Quadrupolar forces and aggregation of nematic droplets

The electrostatic quadrupolar interaction between spherical nematic droplet s in an isotropic (and nonconducting) liquid is calculated. It is found to have an anisotropic form U(q)proportional to 1/R-5, where R is the distance between droplets, with repulsion for droplets having parallel orientation of the quadrupole moments and attraction at oblique angles around the ortho gonal orientation. In an external magnetic field aligning the orientations of the quadrupole moments, a competition of the quadrupolar repulsion and v an der Waals attraction (U(vdW)proportional to 1/R-6) leads to a specific s patial organization of droplets which is in fact often reported in experime ntal observations (see the monograph by P. Drzaic, Liquid Crystal Dispersio ns, World Scientific, Singapore (1995) and references cited therein). (C) 1 998 American Institute of Physics. [S0021-3640(98)00622-7].