Apparent line tension induced by surface-active impurities

Line tension in wetting processes is of high scientific and technological relevance, but its understanding remains vague, mainly because it is difficult to determine. A widely used method to extract line tension relies on the variation of a droplet's contact angle with the droplet's size. Such an approach yields the apparent line tension, which is an effective parameter that factors in numerous contributions to the finite-size dependence, thus masking the actual line tension in terms of the excess free energy of the three-phase contact line. Based on our recent computer simulation study, we investigate how small amounts of nonionic surfactants, such as surface-active impurities, contribute to the apparent line tension in aqueous droplets. When depositing polydisperse droplets, their different surface area-to-volume ratios can result in different final bulk concentrations of surfactants, different excess adsorptions to the interfaces, and, consequently, different contact angles. We show that already trace amounts of longer-chained surfactants in a pre-contaminated liquid are enough to affect measurements of the apparent line tension. Our analysis quantifies to what extent "background" impurities, inevitably present in all kinds of experimental settings, limit the resolution of line tension measurements, which is crucial for avoiding data misinterpretation.