  Synthetic microswimmers mimicking biological movements at the microscale have
been developed in recent years. Actuating helical magnetic materials with a
homogeneous rotating magnetic field is one of the most widespread techniques
for propulsion at the microscale, partly because the actuation strategy
revolves around a simple linear relationship between the actuating field
frequency and the propeller velocity. However, the full control of the
swimmers' motion has remained a challenge. Increasing the controllability of
micropropellers is crucial to achieve complex actuation schemes that in turn
are directly relevant for numerous applications. The simplicity of the linear
relationship though limits the possibilities and flexibilities of swarm
control. Using a pool of randomly-shaped magnetic microswimmers, we show that
the complexity of shape can advantageously be translated into enhanced control.
In particular, directional reversal of sorted micropropellers is controlled by
the frequency of the actuating field. This directionality change is linked to
the balance between magnetic and hydrodynamic forces. We further show an
example how this behavior can experimentally lead to simple and effective
sorting of individual swimmers from a group. The ability of these propellers to
reverse swimming direction solely by frequency increases the control
possibilities and is an example for propeller designs, where the complexity
needed for many applications is embedded directly in the propeller geometry
rather than external factors such as actuation sequences.
