We studied cylindrical microparticles at fluid interfaces, which orient and assemble with high reliability owing to anisotropic capillary interactions. An isolated cylinder has two possible equilibrium states: end-on (the cylinder axis is perpendicular to the interface) or side-on (the cylinder axis is parallel to the interface). Cylinders in the side-on state create distortions of the interface, and overlapping deformations by neighboring particles drive oriented capillary assembly. The interfacial deformations are measured with interferometry and compared with numerical simulations, to corroborate the hypothesis that it is solely the wetting condition at the particle to determine the interfacial deformation. To model the interactions between cylinders, an anisotropic potential is derived based on elliptical quadrupoles, which predicts an attractive force and a torque, both of which depend strongly on aspect ratio, in keeping with experiment. Particle trajectories and angular orientations recorded by video microscopy agree with the predicted potential. To understand interactions near contact, the concentrated excess area near the cylinder ends is quantified and its role in creating stable end-to-end assemblies is discussed. When a pair of cylinders is near contact, these high excess area regions overlap to form a capillary bridge between the particles.

E.P. Lewandowski, M. Cavallaro, Jr., L. Botto, J.C. Bernate, V. Garbin and K.J. Stebe, Langmuir (2010)