Dendritic Growth Dynamics: Steady And Oscillatory States

摘要

Microgravity dendritic growth experiments, conducted aboard the space shuttle Columbia (STS-87) in November/December 1997, are analyzed and discussed. In-situ video images now reveal that pivalic acid (PVA) dendrites growing in the diffusion-controlled environment of low-earth orbit exhibit a range of growth behaviors, including steady, transient, and oscillatory states. The observed transient features of the growth process are being studied with the objective of understanding their physical mechanisms. Some transients in the observed growth speed are thought to arise as an intrinsic aspect of the evolving dendritic pattern. Variability in the growth speed observed from a sequence of identical runs at equal supercooling suggests that self-interactions of the dendrite remain important throughout the development of the dendritic pattern. A Greens function analysis of the near-tip diffusion sources contributing to the local field at the tip suggests that strong non-local interactions exist well into the time-dependent side-branch region of real dendrites. Video data obtained at 30 fps allow the first application of discrete Fourier transform methods (Lomb periodograms) to the digitized images of dendritic growths under quiescent microgravity conditions. These observations provide evidence for the appearance of characteristic frequencies in the tip shape and its dynamical behavior. Some of the frequency bands observed coincide closely with the ratio of the dendritic tip growth speed divided by the side branch spacing. Other observed lower frequencies remain as yet unexplained. These data, and their interpretations, are discussed in this paper.