We present a novel approach that allows us to directly determine the force-distance law between tiny colloidal particles. Our technique exploits the fact that the anisotropy of the forces between dipoles causes particles to form chains. Because chains give rise to a strong Bragg diffraction of the visible light, the interdroplet spacing is accurately measured. Moreover, because the attractive dipolar magnetic force can be varied through the intensity of the external field, the balancing repulsive force can also be measured at various spacings. We give first evidence of the feasibility of this approach by using monodisperse magnetic emulsions under magnetic field.

• Received 6 December 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.2959