Speaker
Description
We investigate the adsorption of surfactants in sessile water nanodroplets on solid hydrophobic substrates. Using molecular dynamics simulations, we provide evidence that surfactants, which in our case are linear alcohols or aromatic molecules, show an excess of adsorption close to the three-phase contact line. This result reveals that surfactants have a higher affinity for the contact line than for the water-vapor and water-substrate interfaces.
We characterize this phenomenon by investigating the surface and line adsorption isotherms of the different species and show that, at small concentrations, they follow a qualitatively similar behavior. This allows us to extract the coefficients quantifying their affinity for the two interfaces and for the contact line, and derive general rules describing their dependence on the chemical characteristics. In particular, for linear surfactants, we find an exponential dependence on the number of carbon atoms.
At higher concentrations, the behavior of surfactants starts differentiating, and the adsorption isotherms exhibit a rich phenomenology, characterized by a complex dependence of the partitioning between the different regions of the droplets (bulk, interfaces, and contact line) on concentration.
We then investigate the dependence of the partitioning on droplet size, and calculate the influence of line adsorption on line tension, showing that it has the effect of reducing it, in a similar fashion as surface adsorption does with surface tension. We finally investigate the implications of this effect on the determination of contact angles.