Interactions of magnetic holes in ferrofluid layers

Abstract

Nonmagnetic microspheres in a ferrofluid layer interact as dipoles in an external magnetic field. With a constant field normal to the layer and an oscillating in-plane field, the spheres stabilize at fixed separations, and can thus be trapped at tunable finite distances. We show analytically how the susceptibility contrast at the system boundary is responsible for this secondary minimum of a pair interaction potential, obtain the effective interaction potential and equilibrium separation as a function of the applied field, and experimentally validate this theory.