Shape-preserving conversion of calcium carbonate tubes to self-propelled micromotors†
The self-assembly of inorganic structures beyond the euhedral shape repertoire is a powerful approach to grow hierarchically ordered materials and mesoscopic devices. The hollow precipitate tubes in chemical gardens are a classic example, which we produce on Nafion membranes separating a CaCl2-containing gel from a Na2CO3 solution. The resulting CaCO3 microtubes are conical and consist of either pure vaterite or calcite. The process also forms branched T- and Y-shaped structures. The metastable vaterite polymorph can be converted to Mn-based structures without loss of the macroscopic shape. In H2O2 solution, the resulting tubes self-propel by the release of O2 bubbles, which for branched structures causes rotation. The tubes can contain multiple bubbles which are ejected in a quasi-periodic fashion (e.g. in groups of four). The addition of surfactants causes the accumulation of bubble trails and bubble rafts that interact with the moving tubes and give rise to distinct motion patterns.