Stability criteria for two-dimensional wetting in monolayers

Two-dimensional pendant liquid expanded droplets partially wet the liquid condensed/gas-phase boundaries in methyl octadecanoate Langmuir monolayers. Their shape is described by the Young-Laplace equation including long-range electrostatic interactions on a scale Delta. It is invariant under shape-invariant scale transformations. We show that the local stability at the three-phase intersection point is described by Young's equation for the contact angle. The contact angle is not invariant under shape-invariant scale transformations but is a materials constant at a fixed scale parameter Delta. By comparison of numerically simulated droplets with experimental droplets observed with a fluorescence microscope, we determine the spreading coefficient of wetting Langmuir monolayer phases as well as a lower limit for Delta. We find 0.12 mum < Delta and suggest that the scale parameter shall be interpreted as a dipolar correlation length. not as a molecular cutoff length.