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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.
Solvent evaporation causes concentration and temperature gradients at the free surface of a coating polymer solution. Thereby surface-tension-driven flows can result in a surface instability with regular structures within the layer. Different types of surface structures can occur at spots with thickness differences. One can find fractal-like structures especially in the edge zone of a layer. The evolution of such fractal-like structures at the surfaces of coating solutions of polyacrylonitrile (PAN) in dimethylformamide (DMF) during layer hardening is examined. Condensation of water on the surface of the fluid layer exerts a strong influence on the formation of these structures. Solutions of PAN in DMF are important for the production of ultrafiltration membranes. Such membranes are produced by precipitation in a water bath after a short open time.
Surface tension-driven Marangoni convection causes the formation of regular surface structures in drying polymer layers. The shape of the surface structures formed during solvent evaporation depends on layer and interfacial dynamic parameters as well as external factors. The influence of a horizontal radial temperature gradient produced by a point heat source below the polymer layer on the diffusion-controlled Marangoni instability has been studied. In the region of the lateral temperature gradient, radial surface flow coupled with the interfacial instability leads to stripe, ladder, chevron and/or labyrinthine surface structures.
Defekte und störende Oberflächenstrukturen keramischer Bauelemente, die aus keramischen Folien gefertigt werden, haben neben keramtechnologischen Prozessfehlern vielfach ihre Ursachen in dynamischen Prozessen bei der Trocknung der Nassschichten. Neben Einzeldefekten wie Poren oder Rissen können in Grünfolien während der Trocknung auch oberflächenspannungsinduzierte periodische Schichtdickenunterschiede auftreten. Grenzflächenendynamische Bewegungen wie auch die Bildung von Blasen oder Agglomeraten treten gleichermaßen in gegossenen keramischen Schichten und in keramikfreien Polyvinylbutyral/Weichmacher-Schichten auf. Das analoge Verhalten der untersuchten gegossenen keramischen Schichten und der entsprechenden keramikfreien Polymerschichten bezüglich der Entstehung dieser Oberflächenfehler sowie Möglichkeiten zur Fehlerreduzierung werden aufgezeigt.