Preface

Since 2014, Russian Federation under the authoritarian regime of President Putin conducts a war against Ukraine, stepping it up to broad-scale military invasion on February 24, 2022. Scientific communities are among the most vulnerable during war times. Only between February 24 and July 2022, about 22,000, or one-quarter of the country’s scientists, mostly women and men older than 60, left the country (1). In a recent publication, Richard Stone describes the extensive damage to academic centres in the Kharkiv region, inflicted by Russian bombardment and by Russian soldiers looting during a brief occupation of the city outskirts (2). Among these are the Kharkiv Institute of Physics and Technology, whose list of alumni includes Lev Landau, Igor Kurchatov, Boris Podolsky, Lev Shubnikov, Victor Weisskopf (3), and the Institute for Single Crystals (ISC) of the National Academy of Sciences of Ukraine (NASU), one of the leading centres in the studies of advanced materials, including liquid crystals. Crystals and plastic scintillators produced at the Institute were used at the Large Hadron Collider in Switzerland and in the Bell experiment in Japan, helping in the discoveries of the Higgs boson and asymmetries between matter and antimatter (2). Incredibly, war does not stop research and educational activities in Ukraine. International help arrives in different forms; one of the most important and immediate measures was that numerous academic institutions around the world, most notably in Poland, opened their doors to Ukrainian scientists who had to leave the country in 2022.

Taylor & Francis initiated a special issue of its Liquid Crystals journal to recognize and celebrate the impact made by Ukrainian scientists on liquid crystal science and technology. The first documented explorations of liquid crystals could be traced to Julius Planer, a professor of anatomy at the University of Lemberg; nowadays known as Lviv National University. In 1861, Planer published the first paper on a material later identified as a liquid crystal. Planer synthesised cholesterol chloride, a derivative of cholesterol, and noted that “during the cooling process, the molten substance exhibits a lively violet color in incident light, a yellow-green color in transmitted light” (4) (cited from the English translation (5)). The description fits nicely the modern understanding of a cholesteric liquid crystal and its ability to selectively reflect light in a certain spectral range because of the periodic helicoidal structure of the molecular orientation. In 2010, Longin Lisetski, a scientist at the ISC, prepared planar cells of cholesteryl chloride and explored their spectroscopic properties to confirm this interpretation of structural colors observed by Planer (6). Figure 1 shows the entry to the building at 4 Hrushevskyy Street, where Planer conducted his experiments. Today, the building serves as a home to the departments of biology and geology of Lviv National University. Its façade is decorated with multiple memorial plaques commemorating prominent scientists that used to work here. Among these are Mykhailo Hrushevsky, a historian and politicians, who led the Ukrainian national revival in 1910-ies, and served as the head of Centralna Rada, the Ukraine’s 1917-1918 parliament. The creator of the first effective vaccine against epidemic typhus, Rudolf S. Weigl and the founder of modern functional analysis, Stefan Banach, also worked in this building; Ivan Franko, a prominent Ukrainian writer and political activist, spent his student years here.

Figure 1. (Colour online) Entry to the building where Julius Planer worked. Photo courtesy of Oleksandr Kyrychuk and Yuriy A. Nastishin; February 14, 2023.

The modern chapter of liquid crystal research in Ukraine started in 1960-ies: Z.M. Mukutyuk at Lviv University reconnected with the century-old exploits of Julius Planer by synthesizing a number of cholesterol derivatives. Lviv became one of the important centres in liquid crystal research, thanks to Ya. Yo. Dutchak, O. Dovhyi, T.H. Dudok, Z.Yu. Gotra, M. F. Holovko, Yu. A. Nastishin, S. Yu. Nastishin, J. Ilnytskii, I. Smalyukh, T.G. Sokolovska, R.O. Sokolovskii, A. Trokhymchuk, R.O. Vlokh. Other centres emerged, among which the Institute of Physics of the Ukrainian Academy of Sciences has risen as a lead, with researchers (D. Andrienko, N. S. Aryasova, O.P. Boiko, V. Bodnar, O. Brodin, O. Buchnev, S. Chernyshuk, A.G. Dyadyusha, Yu. Garbovskiy, V. Y. Gaivoronsky, A. Glushchenko, N.M. Golovataya, O.K. Frolova, I.P. Ilchishin, A.I. Khizhnyak, G.V. Klimusheva, A.V. Kovalchuk, M.V. Kukhtarev, M.V. Kurik, Yu. Kurioz, O.D. Lavrentovich, B.I. Lev, T. Y. Marusii, V. G. Nazarenko, A.B. Nych, S.G. Odulov, U. Ognysta, V.M. Pergamenshchik, A.P. Polishchuk, Yu. A. Reznikov, S.S. Rozhkov, O.G. Sarbei, V. Sergan, M.T. Shpak, M.S. Soskin, E.O. Tikhonov, A. Tolochko, P.M. Tomchuk, O. M. Tovkach, R.M. Vasyuta, D. Voloshchenko, O. Uskova, V.A. Uzunova, O.V. Yaroshchuk, L.I. Zagainova) exploring different facets of physics, optics, and materials science. Among the notable achievements of the Institute are the discoveries of lasing in cholesterics doped with luminescent dyes (7) and of photoalignment of liquid crystals at polymer substrates (8).

Researchers at the Institute of Physics of Semiconductors in Kyiv (V.M. Sorokin, R. Ya. Zelinsky, Yu. V. Kolomzarov, V.G. Nazarenko) and ISC in Kharkiv (L.A. Kutulya, L.M. Lisetski, V.G. Tyshchenko, A. Tolmachev) developed cholesteric liquid crystal materials and devices based on them, such as bistable displays. Theorists at the Institute of Nuclear Research (O. Kuksenok, S.V. Shiyanovskii, V.I. Sugakov) and Taras Shevchenko Kyiv State University (I. Pinkevich, V. Yu. Reshetnyak) proposed models of phase transitions, confined liquid crystals, optics of photorefractive cells, photonic and plasmonic effects in liquid crystals. Scientists at the Institute of Organic Chemistry (N.S. Pivovarova, S.V. Shelyazhenko, Yu. A. Fialkov) are credited with the synthesis of early fluorinated mesogens, while the extensive studies of polymer liquid crystals at the Institute of Macromolecular Chemistry (Yu. Lipatov, V.V. Shilov, V.V. Tsukruk) resulted in a detailed monograph on the subject (9). Liquid crystal science also flourished in Chernihiv, Dnipro, and Odesa. Kyiv plant “RIAP” in 1970-ies established mass production of cyanobiphenyls, while the industrial conglomerate “Zhovten’” in cooperation with the research institute “Helium” in Vinnytsya produced liquid crystal indicators. These and other achievements were described in a recent paper by Vasyl G. Nazarenko et al. (10).

In 1960-1980-ies, researchers could enjoy extensive collaboration within the Soviet Union; strong liquid crystal research was conducted at the Institute of Crystallography in Moscow, Vilnius University, Leningrad University, and Institute of Cybernetics in Tbilisi. Scientists from other Soviet republics would often choose to defend their dissertations at the Institute of Physics, as the Scientific Council of the Institute was comprised of many experts in the physics of liquid crystals. In late 1970-ies, international cooperation started to emerge, thanks to the regular Liquid Crystal Conferences of the Socialist Countries, which attracted scientists from the West, including the USA, France, Italy, both German States. Bilateral meetings, such as USSR-Italy meetings, became another important venue.

Disintegration of the Soviet Union changed the scientific landscape. Reconstruction of economy and the possibility to find research-related employment in the West drove a large-scale migration of scientists. Many moved to scientific centres in the United States, Germany, France, United Kingdom, Italy. International support from Soros Foundation, US CRDF and other sources was of prime importance in the transition period for those who opted to stay in Ukraine. Eventually, the state infrastructure in Ukraine strengthened with the formation of the Scientific Committee of the National Council for the Development of Science and Technology, the 24 members of which are nominated by groups of scientists, academic institutions, scientific societies, and then selected by an independent Identification Committee, led by Sergiy M. Ryabchenko (Chair), and George (Yurij) Gamota (vice-Chair), with participation of renown foreign scientists such as Carlo Beenakker, Bertrand Halperin, and Mats Larson. A strong degree of integration of Ukrainian scientists in the world-wide research on liquid crystals is well illustrated by numerous research connections with Kent State University in Ohio, through its graduate programs in chemical physics, physics, and materials science and through the Advanced Materials and Liquid Crystal Institute (formerly Liquid Crystal Institute). Kent alumni of the Ukrainian origin work now in academia in both the US (O. Iadlovska, A. Glushchenko, A. Golovin, L. Kreminska, B. Senyuk, T. Sergan, V. Sergan, S.V. Shiyanovskii, I. Smalyukh) and Ukraine (O. P. Boiko, Yu. A. Nastishin, V. G. Nazarenko, M. M. Omelchenko, V. I. Savaryn) and in US industry (V. Bodnar, V. Borshch, M. Pevnyi, O. Pyshnyak, D. Reznikov, A. Varanytsya, D. Voloshchenko, T. Turiv, O. Yaroshchuk).

The special issue reflects the current state of Ukrainian integration into the world of science. The contributions are submitted by scientists residing in Ukraine and by those who work in other countries but have been educated in Ukraine. Some of the research data were collected during the war time, by scientists who were constantly concerned about their safety and safety of their families, colleagues, and friends. The recent targeting by the Russian military of energy infrastructure all over Ukraine meant that many experiments were difficult to plan and perform because of frequent and unpredictable power outages. Imagine laboratory life when the electricity, water, internet, heating, are not a given and can disappear any moment without warning; imagine air raid sirens and the need to stay in shelters for hours. Imagine public transport not functioning, no service at gas stations, and stores being closed. Nevertheless, war does not stop the research in Ukraine.

This Special Issue reflects a broad spectrum of research directions, both theoretical and experimental.

1. Stanislav B. Chernyshuk (Institute of Physics, NANU) and E. G. Rudnikov (Taras Shevchenko National University of Kyiv and Igor Sikorsky Kyiv Polytechnic Institute) present a theoretical model of elastic interactions of skyrmion-like director configurations in chiral nematics confined between two plates with homeotropic anchoring (11). The authors model the strongly deformed director field of skyrmions as colloidal particles dispersed in a liquid crystal; outside the “particle” the director gradients are weak and corresponding elastic interactions could be determined analytically.

2. O.S. Tarnavsky and Mykhailo F. Ledney (both at Taras Shevchenko National University of Kyiv) present a conformal mapping approach to find analytical solutions of complex two-dimensional director fields with topological defects caused by the presence of colloidal inclusions such as disks with inhomogeneous surface anchoring (12). The work might serve as a guiding rail in the experimental studies of composite colloid-liquid crystal systems.

3. Victor Yu. Reshetnyak, Igor P. Pinkevych (both at Taras Shevchenko National University of Kyiv) and their colleagues at the Air Force Research Laboratory at Wright-Patterson Air Force base in Ohio model a hybrid device comprised of a thin layer of black phosphorus and a rugate filter, separated by a nematic liquid crystal; the effective refractive index of the latter is controlled by an electric field which imposes a dielectric torque on the director (13). The structure generates the so-called Tamm plasmon-polariton electromagnetic excitations which manifest themselves as transmission peaks and reflection dips in the THz spectral regions. The spectral positions of these peaks and dips could be shifted by an electric field that realigns the nematic director.

4. In a related paper, Victor Yu. Reshetnyak and Igor P. Pinkevych with their collaborators explore an alternative system to generate Tamm states in the visible part of the spectrum, by using a holographic polymer-liquid crystal grating with a periodic modulation of refractive index as a Bragg mirror (14); De Sio et al. reviewed preparation techniques and other applications of these gratings (15). The paper predicts spectral tuning of the reflection dips by an electric or magnetic realignment of the director in nematic layers.

5. Victor M. Pergamenshchik, affiliated with Institute of Physics in Kyiv and, since March 2022, also with the Centre for Theoretical Physics of Polish Academy of Sciences in Warsaw, reviews the theory of surface-like elasticity of a nematic in contact with a surface near which the order parameter and density deviates from their bulk values (16). These deviations induce additional contributions to the surface elastic constants K₂₄ and K₁₃. The paper shows how the Ericksen’s inequalities involving elastic moduli should be modified in the presence of nonvanishing K₂₄ and K₁₃ and how these terms could be incorporated in the elastic theory. The work discusses relevant experiments on thin films of a nematic confined between two isotropic media.

6. Jaroslav Ilnytski at the Institute for Condensed Matter Physics in Lviv, A. Slyusarchuk (Lviv Polytechnic National University) and D. Yaremchuk (Institute for Condensed Matter Physics), in collaboration with colleagues in France, United Kingdom, Germany, and Poland describe numerical simulations of nanoparticles assembly (17). The nanoparticles are decorated with mesogenic ligands, which allows one to dramatically expand the potential of aided- and self-assembly into functional structures. The architecture of the assemblies depends on a large number of parameters, ranging from chemical composition of the particles (18) and the surrounding solvent (19),(20) to physical effects of concentration and external temperature, pressure, flows (21), or electromagnetic fields, thus numerical simulations are an indispensable tool. The paper demonstrates rich morphology of nanoarchitectures, such as smectic and hexagonal discotic phases formed by nanoparticles that adopt a rod-like or disc-like shape. The possibility of light control over the nanoscale assembly with a monodomain smectic morphology is illustrated in the simulations of nanoparticles functionalized with azobenzene derivatives, which are known to experience a trans-to-cis isomerization under irradiation within a certain spectral region, usually in ultraviolet.

7. Konstantyn G. Nazarenko and colleagues affiliated with the Institute of Organic chemistry, ISC, Institute of Physics, and synthesis-on-demand company Enamine, present a paper on synthesis and characterization of chiral ferroelectric nematic composition (22). They use a mixture of two recently discovered ferroelectric nematics, RM734 and DIO (weight proportion 70:30) and demonstrate that the addition of a nematic pentylcyanobiphenyl (5CB) or its chiral isomer CB15 lowers the temperature range in which the ferroelectric nematic is stable. When the chiral dopant CB15 is present (weight concentration 11.1%), the cholesteric mixture exhibits a temperature-dependent pitch which decreases from 1.3 $\mu m$ to 0.7 $\mu m$ as the material is cooled down from the chiral nematic to chiral ferroelectric nematic.

8. O. Kurochkin and collaborators at the Institute of Physics, Institute of Organic Chemistry, and Enamine explore chiral structures induced in a nematic lyotropic chromonic liquid crystal (disodium cromoglycate) by small-molecular-weight chiral additives of hydrophobic and hydrophilic types (23). Induction of cholesteric structures in lyotropic nematics by chiral additives is an intriguing problem, since it is not always clear what are the preferred residences of these additives, within the aggregates or in the solvent between the aggregates and how the information about chiral interactions propagates through the medium. In this study, hydrophobic additives demonstrate a much higher twisting power than their hydrophilic counterparts. The finding could be interpreted as the result of intercalation of the hydrophobic molecules within the chromonic aggregates, which might cause a structural twist and also a reduction of the average length of the aggregates, since the experiments demonstrate phase separation of the system into coexisting mesomorphic and isotropic regions. In contrast, hydrophilic additives are more likely dispersed in water, thus their effect is weak.

9. Volodymyr V. Tsukruk, who leads the research team at Georgia Institute of Technology, co-authors a short overview of chiral nematic organisation of natural materials, such as one-dimensional cellulose needles (24). The paper describes how one could tailor the concentration, ionic strength, hydrophilic/hydrophobic balance, external magnetic field and flows to produce predictable orientational order and pitch of the ensuing cholesteric in solutions and in dried thin films.

10. The theme of chiral structures is continued by Olena Uskova et al. (Beam Engineering for Advanced Measurements Company, Orlando, FL; formerly at the Institute of Physics and graduate of Taras Shevchenko Kyiv State University) (25). Her contribution describes high-quality free-standing films of cholesteric polymers of a large area, in the range of hundreds of cm², featuring selective reflection of light in the visible part of the spectrum. A unique approach allows one to combine films of different spectral properties in a single RGB film with broadband reflection. Various applications are discussed, such as “optical diodes,” which transmit circularly polarized light impingent from one side of the film but block the same light entering from the opposite direction.

11. Oleksandr Kovalchuk of the Kyiv National University of Technologies and Design in collaboration with A. Glushchenko (University of Colorado, Colorado Springs, CO) and Yu. Garbovskiy (Central Connecticut State University, CT) describe experimental approaches to characterize electric conductivity of liquid crystal and the role of ionic impurities in it, a topic that often represents a bottleneck in research, exasperated by complicated processes of ion generation and trapping (26). The author present a detailed exploration of how the DC electric conductivity depends on the cell thickness and attribute the thickness dependence of the electric conductivity to the competition between ion-releasing and ion-capturing processes at the bounding plates of the cells. The authors also propose a combined electro-optical measurements and textural observations to visualize the effect of ionic screening of the applied electric field.

12. Natalia Podolyak and her colleagues at the Institute of Physics, Czech Academy of Sciences, together with collaborators at the Warsaw University prepare and explore a series of lactic acid derivatives that exhibit a chiral smectic C (SmC*) phase with a broadened temperature range of stability, which includes room temperature, and enhanced spontaneous electric polarization, on the order of 10⁻³ C/m² (27). The future plans include preparation of hybrid structures in which the newly synthesized molecules are combined with nanoparticles.

13. A collaborative team of scientists in France, USA, and Croatia, with the participation of Tetyana Sergan and Vasyl Sergan, formerly at the Institute of Physics in Kyiv, Liquid Crystal Institute in Kent and now at California State University, Sacramento, describe nematic-like elastic coefficients of a biaxial smectic A_b (SmA_b) (28). SmA_b features a nematic-like ordering of molecular projections onto the plane of layers. As well known, bend and twist of the principal director in smectic A is prohibited by the requirement of the layers’ equidistance. Bend requires reorganization of layers, as in focal conic domains. If the layers remain flat, the only deformations of SmA_b are those of the secondary director $m$ within the layers. These deformations are triggered by an external electric field, which produces a “biaxial Frederiks transition”. The Frederiks effect allows one to measure the splay and bend elastic moduli but not the twist. The authors then resorted to the old observations that the Frederiks transition in a nematic cell with zero pretilt could produce domains of opposite direction of molecular tilt, separated by an elliptical domain wall, shaped by the ratio of elastic constants. In the present case, the elliptical shape yields the ratio of the bend to twist elastic constants; the latter turns out to be anomalously small.

14. A team of scientists from the Institute of Physics (V. Rudenko, A. Tolochko, D. Zhulai, S. Bugaychuk, G. Klimusheva) in collaboration with G. Yaremchuk and T. Mirnaya (V.I. Vernadsky Institute of General and Inorganic Chemistry) and Yu. Garbovskiy (Central Connecticut State University, CT) describes an approach to produce anisotropic glass using smectic structures of mesogenic ionic metal alkanoates (29). Studies and applications of glasses in general and anisotropic glasses in particular enjoy a rapid expansion; 2022 has been declared the UN International Year of Glass. The produced liquid crystal glass shows a long-term stability at room temperature. When glass incorporates bimetallic Ag/Au nanoparticles, it exhibits a strong nonlinear optical response that depends on the intensity of light. This dependency, rooted in complex mechanisms, could potentially be used to tailor the glass materials for targeted photonic and optoelectronic applications.

15. Michael Shribak (the Marine Biological laboratory, University of Chicago, graduate of the Lviv National University, formerly at the Lviv Radio Engineering Research Institute), together with Mojtaba Rajabi (Kent State University, OH) and the present author describe an approach for a single-shot mapping of a liquid crystal director pattern using a polychromatic polarizing microscope (30). Recent developments in the field of microscopy resulted in PolScope technique, which allows one to map spatially-varying optic axis of anisotropic materials. The conventional PolScope approach relies on multiple exposures involving different settings of an optical compensator, which usually require a few seconds. The single-shot exposure allows one to visualise dynamic effects, such as director reorientation under shear, with a speed limited only by the video camera.

The strong international support so far has been directed primarily to researchers who had to leave Ukraine. The task is also to help those who stayed and those who will return. Taylor & Francis have exhibited robust support, providing free access to their journal content to Ukrainian research institutes via Research4Life’s programme, https://www.research4life.org. All contributions to this Special Issue will be free to access for six months after the publication. Kent State University sent collected private donations to support liquid crystal research at the Institute of Physics in Kyiv. These examples need to be expanded to a much larger scale (31). The hope is that the victory in the war will also see Ukraine’s scientific infrastructure rebuilding and advancing to a new level through efforts within the country and strong international support and collaboration.