Micromechanics of colloidal aggregates at the oil–water interface

Abstract

The micromechanics of two-dimensional (2D) colloidal aggregates at the oil–water interface are measured using optical tweezers. Aggregates form from stable 2D suspensions after introducing either 0.25 M NaCl/0.1 mM SDS in the aqueous sub-phase or 25 μM sorbitan monooleate (SPAN 80) in the oil super-phase. Aggregates formed with NaCl/SDS have strong bond bending rigidities due to tangential forces between particles, leading to an average aggregate rigidity κ_a = 4.9 ± 3.1 mN m⁻¹. Rigid aggregates are consistent with previously reported open microstructures and irreversible, diffusion-limited cluster aggregation kinetics. In contrast, aggregates formed by SPAN 80 exhibit weak bond rigidities (κ_a = 0.28 ± 0.31 mN m⁻¹), enabling particle rearrangements that lead to a denser microstructure. The micromechanical properties of aggregates that constitute the macrocolloidal structure of 2D suspensions provide a critical link between their colloidal interactions and interfacial rheology.