Dual Marangoni effects and detection of traces of surfactants†
We report controlled and tunable Marangoni flows resulting from concentration gradients that are induced by placing a droplet of volatile solute above the surface of a liquid. Condensation of the solute at the liquid surface results in a surface tension gradient that drives a permanent flow of surface velocity up to a few centimeters per second. Depending on the sign of the variation of surface tension with solute concentration, inward or outward surface flows can be obtained. We show that, in the region close to the vertical axis of the droplet, the flow rate varies with the droplet height following a power law, whose exponent depends on the nature of the transfer of the solute in air. In the case of a purely diffusive transfer we establish an analytical law for the velocity rate, which is in very good agreement with the experimental data. In addition, we discuss the effect of convection on the found scaling laws. Finally, we demonstrate that the Marangoni flow is modified by the addition of a very small quantity of surfactants, which themselves induce a Marangoni flow opposing the primary one. We suggest it could provide a simple method to detect traces of surfactants, of increasing sensitivity with decreasing surfactant solubility.