  A mixture of two types of super-paramagnetic colloidal particles with long
range dipolar interaction is confined by gravity to a flat interface of a
hanging water droplet. The particles are observed by video microscopy and the
dipolar interaction strength is controlled via an external magnetic field. The
system is a model system to study the glass transition in 2D, and it exhibits
partial clustering of the small particles. This clustering is strongly
dependent on the relative concentration $\xi$ of big and small particles.
However, changing the interaction strength $\Gamma$ reveals that the clustering
does not depend on the interaction strength. The partial clustering scenario is
quantified using Minkowski functionals and partial structure factors. Evidence
that partial clustering prevents global crystallization is discussed.
