  Combining experiments and computer simulations, we use a spatially periodic
and flashing light-field to direct the motion of phototactic active colloids.
Here, the colloids self-organize into a density spike pattern, which resembles
a shock wave and propagates over long distances, almost without dispersing. The
underlying mechanism involves a synchronization of the colloids with the
light-field, so that particles see the same intensity gradient each time the
light-pattern is switched on, but no gradient in between (for example). This
creates a pulsating transport whose strength and direction can be controlled
via the flashing protocol and the self-propulsion speed of the colloids. Our
results might be useful for drug delivery applications and can be used to
segregate active colloids by their speed.
