We performed micro-computed x-ray tomography on desiccated corn starch slurries. This movie shows an approximately 1 cm x 2 cm area of corn starch, in cross-section. We pan through approximately 3 cm of depth during the two sequences in the movie, moving in the direction of drying. The thin black lines are cracks, which form a network. The circular black dots are bubbles in the slurry, and the concentric ring features are artifacts of the tomography. Columnar jointing arises from a thin propagating fracture front. The movie displays the time evolution of the fracture network which leaves behind the columns. In the uncontrolled desiccation sequence, we observe how the columns initially order, and then continue to coarsen, maintaining statistically self-similar properties. In the controlled desiccation sequence, the evaporation rate was kept fixed at 1g/h in the final phase of the desiccation. Here, coarsening is halted by a balance between the increase in the average cross sectional area due to the termination of single cracks or vertices, and the decreases due to the creation of new columns at vertices. There is a strong tendency for adjacent columns to evolve toward equal areas.
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