Light regulated molecular diffusion in a liquid crystal network, A. Cao et al., Soft Matter, 15, 4737 (2019).
Oscillating dynamics of cis-trans isomerisation of the azobenzene can boost free volume generation in liquid crystal polymer networks. As such excess volume can influence the guest molecules’ motion by influencing the space around them, the isomerisation process could be used for applications involving molecular transport. The paper by Cao et al., reports the investigation of a photomechanical effect in a polymer network for molecular transport purposes. The transport of 5CB liquid crystal through a chiral nematic polymer network consisting of azobenzene units is investigated. ~28% increase in diffusion is observed from light OFF to light ON state. The influence of polarisation of the incident light on the diffusion process is studied.
Investigation of the heliconical smectic SmCSPhelF phase in achiral bent-core mesogens derived from 4-cyanoresorcinol, J. K. Vij et al., Phys. Rev. Mater. 3, 045603 (2019).
Bent core liquid crystals with 4-cyanoresorcinol core and two terephthalate wings exhibit a number of interesting polar smectic liquid crystal phases. This paper reports a new phase that has a synclinic tilted polar smectic arrangement with a helical axis running parallel to the layer normal (SmCSPhelF) in a 4-cyanoresorcinol liquid crystal. The pitch of the helix is calculated to be ~15 nm, which is interestingly comparable to the helical pitch observed in twist-bend nematic (NTB) phase. The new phase is observed on cooling from a random polar synclinic smectic C phase (SmCSPR). Experimental evidences from electro-optics, conoscopy and AFM are presented, which prove the presence of helical structure in the SmCSPhelF phase.
Colloids in confined liquid crystals: a plot twist in the lock-and-key mechanism, G. Boniello et al., Soft Matter, 15, 5220 (2019).
Liquid crystals offer a great medium to address the dynamics of colloidal particles and localise single particle at specific sites. This paper investigates the influence of an undulating structure/wavy wall on the confinement of spherical particles. In such structures, the waviness of the wall introduces sites of various splay and bend distortion, which could potentially couple with the complementary distortions around the particles (docking). Boniello et al., introduce a twist distortion to such particle and wavy wall set up by controlling the anchoring directions at the boundaries and by using chiral nematic liquid crystals. They show that by tuning the pitch of the chiral nematics, it is possible to selectively promote or inhibit particle trapping at the docking sites. Simulation of homeotropic particles close to the wavy wall is carried out and compared with experimental results.
Direct measurement of structural forces and twist transitions in cholesteric liquid crystal films with a surface force apparatus, W. Zheng et al., Soft Matter 15, 4905, 2019.
This paper introduces a surface force apparatus (SFA) as a tool to study the structural and mechanical response of liquid crystal films. The main advantages of this technique over atomic force microscope are that SFA allows active control over the film thickness and can be used to induce structural transition in the liquid crystal film while simultaneously measuring structural forces acting on the confining forces. The apparatus is claimed to give precise measurement of film thickness, surface curvature and anchoring transitions. Experimental results of a chrial nematic film confined between two cylindrical glass lenses covered with mica sheets are provided, where an azimuthal strength of planar anchoring on mica was found to be significantly higher than values measured on polymers and other aligning surfaces. The measurement of structural forces are compared with that of values obtained from theoretical models.
Fast and giant photo-rheological effect in a liquid crystal dimer, S. Aya et al., Adv. Mater. Interfaces, 1802032, 2019.
Materials which exhibit fast and reversible switching of viscoelastic properties in a high contrast ratio are interesting as mechanically switchable systems. The paper by Aya et al., presents the viscoelastic properties of a photo-switchable azo-benzene based liquid crystal dimer, which exhibits a twist bend nematic phase. The material shows a contrast in viscoelastic parameters of up to six orders of magnitude in the crystal phase and a contrast of one order is observed between the nematic and twist bend nematic phases. The twist-bend nematic and nematic phase transitions are driven by UV-vis irradiation leading to transition between twisted trans and the double-kinked cis conformations. As expected the nematic phase is found behaves like a Newtonian fluid, the viscosity is independent of shear stress whereas a strong shear thinning is observed in the twist-bend nematic phase. A detailed study of the dynamics and repeatability of the photo-switching processes in combination with structural studies from X-ray diffraction experiments are given.