Movie S1.

A movie in real time of the breakup of six parallel cores embedded in a fiber during heating. The sample is a 2-cm-long PES polymer fiber with a rectangular cross-section (0.3 × 1 mm2) containing six equally spaced G2 cores, each 20 µm in diameter and separated by 100 µm from its nearest neighbor. A rectangular—rather than a cylindrical—fiber was used to eliminate optical lensing effects resulting from a curved outer surface and to reduce the fiber movement or disturbance during thermal processing.