Instabilities during Directional Solidification of a Transparent Material
Directional solidification of a transparent material is not only a useful model system for solidification in general, but also a fascinating one-dimensional pattern forming system. In this dissertation I describe our novel experiments on the solid-liquid interface of directionally solidifying succinonitrile. These experiments employed a high-resolution directional solidification stage. We used image processing techniques extensively to extract quantitative characterizations of patterns exhibited by the interface's shape. We measured the planar interface's position in the temperature gradient and found significant departures from expected behavior as well as apparent conflict with our direct interface temperature measurements. Different causes for this behavior are explored. We have found that breaking the continuous rotational symmetry about the pulling direction results in a host of new behavior. The first instability in this case is to a long wavelength mode believed to arise from an impurity concentration gradient in the plane of the interface. This can lead to nucleation and growth of the Mullins-Sekerka instability at the less pure end of the interface. Growth is described in the context of front propagation, which inherently has broken time reversal symmetry. We also use front propagation as a powerful, qualitative test of the nature of the planar to cellular bifurcation. Our measurements of the groove shape in the front region reveal that they are Gaussian in shape with negligible overlap between neighboring grooves. This has profound implications for the wavelength selection problem in directional solidification. We find that the cellular array left behind after the transit of the front exhibits enhanced spatial and temporal order compared to the case when there is no front propagation. We attribute this to an effective "soft" boundary on the cellular array where the cell spacing can be freely adjusted. I describe new, time-dependent behavior we have observed and quantitatively characterized: Specifically, we observed traveling wave states comprised of reflection asymmetric cell tips when the seed-crystal contains a crystal grain boundary. Our measurements of the asymmetry and phase speed are compared with recent theory. We find the crystal grain boundary to be the site of a spatio-temporal dislocation.
- Pub Date:
- February 1991
- CRYSTAL GROWTH;
- Physics: Condensed Matter; Physics: General