Soluble surfactant spreading: How the amphiphilicity sets the Marangoni hydrodynamics
Résumé
Amphiphiles are molecules combining hydrophilic and hydrophobic parts. The way they arrange in bulkand at interfaces is related to the balance between these two parts, and can be quantified by introducing thecritical micellar concentration (cmc). Amphiphiles (also named “surfactants”) are also at the origin of dynamicaleffects: local gradients of interfacial concentrations create the so-called Marangoni flows. Here we study thecoupling between the molecule amphiphilicity and these Marangoni flows. We investigate in detail a spreadingconfiguration, where a local excess of surfactants is locally sustained, and follow how these surfactants spreadat the interface and diffuse in bulk. We have measured the features of this flow (maximal distance and maximalspeed), for different types of surfactant, and as a function of all experimentally available parameters, as well asfor two different configurations. In parallel, we propose a detailed hydrodynamical model. For all the measuredquantities, we have found a good agreement between the data and the model, evidencing that we have capturedthe key mechanisms under these spreading experiments. In particular, the cmc turns out to be—as for the staticpicture of a surfactant—a key element even under dynamical conditions, allowing us to connect the moleculeamphiphilicity to its ability to create Marangoni flows.