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

Experimental results on the evolution of induced patterns in Benard-Ma rangoni convection are reported. These patterns are initially forced b y means of a thermal technique which allows the formation of a regular hexagonal pattern with a chosen wavelength. Several series of measure ments have been performed in square vessels with different aspect rati os 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 i s the generation of defects which increase the disorder in the pattern . This disorder is mainly due to nucleation of new cells when the forc ed ones are too large, or by fusion of cells when the original ones ar e too small. Another interesting phenomenon occurs when the forced wav elength lambda is close to the optimal one. In large-aspect-ratio vess els the disorder rises initially at the center of the pattern, leading to a relaxation of the mean wavelength. However, in small-aspect-rati o vessels, the behavior can be nonmonotonous. Under well-chosen condit ions (the initial pattern has a mean wavelength slightly smaller than the optimal one), lambda increases initially as a consequence of sidew all 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 pr ovide an effective wavelength selection mechanism. Using a Ginzburg-La ndau model adapted to the hexagonal lattice, the relative importance o f local wavelength variations, disalignment of polygon lines, defects, and sidewalls have been determined.