Darren R. Link, Alberto Fernandez de las Nieves, David A. Weitz

Monodisperse emulsions of nematic liquid crystal (5CB) in a continuous phase of water are prepared by drop break-off in a rotating bath. The water in this case has 1% PVA added to give planar boundary conditions on the nematic director. The minimum free energy director configuration within the droplets in this case is bipolar, having two disclinations located opposite one another on the drops surface. A disclination at an interface may also be called a boojum. The 30 microns liquid crystal drops are then injected into microfluidic channels that are only 10 microns in height and 100 microns in width to create a pancake shaped objects.

In the absence of flow, the pancake shaped drops have an equilibrium director configuration with the two disclinations located opposite one another in a plane parallel to the confining surfaces. In a flow field, however, the disclinations move forward along the edge of the drops, come together to make a strength +2 disclination. The +2 disclination then breaks into two strength +1 disclinations that move back along the top and bottom of the drop to recombine into a +2 disclination at the back of the drop. They then split into two again and move up the sides of the drops to repeat the cycle. Schematics from the first half of the cycle are shown below.

One of the surprising things about this topological evolution of the director field, is that the radial splay type +1 disclinations that are initially on the sides of the drops are converted into bend type disclinations that are connected by a disclination line when they move back across the top of the drop. From this bend state, there is no direct route for relaxation to the minimum free energy structure and hence the state is metastable. However, if the drop is heated into the isotropic phase, on cooling is will adapt the minimum free energy configuration of a bipolar drop.