Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Movie of droplets

Two small water droplets in a nematic liquid crystal viewed through crossed polarizers in an optical microscope. The orientational elasticity of the liquid crystal results in an attractive interaction between the droplets, and this movie shows a simple scheme devised by Philippe Poulin, a former post doc in our group, to measure the nature of the attractive interaction.

The water droplets are filled with a ferrofluid, which is superparamagnetic. When a magnetic field is applied normal to the plane of observation, magnetic dipoles are induced in the droplets which repel, and force the droplets apart. When the field is removed, the attractive interaction pulls the droplets together. Since the motion is viscously damped, the attractive force is exactly balanced by the viscous drag. By measuring the velocity as a function of separation, we are able to determine the force-distance behavior, and show that it is dipolar in nature, as predicted for these structures in a nematic host. Read more about it.