Colloidal Aggregation in Microgravity by Critical Casimir Forces

By using the critical Casimir force, we study the attractive strength dependent aggregation of colloids with and without gravity by means of near field scattering. Significant differences were seen between microgravity and ground experiments, both in the structure of the formed fractal aggregates as well as in the kinetics of growth. In microgravity purely diffusive aggregation is observed. By using the continuously variable particle interaction potential we can for the first time experimentally relate the strength of attraction between the particles and the structure of the aggregates.

Figure 1

Typical light scattering result of the aggregation of colloids in microgravity. Different curves represent different points in time during the aggregation process. Inset: Normalized variance of the scattering intensity (NVI) in time (sample: $0.31\mathrm{mmol}/\mathrm{L}$ NaCl at $T_{\mathrm{agg}}$).

Figure 2

Collapse of scattering curves in microgravity. Rescaled scattering intensity as a function of rescaled wave vector $q$ (sample: $0.31\mathrm{mmol}/\mathrm{L}$ NaCl). Lines are drawn as a guide to the eye to demonstrate the decrease at low $q$.

Figure 3

Growth of structures in time at different temperatures. Results from both microgravity (closed symbols) and ground experiments (open symbols) are shown (sample: $1.5\mathrm{mmol}/\mathrm{L}$ NaCl). Inset: Enlargement of the start of the aggregation process.