of minute droplets. The size of the expelled droplets strongly depends on the initial concentration of alcohol. As in the case of the famous “tears” that wine lovers appreciate, Marangoni flows induced by the evaporation of alcohol play a key role in the overall dynamics. The intricate coupling of hydrodynamics, wetting, and evaporation can be captured by analytical scaling laws, although the detail of the instability remains to be understood.

2. Moving a wide sphere in a narrow elastic tube What is the force required to move an object inside a narrow elastic pipe? The constriction by the tube induces a normal force on the sphere. In the case of solid friction, the pulling force may be simply deduced from Coulomb’s law. How is such force modified by the addition of a lubricant liquid? This coupled problem between elasticity and viscous flow shares some analogies with the seminal works of Bretherton on liquid coating. In particular, such coupling results in a non-linear dependence of the force on the traction speed.

This talk is part of the Fluid Mechanics (DAMTP) series.

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