Colloidal rings in a liquid mixture

We investigate the self-assembly of colloidal particles on microscopic decane droplets in water and show that, by use of paramagnetic colloids, it is possible to assemble ringlike structures that can be controlled with a magnetic field. Moreover, the use of paramagnetic colloids allows us to determine the attractive forces between the colloids located at the three-phase contact line between decane, water, and air. The attractive force is in the femtonewton range and is attributed to capillary interactions due to interface deformations. When the liquid emulsion dries on a glass slide, we observe solid deposits in the form of microscopic rings of varying diameters.