The pair interactions of polystyrene particles were measured at the decane-water interface using laser tweezers. In the absence of sodium dodecylsulfate (SDS) in the aqueous subphase, the particle interactions show a long-range repulsion, consistent with models proposed by Hurd [1] and Aveyard et al. [2, 3]. As SDS is added to the sub-phase, the repulsive force between particles decreases. Surprisingly, we find that the repulsive interactions also decrease with time at fixed surfactant concentrations. This provides a possible mechanism for the reported flocculation behavior of 2D suspensions [4]. Finally, as the repulsion decreases, a long-range attractive force emerges, creating a secondary minimum in the potential. The attractive interaction is consistent with attractive capillary forces between particles. Overall, our experiments demonstrate the richness of particle interactions at the oil-water interface, which can be tailored to alter the stability and interfacial rheology of 2D systems.

[1] A. J. Hurd, J. Phys. A: Math. Gen. 1985, 18, L1055. [2] R. Aveyard et al. Langmuir, 2000, 16, 1969. [3] R. Aveyard et al. Phys. Rev. Lett. 2002, 88, 246102-1. [4] S. Reynaert et al. Langmuir, 2006, 22, 4936.