Application of small-angle neutron scattering to the study of forces between magnetically chained monodisperse ferrofluid emulsion droplets

The optical magnetic chaining technique (MCT) developed by Leal-Calderon, Stora, Mondain-Monval, Poulin & Bibette [Phys. Rev. Lett. (1994), 72, 2959-2962] allows precise measurements of force profiles between droplets in monodisperse ferrofluid emulsions. However, the method lacks an in situ determination of droplet size and, therefore, requires a combination of separately acquired measurements of droplet chain periodicity versus an applied magnetic field from optical Bragg scattering and droplet diameter inferred from dynamic light scattering (DLS) to recover surface force-distance profiles between the colloidal particles. Compound refractive lens (CRL) focused small-angle neutron scattering (SANS) MCT should result in more consistent measurements of droplet size (form factor measurements in the absence of field) and droplet chaining period (from structure factor peaks when the magnetic field is applied), and, with access to shorter length scales, extend force measurements to closer approaches than possible by optical measurements. This article reports on CRL-SANS measurements of monodisperse ferrofluid emulsion droplets aligned in straight chains by an applied field perpendicular to the incident beam direction. Analysis of the scattering from the closely spaced droplets required algorithms that carefully treated resolution and its effect on mean scattering vector magnitudes in order to determine droplet size and chain periods to sufficient accuracy. At lower applied fields, scattering patterns indicate structural correlations transverse to the magnetic field direction owing to the formation of intermediate structures in early chain growth.