Planar magnetic colloidal crystals

Abstract

This paper reports the unexpected discovery that in a certain parameter range of monodispersed magnetic particles, two-dimensional (2D) crystals can be formed on a fluid surface with not just the (hexagonal) triangular lattice, but also with all the other planar crystal symmetries such as the oblique, centered-rectangular, rectangular, and square lattices. These lattice structures, some of which are metastable, can be reversibly tuned by adjusting the polar and azimuthal angles of the magnetic field relative to the surface normal and the symmetry direction of the 2D lattices. Furthermore, by using two different sized magnetic particles, local formations of 2D quasicrystallites with fivefold symmetry were observed. Theoretical predictions based on energy considerations are shown to be in good agreement with the experiments.