Biomorph Oscillations Self-organize Micrometer-Scale Patterns and Nanorod Alignment Waves

Abstract

The coprecipitation of barium carbonate and silica spontaneously creates complex micrometer-scale objects such as sheets and helices. These structures consist of densely packed crystalline nanorods that in the case of sheets align in radial direction. We report the existence of an additional level of self-organization that creates oscillatory height variations in biomorph sheets. These topographic features take the form of either concentric rings or disordered, patchy patterns and form immediately in the wake of the crystallization front. Their wavelength varies around 6.5 μm and shows no pronounced dependence on the reactant concentrations. Atomic force microscopy reveals height variations of up to 500 nm which equal 45% of the average sheet thickness. These undulations are accompanied by a systematic out-of-plane displacement of the nanorods. Our results are discussed in the context of an earlier hypothesis that predicts pH oscillations near the crystallization front.