The coupling between evaporation and adsorbed surfactant accumulation and its effect on the wetting and spreading behaviour of volatile drops on a hot surface

The role played by surfactants in the wetting process has been extensively studied because of its huge potential applications. The effect of surfactants on wetting behaviour of volatile liquids has not been widely investigated. Superspreading phenomenon has been recently subject to numerous research works. It has been pointed out that Marangoni effect could be a key mechanism behind the superspreading phenomenon. In the present paper, the case of volatile wetting liquids is discussed; it is argued that there is a strong coupling (competition) between spreading and evaporation.The present work presents an experimental investigation of the temperature dependence of the wetting behaviour of droplets having different concentrations of surfactant (Triton X-100) on a heated aluminum surface. The need to investigate the temperature dependence of the contact angle of water stems from several applications involving droplets evaporation. The evaporation of the sessile drops was recorded using a CCD camera and the images were analyzed using the drop shape analysis software. The water drops are found to have a larger initial contact angle than the surfactant sessile drops. That was attributed to surfactants lowering the surface tension. The evaporation rate is strongly affected by the surfactant concentration; in some cases, almost around 75% from the water evaporation rate enhancement is observed. This is attributed to increased heat transfer area of the surfactant drops, as they increase the wetting area. The accumulation of the surfactant near the interface plays a major role in both wetting and evaporation. The concentration of the surfactant along the liquid–vapour interface is not uniform because of non-uniform evaporation; this leads to a solutal Maranfoni effect. The final spreading area when plotted as a function of surfactant concentration is found to exhibit a maximum when surfactant is present. It is thought that evaporation induces gradients in surfactant interfacial concentration leading to observed behaviour. Another interesting result was found in the presence of the surfactant; the spreading area exhibits a maximum when plotted against the substrate temperature. It seems that the evaporation can affect the interfacial surfactant distribution and accumulation leading to the observed results.