Dynamics of Low Anisotropy Morphologies in Directional Solidification

TL;DR

This study investigates how small anisotropies in surface tension influence the morphology of diffusion-limited growth in directional solidification, revealing various seaweed structures and transitions between growth forms.

Contribution

It provides experimental insights into the effects of minor anisotropies on seaweed morphologies in directional solidification, highlighting transitions and the role of misorientation.

Findings

Different seaweed morphologies depend on surface tension anisotropy.

Transition from fractal to compact seaweed with increased undercooling.

Observation of seaweed-dendrite morphological transitions.

Abstract

We report experimental results on quasi-two-dimensional diffusion limited growth in directionally solidified succinonitrile with small amounts of poly(ethylene oxide), acetone, or camphor as a solute. Seaweed growth, or dense branching morphology, is selected by growing grains close to the $\{111\}$ plane, where the in-plane surface tension is nearly isotropic. The observed growth morphologies are very sensitive to small anisotropies in surface tension caused by misorientations from the $\{111\}$ plane. Different seaweed morphologies are found, including the degenerate, the stabilized, and the strongly tilted seaweeds. The degenerate seaweeds show a limited fractal scaling range and, with increased undercooling, suggests a transition from "fractal" to "compact" seaweed. Strongly tilted seaweeds demonstrate a significant twofold anisotropy. In addition, seaweed-dendrite transitions are observed in low anisotropy growth.