Equilibrium clustering of colloidal particles at an oil/water interface due to competing long-range interactions

Hypothesis: Colloids at fluid interfaces organize according to inter-particle interactions. The main contributions to an effective interaction potential are expected to be electrostatic dipole-dipole repulsion and capillary attraction due to fluid interface deformation. When these interactions are weak, a secondary minimum in the particle pair interaction potential is expected. Experiments: Clean bare silica particles were deposited at an oil/water interface and their organization as well as dynamics were observed under a light microscope and analyzed in terms of radial distribution function and mean squared displacement. Findings: Weak long-range competing interactions between colloids at an oil/water interface result in cluster formation. The clusters have a liquid-like structure and grow with increasing particle packing fraction. System 'ergodicity' suggests near-equilibrium assembly, which is confirmed by free particle dynamics outside the clusters. The interplay between dipole-dipole repulsion and capillary attraction responsible for the cluster formation is reflected in a secondary minimum of the effective interaction potential predicted theoretically but inaccessible experimentally from collective particle properties prior to this work. (C) 2020 Elsevier Inc. All rights reserved.