Line tension in Langmuir monolayers probed by point forces

A Langmuir monolayer of methyl octadecanoate in the phase coexistence region liquid expanded/liquid condensed is observed with fluorescence microscopy and mechanically manipulated using optical tweezers. A circular liquid condensed droplet is locally fixed with the tweezers and deformed by hydrodynamic flow of the surroundings. The stationary shape is determined by the competition of the bare line tension lambda, dipole and hydrodynamic forces. The dipole contributions to the shape can be accounted for by introducing an effective line tension lambda(eff). At the front of the droplet a wedge angle theta is formed, from which lambda(eff) can be deduced. The electrostatic contribution lambda(eff)-lambda is calculated for the experimental shape in the limit of weak deformations. The range of deformation where this approach holds is determined for a triangular shape. (C) 2000 American Institute of Physics. [S0021-9606(00)71633-3].