Observed propulsion mechanisms of Mg-TiO2 micromotors. (i) Micrograph, (ii) schematic illustration, and (iii) the path length and net displacement versus time. Vertical lines in (iii) represent the appearance of a new bubble for (a) an axisymmetric and (b) a typical Mg-TiO2 micromotor. (c) Sudden fluid jet mechanism for propulsion upon bubble nucleation inside a micromotor. Colored arrows track sequential bubbles in order of ejection.