Arrangement dependence of interparticle force in nematic colloids

We have experimentally and theoretically studied the interparticle force between two colloidal particles with different sizes accompanied by hyperbolic hedgehog defects in a nematic liquid crystal. The force f was directly measured using dual-beam optical tweezers and calculated theoretically from the equilibrium tensor field around the particles. The dependence of f on the center-to-center distance between particles of different sizes R is different from that for particles with the same size. The magnitude of f depends on the relative arrangement of the particles: f is larger when a defect between the particles belongs to the larger particle. From the theoretical calculation, the difference in f between the two arrangements, delta f, monotonically increases with increasing size difference. The difference delta f was experimentally and theoretically found to be proportional to R-4.6 at large R. The obtained exponent is comparable to the exponent of -5 predicted by electrostatic analogy.