The Water Droplet Contact Line Probed with Multiwalled Carbon Nanotubes at the Air–Water Interface

Within the issue of sessile droplet evaporation, particularly important is the behavior of the triple phase contact line governed by pinning phenomena. We demonstrate how pristine, insoluble multiwalled carbon nanotubes (MWNT) can exhibit ordering phenomena at the air–water interface, contribute to droplet pinning, and are also responsive to the evolving shape of evaporating water droplets at the contact line. The arc-discharge-synthesized MWNTs were of high quality, but they were mixed with graphitic impurity particles in the 10–100 nm size ranges. The MWNTs were ordered into chain structures by capillary interactions at the air–water interface. Moreover, we observed how the chain structures regularly turned perpendicular to the contact line a short time prior to the withdrawal of the strongly pinned contact line, which we explain with the capillary force acting in a region with nonconstant interface curvature. The MWNT chains thus offer a unique way to probe the local behavior of the droplet. We modeled the van der Waals interactions of MWNTs and graphitic impurities in the vicinity of the contact line. According to these, the related energies are large enough to explain issues related to the transfer onto the air–water interface. Capillary effects can be qualitatively explained by existing theories. As the MWNTs are strictly confined to the air–water interface, these results are complementary to the separate but closely related coffee ring effect.