School of Physics

Home people people pubs pubs
seminars seminars announcements location wiki

Jayalakshmi Vallamkondu, Ekapop Pairam, Perry Ellis, Alberto Fernandez-Nieves

Reversible handedness in a nematic torus

Drops and bubbles are in spherical shape and surface tension gives them perfect spherical shape to minimize surface area for a given volume. However, to make non-spherical shapes we need to apply force to overcome surface tension. Pairam et al. [1] had developed a technique to produce toroidal droplets in a viscous outer medium but, the torus is unstable eventually collapses to a droplet which is a stable configuration. However, by replacing yield stress material as a the outer medium we successfully made first stable nematic toroidal droplets [2].

Remarkably, these nematic droplets have a double twist configuration. To quantify the twist we measured the twist angle in our nematic toroidal droplets. The method we used to measure twist angle was very well described in our recent article [2]. The method relies on the fact that linearly polarized light follows the twist of a nematic liquid crystal if the polarization direction is either parallel or perpendicular to the nematic director at the entrance of a sample. We then image the torus from above keep the polarizer parallel and then rotate analyzer an angle φexit with respect to the polarizer while monitoring the transmitted intensity. The minimum in T tells us the lack of light propagation through the analyzer suggesting that the incident polarization direction has rotated by an amount τ such that it is perpendicular to the analyzer after exiting the torus. The image of the torus at minimum T exhibits four dark regions where extinction occurs and the sense of rotation of analyzer tells the nematic arrangment.


Using this technique we measured the twist angle of a torus (Figure 1 shows the bright field and cross polarized image of a torus). The torus has right handed arrangement right after torus was made as shown in Figure 2. Interestingly, after 20 days the same torus is having a left handed arrangement as shown in Figure 3. In general, handedness inversion occurs when the helix of one handedness unwinds disappears at a certain point and then forms a helix of opposite handedness. In Figure 4, you can see the free energy as a function of variational parameter ω, for two different (K2-K24)/K3 values. For (K2-K24)/K3=0.02 there is only one energy minimum at ω=0 which corresponds to axial texture. On the other hand for (K2-K24)/K3=0.01, there are two minima of equal energy corresponding to the two possible handednesses however, both are separated by a energy barrier. We are trying to understand the mechanism behind the inversion of handedness and also doing kinetics to explore whether inversion can be accelerated by taking to the isotropic temperature of nematic liquid crystal. Figure 5 shows the schematic representation of left and right handed arrangement of nematic liquid crystal.




1. E. Pairam and A. Fernandez-Nieves, Phys. Rev. Lett., 102, 234501 (2009).

2. E. Pairam, J. Vallamkondu, V. Koning, B. C. van Zuiden, P. W. Ellis, M. A. Bates, V. Vitelli, and A. Fernandez-Nieves, PNAS, 110, 9295 (2013).