Ordered molecular assemblies spontaneously appeared from evaporating droplets of collagen solutions on glass substrates. Nanoscopic structures of the assemblies were examined using an atomic force microscope; well aligned collagen molecules oriented their ends toward the assembly surface and formed layered structures for concentrations >0.05 mg/mL, while the molecules, mostly curved, lay for concentrations <0.005 mg/mL. The layers exhibited a variety of morphological forms (e.g., regular, undulated, and rolled patterns) indicating their flexibility. For concentrations >5 mg/mL, concentric ring patterns additionally appeared in the assemblies; each ring consisted of multilayers of well-aligned tilting collagen molecules. The color of the rings alternatingly changed from blue to orange when using a color-sensitive plate. The ring width increased from 3 to 120 μm with increasing collagen concentration and also increased toward the center of the patterns. The observations of temporal and spatial evolution of the concentric ring patterns with a polarizing-light microscope showed the repetitive appearance of the multilayered region which underwent a subsequent transition into a pair of blue and orange concentric rings near the droplet edge during evaporation. The formation time of one concentric ring was dependent on collagen concentration; it was 37−50 s at an early stage of the formation process.
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Hideatsu Maeda (1999) studied this question.