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The movable contact line between two liquids and a gas phase sensitively reacts to small disturbances in the force equilibrium. The shape of the contact line and the adjoining interfaces is determined by the interface and surface tensions, the contact angles, the density differences (hydrostatic pressure), and the Laplace capillary pressure. When these change, the three-phase contact line can deform and even become unstable. Interface and surface tension depend on the concentration and temperature. During mass transport processes (concentration changes) various forms of the instability of the contact line can be observed:
Oscillations of a circular contact line (regular expansion and reduction);
Single deformations (bulges) which quickly disappear again;
Deformations (bulges) which run along the boundary line;
Periodically generated and damped deformations with different modes.
The behavior of the three-phase contact line is of practical importance for coalescence processes and for spontaneous emulsification on liquid surfaces.
The liquid-crystalline side-group polyacrylate PAC6 forms two mesophases in thin layers within the temperature range between 90 and 100 °C. The structures developing by self-organization can be made clearly visible by Schlieren-optical methods. If a low lateral temperature gradient is maintained in the temperature range mentioned above one can simultaneously and abreast observe four phases in PAC6 layers. Both new phases occur between the smectic and the nematic phase. Layer surface deformations arise during this phase transition. They were analyzed with a microscope interferometer. The PAC6 melt shows a considerable change of surface tension in this temperature range.
When thin layers of polymer solutions dry by solvent evaporation and/or heating from below, the layers can become hydrodynamically instable. As a result various surface structures occur. A great pattern manifold of such structures has been found. By self-organization small surface motions caused by surface tension differences can be amplified, when the ratio of the driving forces to the damping ones characterized by the Marangoni-number exceeds a critical value. Besides greater and hierarchic subdivided cell patterns, periodic line structures with peak-valley-differences of down to 1 nm have been found in the solidified layers. The surface profiles (3D- and line profiles) have been measured by means of a microscope interferometer.