Gels are abundant in nature; yet, the physics of gel formation is still poorly understood.. Many of the common gel formers exhibit strong attractive interactions, which are also highly directional. Spherical colloids, micron-sized particles in a solvent, which have sticky patches on their surface, represent a very simple experimental model for gel formers with directional interactions.

We employ confocal microscopy, to follow our patchy colloids, as they form gels. To further simplify our system, we confine our particles to a two dimensional space. Strikingly, the clusters of sticking particles do not grow indefinitely; once a cluster reaches its limiting size, free particles which occasionally bump into it never get bound. To understand this behavior, we measure the morphology and size distribution of these clusters, and describe the experimental data by a simple statistical lattice-model.