Colloids in inhomogeneous external magnetic fields: particle tweezing, trapping and void formation

Two-dimensional super-paramagnetic suspensions that are confined to a planar liquid–gas interface and exposed to an inhomogeneous external magnetic field directed perpendicular to the interface are studied by extensive Monte Carlo computer simulations. The external field is a superposition of a homogeneous field and a localized inhomogeneity, modelled by a Gaussian function. The inhomogeneity causes two combined effects that compete against each other: it provides an external potential, modifying at the same time the mutual interparticle repulsion. If the inhomogeneity enhances the strength of the homogeneous profile, the inhomogeneous field is a ‘magnetic tweezer’ for low particle densities. At higher densities, on the other hand, there is a small accumulation in the centre of the inhomogeneous field, which leads to a depletion zone outside the inhomogeneity due to the mutual interparticle repulsion. Very large inhomogeneities produce local crystallites surrounded by a depletion ring. If the inhomogeneity reduces the total field strength, particles are repelled from the inhomogeneity and voids are generated in the suspension. Our predictions are of relevance to the direct transport of magnetic particles and can be verified in real-space experiments of super-paramagnetic suspensions.