Interactions and Pattern Formation in a Macroscopic Magnetocapillary SALR System

When particles are deposited at a fluid interface they tend to aggregate due to capillary attraction, in an effort to minimize the overall potential energy of the system. In this work, we embed floating millimetric disks with permanent magnets to introduce a competing repulsion effect. The pairwise energy landscape of two disks is described by a Short-range Attraction and Long-range Repulsion (SALR) interaction potential, previously documented in a number of microscopic systems. The simple interaction model predicts a variety of equilibrium states, including the possibility of a local minimum energy corresponding to a finite disk spacing as observed in experiment. 2D experiments and simulations in confined geometries show that as the area packing fraction is increased, the uniform lattice state becomes unstable and localized clusters spontaneously form which eventually merge into a striped pattern. Finally, we show that the pattern can be dynamically switched via application of an external magnetic field.