Marangoni Flow with Water-Soluble Surfactants
Résumé
Amphiphiles, such as alcohols, lipids and soaps can decrease the interfacial tension
between water and other fluids, thus helping for the compartmentalization of matter in
emulsions, foams, vesicles and cells. A difference in the concentration of amphiphiles
along an interface between two media triggers an interfacial-tension-induced Marangoni
flow whose occurrence is critical for transport phenomena in lipid nanotubes, the stability
of emulsions and foams, pulmonary surfactant replacement therapy, insect locomotion
and many industrial applications. Although most amphiphiles found in nature and used in
the industry have a finite solubility in water, only few studies deal with Marangoni flows
induced by water-soluble amphiphiles, while simultaneously considering the effect of the
amphiphile finite solubility.
Here we provide a quantitative description of a model Marangoni flow, induced by the
spreading and dilution of a local surfactant excess, both in terms of surfactant
physicochemistry and hydrodynamics. We show experimentally that i) the spreading
Marangoni flow induced by surfactants of finite solubility in water extends over a finite
distance and ii) this distance varies monotonically with the critical micellar concentration
(CMC). We provide evidence that the velocity field of this flow has universal features, and
we identify the scaling laws that capture both the extent over which the Marangoni flow is
observed and the magnitude of the maximum velocity.
These results are important for a better understanding of transport phenomena involving
amphiphiles at liquid interfaces in out-of-equilibrium systems. In addition, our
experimental approach could be adapted to design a new method to measure the critical
micellar concentration of an amphiphile.