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 CaCl 2 -containing gel from a Na 2 CO 3 solution. The resulting CaCO 3 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 H 2 O 2 solution, the resulting tubes self-propel by the release of O 2 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.