Rainbows in a vial: controlled assembly of 2D colloids in two perpendicular external fields

The ability to control the dynamics of self-assembly of an isotropic nanoparticles over multiple length scales by application of external fields is an important technological and fundamental challenge in colloidal science and soft condensed matter physics. Here we demonstrate the controlled assembly of two-dimensional (2D) colloidal liquid crystals under the influence of gravity and an external magnetic field. Due to the positive anisotropy of diamagnetic susceptibility (Delta chi > 0), the colloidal nanoplates of monolayer zirconium phosphate could exhibit intriguing alignment with their normals parallel to the magnetic field, thus transforming the multi-domain nematic phase into a mono-domain superstructure. Interestingly, the quasi-equilibrium states of such 2D soft materials under the prolonged gravity-induced sedimentation were found to generate a rich pattern of interference color bands (referred to as rainbows in a vial) upon under the effect of an aligning magnetic field. The birefringence measurement of the resulting color bands suggested quadratically decreasing volume fraction with sample height. The osmotic pressure (Pi) as a function of nanoplate volume fraction (phi) is found to agree with the theoretical prediction of the equation of state of the nematic phase of hard nanoplates, with a positive deviation at higher volume fractions. To the best of our knowledge, such field-driven dynamic formation of rich interference color bands, imitating almost the full spectrum of Michel-Levy interference bands, which was a pure mathematical representation, is experimentally achieved for the first time in a single sample.