  Active systems such as microorganisms and self-propelled particles show a
plethora of collective phenomena, including swarming, clustering, and phase
separation. Control over the propulsion direction and switchability of the
interactions between the individual self-propelled units may open new avenues
in designing of materials from within. Here, we present a self-propelled
particle system, consisting of half-gold coated titania ($\mathrm{TiO}_2$)
particles, in which we can fast and on-demand reverse the propulsion direction,
by exploiting the different photocatalytic activities on both sides. We
demonstrate that the reversal in propulsion direction changes the nature of the
hydrodynamic interaction from attractive to repulsive and can drive the
particle assemblies to undergo both fusion and fission transitions. Moreover,
we show these active colloids can act as nucleation sites, and switch rapidly
the interactions between active and passive particles, leading to
reconfigurable assembly and disassembly. Our experiments are qualitatively
described by a minimal hydrodynamic model.
