  The self-propulsion mechanism of active colloidal particles often generates
not only translational but also rotational motion. For particles with an
anisotropic mass density under gravity, the motion is usually influenced by a
downwards oriented force and an aligning torque. Here we study the trajectories
of self-propelled bottom-heavy Janus particles in three spatial dimensions both
in experiments and by theory. For a sufficiently large mass anisotropy, the
particles typically move along helical trajectories whose axis is oriented
either parallel or antiparallel to the direction of gravity (i.e., they show
gravitaxis). In contrast, if the mass anisotropy is small and rotational
diffusion is dominant, gravitational alignment of the trajectories is not
possible. Furthermore, the trajectories depend on the angular self-propulsion
velocity of the particles. If this component of the active motion is strong and
rotates the direction of translational self-propulsion of the particles, their
trajectories have many loops, whereas elongated swimming paths occur if the
angular self-propulsion is weak. We show that the observed gravitational
alignment mechanism and the dependence of the trajectory shape on the angular
self-propulsion can be used to separate active colloidal particles with respect
to their mass anisotropy and angular self-propulsion, respectively.
