We report an experimental study of the shape of large liquid drops spreading on surfaces with patterns of wettability. A heterogeneous surface exhibits multiple energy minima of free energy for the three-phase system. In particular, when the distribution of defects on the substrate is periodic (square or hexagonal), it is possible to initiate a transition between circular and faceted drops. We describe the surface growth of these liquid drops as a geometric avalanche process released by surface tension. c EDP Sciences