Near field capillary repulsion

Anisotropic microparticles adsorbed at fluid-fluid interfaces create interface deformations and interact because of capillarity. Thus far, much of the work related to this phenomenon has focused on capillary attraction, which is ubiquitous in the far field for microparticles at interfaces. In this paper, we explore capillary repulsion. We study particles at interfaces with contact line undulations having wavelength significantly smaller than the characteristic particle size. By a combination of simulation and experiment, we show that identical microparticles with features in phase attract each other, and microparticles with different wavelengths, under certain conditions, repel each other in the near field, leading to a measurable equilibrium separation. We study these assemblies at air-water and oil-water interfaces. The capillary bond between particles at air-water interfaces is rigid, whereas at oil-water interfaces, the bond between particles with near field repulsion is elastic under perturbation. These results have implications for the capillary assembly of rough microparticles at interfaces, and for the tailoring of mechanics of particle monolayers.