Self-assembly of Rod-like particles into 2D lattices

It was recently shown by us that spherical particles floating on a fluid-fluid interface can be self-assembled, and the lattice between them can be controlled, using an electric field. In this paper we show that the technique can also be used to self assemble rod-like particles on fluid-fluid interfaces. The method consists of sprinkling particles at a liquid interface and applying an electric field normal to the interface, thus resulting in a combination of hydrodynamic (capillary) and electrostatic forces acting on the particles. A rod floating on the fluid interface experiences both a lateral force and a torque normal to the interface due to capillarity, and in the presence of an electric field, it is also subjected to an electric force and torque. The electric force affects the rods' approach velocity and the torque aligns the rods parallel to each other. In the absence of an electric field, two rods that are initially more than one rod length away from each other come in contact so that they are either perpendicular or parallel to the line joining their centers, depending on their initial orientations. In the latter case, their ends are touching. Our experiments show that in an electric field of sufficiently large strength, only the latter arrangement is stable. Experiments also show that in this case the electric field causes the rods of the monolayer to align parallel to one another and that the lattice spacing of a self-assembled monolayer of rods increases.