Research Projects

Twist-Bend nematic (NTB) is a new class of nematic (N) liquid crystal (LC) that is theoretically predicted in bent core molecules by Dozov [1] and experimentally reported for the first time by Cestari et al [2] in 2011. The uniaxial NLC phase is made up of typically rod-like molecules that spontaneously acquires an orientational order along its long molecular axis and this direction is represented by a special vector called director n; whereas the NTB phase is formed by long bent molecules that spontaneously self-assemble in a heli-conical structure of nanoscale periodicity as shown in Fig 2 in [3]. This new discovery revived the interest of the researchers working on both basic and applied aspects of LCs. While liquid crystal chemist have since been focusing on designing new materials keeping molecular structural properties as guiding principle that could potentially exhibit the NTB phase [4, 5], physicists have been trying to understand physical principles behind the stability of this phase and its signatures [6-8]. Two important salient features of the NTB phase are a) the bend elastic constant (K33) should decrease as the phase transition occurs from N -NTB phase, and in the NTB phase K33 should approach to negative value (to be precise, bend coefficient proportional to K33) [1], b) there should be an additional contribution to the total flexoelectric polarization due to the coupling between the director n and helical axis direction t [6] and additional contribution to the flexo polarization due to the gradient of the heliconical angle induced by a transverse to the helical axis electric field [7]. The bend elastic constant (K33) and flexo-polarization could be measured by subjecting the NTB phase to an external electric field in different cell geometry, and there have been some experimental reports [9- 14] but not sufficient to conclude anything. We propose the following methods to investigate the electric field effects in a NTB phase. 1. Applying the electric field parallel to the helical axis direction and checking the coupling of n and t. The magnetic field analogous has been reported [15], but to the best of our knowledge no electric field study in this configuration [E parallel to t] has been reported. We expect the coupling of the director to the applied electric field and thereby shift in the N- NTB phase transition temperature and hope to estimate the splay elastic constant. 2. A systematic measurement of K33 (by Freedericksz method as explained in proposer’s paper [16, 17]) close to N- NTB phase transition temperature in different LC compounds in order to arrive at a general pattern of the bend elastic constant behaviour in and near NTB phase. 3. Measuring effective flexo-coefficient by electro-optical method in an in-plane field cell as explained in the proposer’s paper [18]. The measurement detail is elaborated in the technical document. • References are listed in the attached technical document.