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Exciting new insight in soft matter physics

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Publish Date: 18.02.2022

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 Ionically charged topological defects in nematic fluids

The ability to spatially control the electric charge is important in a range of fields, from charged polymers, biological and active substances, to colloidal materials, complex liquids and microelectronics. Many of these materials, called nematic liquids, are characterized by orientational ordering of their building blocks, such as molecules or elongated / disk-like particles (green lines in the figure below) which preferentially align along a certain direction. However, areas can also appear in the form of lines, points, loops (large green loop in the picture below), called topological defects, where the building blocks can not orientationally order and remain disordered. 

 FMF Ravnik

Figure: Ion-charged topological defects in nematic fluids. The topological defect is shown by a green loop, short green bars indicate the orientational arrangement of nematic building blocks around the defect, and red pluses and blue minuses indicate electrically charged particles (ions) interacting with the nematic defect loop. Figure author: Jeffrey C. Everts, Source: Physical Review X (2021)  

Dr. Jeffrey C. Everts, Marie Skłodowska-Curie Fellow at the Faculty of Mathematics and Physics, University of Ljubljana and Assoc. prof. dr. Miha Ravnik from UL FMF and JSI showed by using and performing computer-intensive calculations that topological defects in nematic electrolytes can act as areas for local separation of electric charge. Specifically, the defects get filled with electric charges, electrically charged defect cores are formed and electric multi-layers can emerge, which is a generalization of electrical double layers known in isotropic electrolytes. In particular, the authors showed that ions couple very efficiently with defect cores via ionic solubility, and with the surrounding orientation field through the flexoelectric mechanism. The study was presented in three related scientific publications (Physical Review X (2021), Physical Review Letters (2020) and Science Advances (2021)), with the third publication providing a theoretical explanation of experiments performed at the University of Colorado Boulder (USA). This research achievement contributes to the understanding of electrostatic mechanisms in topological soft matter, it is the first step towards understanding similar phenomena in biological systems, and also can lead to the development of new soft microelectronic or related photonic elements. The content is recognized by the Slovenian Research Agency (ARRS) as one of the most prominent research achievements in the field of natural sciences and technics "Odlični v znanosti 2021".

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