M. Murali, H. Agha, G. Scalia
University of Luxembourg,
Luxembourg
Keywords: liquid crystals, carbon nanotubes, resistance, disclinations, pressure sensing
Summary:
Liquid crystals are anisotropic fluids that can be easily orientationally ordered over macroscopic scale. As in solids, defects can be found also in liquid crystals. Defect lines in the nematic phase, the phase with only orientational order, are called disclination lines, corresponding to places where the orientational order of the nematic phase is broken [1] can be used for trapping single or even multiple particles. The high response of liquid crystal to external stimuli and specific properties of nanoparticles like the high electrical conductivity, can be used for realizing unconventional responsive systems. The confinement into defects can be used to promote the formation of networks since the local particle density increases due to the confinement into the restricted space. Carbon nanotubes (CNTs) are interesting particles for forming conductive networks since they are highly conductive and they network at very low concentrations due to their extremely high aspect ratio. Defects in the liquid crystal can be tailor generated in chosen locations creating conflicting anchoring conditions by three successive rubbing paths along different directions on polymer coating the glass substrate. In our work, the rubbing is obtained by unidirectionally smearing the polytetrafluoroethylene (PTFE) bar onto the glass substrate. Multiwall carbon nanotubes (CNTs) were dispersed using dichloromethane and then later incorporated into pentyl cyanobiphenyl (5CB), a thermotropic liquid crystal that exhibits a room temperature nematic phase. In the presence of a splay-bend disclination line, the CNTs are first attracted towards it and are ultimately trapped in it [2]. The disclination lines work as templates and when a sufficient number of nanotubes are trapped onto the line, they form a vertical interconnect. The network can be easily modified by the flow of liquid crystal generated by e. g. pressure and be restored when the external action is stopped. Such a 3D nanowiring could have potential applications as it allows the spatial control of the interconnects, even at relatively low spatial density, which can be used to monitor the variations in the electrical properties in response to an applied local pressure. Thus, new devices can be realized with this combination as a pressure sensor. References: [1] H. Agha, J.B. Fleury, & Y. Galerne, Micro-wires self-assembled and 3D-connected with the help of a nematic liquid crystal, Eur. Phys. J. E (2012) 35: 82 [2] H. Agha & Y. Galerne, Interactions of carbon nanotubes in a nematic liquid crystal. II Experiment. Phys Rev E. 2016 Apr; 93:042703