Direct measurement of force between colloidal particles in a nematic liquid crystal

The interparticle force between two colloidal particles in a nematic liquid crystal is directly measured as a function of the interparticle distance R by two different experimental methods: the free-release method and the optical tweezing method. The obtained force between an elastic 'dipole', which constitutes a colloidal particle and an accompanying hyperbolic hedgehog defect, confirms previous theoretical predictions that the force is attractive and proportional to R^-4. We also observe that a repulsive component emerges at short distances to preclude direct contact of the particles. We find that the magnitudes of the forces obtained by the two methods are different. The origin of this discrepancy is discussed by a comparison between the static and the non-static measurements.