(518c) Tear Formation at the Unstable Receding Contact Line of an Evaporating Meniscus

We experimentally observe rim formation at the three-phase contact line of an evaporating methanol meniscus. A microliter volume of methanol is placed on top of a Si wafer maintained at a controlled temperature. The highly wetting liquid spreads and forms a thin-film meniscus, which starts to recede as the liquid film evaporates. A liquid rim, thicker than the evaporating bulk meniscus, is formed due to local dewetting of the liquid near the contact line. This local dewetting originates from repulsion of the bulk molecules along the liquid interface by immobile adsorbed molecules attached to the surface. This dewetting phenomenon results from the autophobic behavior of the liquid with its adsorbed layer and is observed in polar liquids such as alcohols. The adsorption of methanol is confirmed via X-ray photoelectron spectroscopy (XPS) and ellipsometry. The rim is unstable, and spatially periodic undulations are observed along its length. The undulation peaks bulge out and eventually grow into small tear-shaped droplets that pinch off from the rim at a consistent frequency, leaving behind a regular pattern of droplet tears as the contact line recedes. The temporal characteristics of the rim width and undulation wavelength are investigated to characterize the nature of this naturally occurring instability. The rim movement follows an intermediate slip model and the undulations grow in an exponential manner prior to tear pinch-off. The rim width and undulation wavelength correspond to the critical unstable Rayleigh ratio for a rim in the intermediate slip regime. Occurrence of this unstable rim during an evaporation-driven dewetting process allows for rapid deposition of nanoliter-size droplets from the contact line, which provides an avenue for rapid deposition of suspended/dissolved species in a regular pattern without involving any complex fabrication techniques.