  Glasses remain an elusive and poorly understood state of matter. For example,
it is not clear how we can design an efficient macroscopic glass former by
tuning the properties of its microscopic building blocks. In this paper, we
propose a simple directional colloidal model that reinforces the optimal
icosahedral local structure of binary hard-sphere glasses. We show that only
this specific symmetry results in a dramatic slowing down of the dynamics. Our
results open the door to controlling the dynamics of dense glassy systems by
selectively promoting specific local structural environments.
