Nucleation of weakly attractive aggregates in microgravity

We investigate the incipient stages of the formation of aggregates of weakly attractive particles in microgravity. Using colloidal particles interacting with tunable critical Casimir forces, we drive the system increasingly out of equilibrium starting from near-equilibrium gas-solid coexistence, and follow the early stages of aggregation with near-field scattering. Contrary to measurements on ground that show a rapid exponential growth of aggregates, our microgravity measurements indicate that under pure diffusion-limited conditions, the formation of aggregates is still governed by a nucleation process over the investigated range of moderate attractive strength of a few k_BT. By studying in detail the decrease of the critical radius with the increase of the attractive strength, we determine a surface tension of 115 nN/m, or 4.6k_BT, with σ the particle diameter, of the order of but larger than that for equilibrium colloidal liquids and crystals. These results highlight the importance of nucleation in the formation of non-equilibrium solid aggregates bound by weakly attractive forces.