Stable equidistant step trains during crystallization of insulin

Bunching of growth steps plagues layerwise crystallization of materials in laboratory, industrial, and geological environments, and theory predicts that equidistant step trains are unstable under a variety of conditions. Searching for an example of stable equidistant step trains, we monitored the generation and spatiotemporal evolution of step trains on length scales from 100 nm to 1 mm during the crystallization of insulin, using atomic force microscopy and phase-shifting interferometry. We show that near-equidistant step trains are generated by single and cooperating screw dislocation. The lack of step-step interaction and the overall transport-controlled growth regime further regularize the step train and ensure the stability of the obtained equidistant arrangement.