Tears of an evaporating methanol meniscus on a silicon substrate

We identify the formation of an unstable rim at the three-phase contact line of an evaporating methanol meniscus due to the autophobic nature of the liquid. Undulations along the rim rupture at a consistent frequency, leaving behind a regular pattern of droplet tears as the contact line recedes. We characterize the geometry and undulation dynamics of the rim. The rim movement exhibits slip at the contact line; the measured ratio of rim width to undulation wavelength matches the critical unstable Rayleigh ratio for an intermediate slip regime. Unlike previously observed rim instabilities, the rim volume here is replenished from the evaporating bulk meniscus and maintains a constant width, such that the instability wavelength remains constant and droplets are generated with a consistent, nano-liter volume and deposited in a regular pattern. Occurrence of this unstable rim during an evaporation-driven dewetting process may allow for rapid, controlled deposition of small droplets from the fast-moving contact line of a highly volatile fluid. Published by AIP Publishing.