  Active colloids, also known as artificial microswimmers, are self-propelled
micro and nanoparticles that convert uniform sources of fuel (e.g. chemical) or
uniform external driving fields (e.g. magnetic or electric) into directed
motion by virtue of asymmetry in their shape or composition. These materials
are currently attracting enormous scientific attention for two main end uses.
First, active colloids are prototypical internally driven, out-of-equilibrium
systems and their study has led to new emergent material properties, such as
swarming and living crystallization. Secondly, they hold the promise to be used
as micro- and nanoscale devices with tremendous potential from medical to
environmental applications. However, the current fabrication of active colloids
is limited in the programmability of materials, geometry, and modes of motion.
Here, we use sequential capillarity- assisted particle assembly (sCAPA) to link
microspheres of different materials into clusters of prescribed shapes
("colloidal molecules") that can actively translate, circulate and rotate
powered by asymmetric electro-hydrodynamic flows. Further engineering of the
geometry and composition provides active colloids that switch motion under
external triggers or perform simple pick-up and transport tasks. Our
fabrication strategy enables both physicists and engineers to design and create
customized active colloids to explore novel fundamental phenomena in active
matter and to investigate materials and propulsion schemes that are compatible
with future applications.
