EVOLUTION OF INDUCED PATTERNS IN SURFACE-TENSION-DRIVEN BENARD CONVECTION

Experimental results on the evolution of induced patterns in Benard-Marangoni convection are reported. These patterns are initially forced by means of a thermal technique which allows the formation of a regular hexagonal pattern with a chosen wavelength. Several series of measurements have been performed in square vessels with different aspect ratios GAMMA. For fixed GAMMA, after inducing a pattern with a wavelength different from the optimal one, an evolution is observed that leads to an evolving mean wave-length. The main mechanism for this evolution is the generation of defects which increase the disorder in the pattern. This disorder is mainly due to nucleation of new cells when the forced ones are too large, or by fusion of cells when the original ones are too small. Another interesting phenomenon occurs when the forced wavelength lambda is close to the optimal one. In large-aspect-ratio vessels the disorder rises initially at the center of the pattern, leading to a relaxation of the mean wavelength. However, in small-aspect-ratio vessels, the behavior can be nonmonotonous. Under well-chosen conditions (the initial pattern has a mean wavelength slightly smaller than the optimal one), lambda increases initially as a consequence of sidewall effects; then it decreases due to the rising and propagation of a dislocation line in the pattern. This evolution has a form similar to the creep function in a viscoelastic material. This effect seems to provide an effective wavelength selection mechanism. Using a Ginzburg-Landau model adapted to the hexagonal lattice, the relative importance of local wavelength variations, disalignment of polygon lines, defects, and sidewalls have been determined.