Electric field induced alignment and self-assembly of rods on fluid-fluid interfaces

In this paper we show that an external electric field normal to a fluid-fluid interface can align rods floating on the interface as well as adjust the lattice spacing of a monolayer of rods. The method consists of sprinkling rods onto a fluid interface, with the fluids contained in a chamber subjected to a vertical electric field. The rods are attracted to one another and cluster under the action of capillary forces. 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. While the electric force affects the approach velocity of the rods' 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 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. The alignment, however, is not complete since the rods physically block one another and the direction of the alignment is arbitrary in the sense that it only depends on the initial orientations of the rods.