The line tension λ of the liquid expanded (LE)/gas (G)-phase boundary of a methyl octadecanoate Langmuir monolayer at the air/water interface is measured using fluorescence microscopy combined with optical tweezers. Silica spheres, immersed into the monolayer and manipulated by the tweezers, deform the phase boundary. After switching off the tweezers, the relaxation of the deformed region is dominated by the competition between line tension and hydrodynamic resistance while dipole–dipole forces between the molecules can be neglected. A linear relation between the deformation length and time is found, from which a line tension of λ=7.5 pN is deduced. The linearity gives an upper bound for the surface potential differences of both phases. It is shown that viscous forces from the two-dimensional LE surroundings dominate the subphase friction. The optical tweezers enable one to observe relaxations on a shorter time scale, extending the range of measurement of previous techniques toward higher line tension or lower viscosities of the monolayer and of the subphase.
A technique for direct observation of particle motion under shear in a Langmuir monolayerThe ratio of the rotational and translational drag coefficient of a circular liquid condensed Langmuir monolayer domain moved in different phases is measured. A single domain is fixed at its boundary and forced to undergo combined translational and rotational motion. It is observed, that the drag force is dominated by the viscous dissipation of the three dimensional subphase and affected by the elasticity of the surrounding monolayer phase.
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 λ, dipole and hydrodynamic forces. The dipole contributions to the shape can be accounted for by introducing an effective line tension λeff. At the front of the droplet a wedge angle θ is formed, from which λeff can be deduced. The electrostatic contribution λeff-λ 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.
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