Fractal aggregates formed by ellipsoidal colloidal particles at the air/water interface

We study the aggregation of charged ellipsoidal colloidal particles at the air/water interface. The particles diffuse on the interface and aggregate over time to form fractal structures. We found that the directionality of the interactions depends on the aspect ratio of the particles. For the smaller aspect ratio, the interaction is stronger along the long axis of the particles, as we observe that the particles tend to aggregate side-by-side along this axis. As the aspect ratio increases, the particles arrange themselves mostly tip-to-tip. Using Monte Carlo simulations we built a simple orientation-dependent potential model for each aspect ratio, in such a way that the resulting structures reproduce the main features of the experimental systems. We also found that the larger aspect ratio ellipsoids require fewer particles to reach a constant fractal dimension. In addition, we measured the fractal dimension of clusters for different aspect ratios and found that it depends on the aspect ratio of the particles in the cluster, contrary to the idea that the fractal dimension is universal and therefore does not depend on the shape of the particles. Our results indicate that the fractal dimension decreases as the aspect ratio increases.