  In many active matter systems, particle trajectories have a well-defined
handedness or chirality. Whether such chiral activity can introduce
stereoselective interactions between particles is not known. Here we developed
a strategy to tune the nature of chiral activity of 3D-printed granular
ellipsoids without altering their shape or size. In vertically agitated
monolayers of these particles, we observed two types of dimers form depending
on the chirality of the pairing monomers. Heterochiral dimers moved
collectively as a single achiral active unit, while homochiral ones formed a
translationally immobile spinner. In active racemic mixtures, the former was
more abundant than the latter indicating stereoselectivity. Through dimer
lifetime measurements, we provide compelling evidence for chiral
self-recognition in mixtures of particles with different chiral activities. We
finally show that changing only the net chirality of a dense active liquid from
a racemic mixture to an enantiopure liquid fundamentally alters its nature of
collective relaxation.
