Rod-like microparticles at interfaces: near-field capillary interactions and implications for mechanics of particle-laden interfaces

Rod-like particles assemble at fluid interfaces owing to anisotropic capillary interactions. Experiments on cylindrical and ellipsoidal particles reveal that preferred orientation, interaction strength, and properties of resulting assemblies depend strongly on particle shape. While cylinders assemble end-to-end forming rigid linear chains, ellipsoids form flexible structures with particles side-by-side. Simulations of pairs of particles at contact reveal that chains of cylinders are rigid. Experiments bear this out: a chain of microcylinders rotated via a magnetic field remains rigid even under torques in excess of $10^5kT$. Above a yield stress, chains snap and dissipate stored capillary energy. Interactions between ellipsoids are comparatively weak, with no energy barrier as particle rotate about each other, consistent with the highly flexible chains formed by these particles. These interactions have profound implications for the mechanics of particle-laden interfaces including their viscoelastic properties.