Effect of Surfactant Mixtures on the Evaporation Rate of Aqueous Sessile Droplets from Slightly Hydrophobic Substrates

The evaporation of surfactant-laden sessile droplets has widespread applications in both natural and technological contexts. This study explores the evaporation of droplets containing a nonionic surfactant (tristyrylphenol ethoxylates (EOT)), an anionic surfactant (sodium benzenesulfonate with alkyl chain lengths of C10–C13 (NaDDBS)), and their mixtures at nEOT/nNaDDBS mole ratios of 0.01, 0.1, 1, and 4, deposited on slightly hydrophobic silane-coated glass slides. The surfactants present significantly different critical micelle concentrations. In all cases studied, surfactants decreased the evaporation time compared to that of the pure water droplets. As the initial surfactant concentration increased, the evaporation time decreased. Interestingly, EOT-laden droplets exhibited longer evaporation times, despite EOT decreasing surface tension more than NaDDBS. For the EOT/NaDDBS mixtures, evaporation times fell between those of the surfactants alone, which was attributed to synergistic effects at the interface. The presence of surfactants tends to flatten the droplet, increase the surface area, and disrupt hydrogen bonds at the surface, which are likely to contribute to reducing the evaporation time; on the other hand, surfactant molecules tend to form layers at the surface, which hinder evaporation. The results were interpreted based on two distinct modes of evaporation, i.e., the constant contact radius and the constant contact angle; while both modes of evaporation were observed in all cases, the duration of the constant contact radius mode increased with surfactant concentration. As the concentration increased, deviations were observed between the results and predicted trends.