Research news

Controlling the surface topographies of polymer materials has been a key area of research in optical display industries, mechanical and photonic devices and microscale structures, to name a few. Photocontrolled polymer surfaces have been well researched on, mainly through the photochemical mass transport method. In this paper, the authors introduce a new method of inducing mass transport via photopolymerisation of liquid crystals using structured light. This method enables direct formation of canal structures and microscale wells on polymer film surfaces. It was found that wells and canals with depths and aspect ratios much different from the previously studied structures were present in the middle of the non-irradiated areas. Another result of the study was the 2D arrangement of well and canal structures by designing light patterns. The formation of canal structures also causes anisotropic molecular diffusion in the photopolymerisation process which causes a directed alignment of the molecules and consequently the formation of surface structures, all of which have great potential applications in optical and mechanical devices.

Traditionally, reconfigurable liquid crystal microwave devices that are externally electrically controlled are made using a liquid crystal sandwiched between two hard substrates with etched metal layers.The problem this poses is of short circuiting between the two electrodes in the bending state in the case of electrodes on flexible substrates.In this paper, the authors come up with a novel method that enables the production of a frequency tunable liquid crystal resonator fabricated on flexible substrates, whereby the resonator is made up of coplanar waveguide structures.Here, for the control of electric field, the liquid crystal sample is placed under the coplanar metals.In order to make the controlled electric field to be parallel to the electrode layers, the metal layers of the coplanar waveguide are placed on the same horizontal plane, and they do not touch each other when bending.The results of the experiment agreed with the theory and simulation, hence verifying the reliability of the resonator.Additionally, the results also revealed that when the flexible resonator is bending, frequency continuous electrically tunable function can be achieved by the applied electric field method.
Disclination lines play a crucial role in many physical processes and understanding and achieving control over them is an essential step in many key applications.In this work, the authors introduce a theoretical method of tailoring three-dimensional disclination architecture in nematic liquid crystals experimentally.It quantitatively predicts the shape and connectivity of disclination lines in nematic filled glass cells with a narrow cell gap and strong planar anchoring.An analogy is drawn between nematics and magnetostatics to reveal two important predictions: defects with the same topological charge but on opposite surfaces are connected by defect lines and regions with the highest twist attract disclination lines.These predictions are experimentally tested using polarised light to pattern the in-plane alignment of the liquid crystal molecules.Furthermore, it is also used to determine the essential parameters that tune the curvature of the disclination lines.Lastly, these predictions are also verified via computer simulations.It is found that non-dimensional parameters enable matching between experiments and simulations at different length scales.
Living liquid crystals, a mixture of active matter and nematic liquid crystals, is an interesting material with self-healing properties enabling it to have an array of potential applications particularly in targeted microcargo and information delivery.In this paper, the authors provide a phenomenological model that studies the system's symbiotic pattern dynamics using the Landau-de Gennes free energy in the case of liquid crystals and Toner-Tu model for active matter.Furthermore, they also introduce a coupling term that favours coalignment of the nematic and active components.In this work, the authors discover two new steady states, namely chimeras and solitons.The induced dynamics of the passive nematic mentioned in this paper are never worked on or discussed by anyone before.This paper showcases how the symbiotic dynamics of the liquid crystal and active matter can be exploited to induce order in a usually disordered system.
Flow patterns and defect dynamics of active nematic liquid crystals under an electric field Kinoshita, Yutaka, and Nariya Uchida, Flow patterns and defect dynamics of active nematic liquid crystals under an electric field.Physical Review E 108, no. 1 (2023): 014605.
In this work, numerical methods are employed to study the effects of an electric field on the defect dynamics and flow patterns of two-dimensional active nematic liquid crystals.It was found that anisotropic active turbulence, which is characterised by enhanced flow, that is perpendicular to the electric field, are caused by field induced director reorientation.
The anisotropy and average flow speed of the active turbulence was found to be maximum at an intermediate field strength.It is also found to have localised topological defects exhibiting typical dynamics such as spontaneous creation of defect pairs.At larger field strengths a laning state is observed, which is characterised by striped domains with alternating flow directions.The authors were able to generate periodic oscillations between active turbulence and laning state, similar to how active nematics are experimentally observed when subject to anisotropic friction.

Maryam Qaiser
Department of Physics and Astronomy, University of Manchester, Manchester, UK maryam.qaiser@postgrad.manchester.ac.uk