A new scheme is investigated for evaluating the temperature dependence and dispersion relation of the Kerr constant (K) of an optically isotropic medium in isotropic and blue phases (BPs) liquid crystals. The scheme employs the measurement of the component of the transmitted light intensity of double modulated frequency using the modified in-plane-switching cell geometry (based on metallic film electrodes). It overcomes to a large extent the problem of a nonuniform electric field, employs relatively small driving voltages, and allows K to be measured directly. It is shown that the dispersion relation based on the single-band birefringence model describes well both blue and isotropic liquid crystal phases. It is found that the experimental data indicate that the temperature-dependent coefficients in this relation have a simple linear form in the isotropic phase, which allows a general model for the temperature and wavelength dependence of the Kerr constant in the isotropic liquid crystal phase to be formulated. In the BPs the temperature dependence of the experimental data deviate from the simple linear trend, but follow well an inverse exponential form.

The analysis of the first, second and third harmonic electro-optical responses (EORs) have been performed for planar samples of the liquid crystal (LC) having a broad temperature range of the phase. It demonstrated strong dependence on the angle between the polariser and the smectic layers normal. The comparison of these measurements for the smectic LC cells with both homogeneously and random orientation of the director agreed with results of the performed theoretical considerations based on the deformation of both the helix and the indicatrix under applied electric field. For the homogeneously aligned cell, the electro-optical effects are dominated by the strong linear process - in contrast to the unaligned cell, where the second-order EOR is important. The study of angular dependence of the nonlinear EOR allowed for easy separation of linear and nonlinear effects. [GRAPHICS] .

Professor Jerzy Małecki, a former member of the editorial board of ‘Phase Transitions’ and a Research Staff Member at the Institute of Molecular Physics of the Polish Academy of Sciences in Poznań, Poland, passed away on 2 July 2018. Professor Małecki was born in 1933 in Swarzędz, a small town not far from Poznań. After the outbreak of World War II his family moved to Warsaw. In 1945, after leaving the Nazi prison, his parents moved with the children to Poznań. In the years 1951–1955 Jerzy Małecki studied physics at the University of Poznań. After graduating, he started working at the Institute of Physics of the Polish Academy of Sciences. At this Institute, he progressed through the stages of seniority until he became a full professor in 1983. His expertise was in the field of molecular and condensed matter physics. Professor Małecki was a world-class authority in the field of molecular interactions in liquids and liquid crystals. He made particular contributions in the field of nonlinear dielectric effect research methods, also known as dielectric saturation. The techniques he developed have been widely used in many national and foreign laboratories. Professor Małecki’s interests focused on the phenomena of molecular orientation and transport in liquids and liquid crystals. This research, carried out with his students, resulted in contributions of great importance in the field of molecular biology, such as in molecular association in liquids and the proton transfer process in molecular complexes with hydrogen bonding. This work has contributed to our understanding of point mutations and the aging process. The results of this research were published in over 140 papers and formed the basis of the scientific careers of many of his students, work which resulted in 13 dissertations and 9 habilitations. Six of his students have received the title of professor. Professor Małecki had a keen interest in technological applications. The project co-directed by him on the technology of electret microphone production was particularly successful. This cooperation with industry resulted in the large scale production of high-quality microphones, which satisfied the needs of the electronics industry of Eastern Europe in the 1970s and 1980s. Following on from this cooperation, a number of well-respected books on the physics and technology of electrets have remained. Professor Jerzy Małecki has left a lasting legacy in both the physics of condensed matter and in its technological applications. As a valued friend, colleague, and teacher he will be greatly missed by those who knew him.

The method of widening of the blue phase (BP) temperature range is presented. By means of polarising optical microscopy, we demonstrate that the temperature regime in which BP is stable is greatly enhanced by the cell thickness changes in mixtures of nematic liquid crystal (LC) and chiral dopant.

A polymer-stabilized liquid crystal based on 4' -(octyloxy) biphenyl-4-carboxylate 2-fluoro-4-((octyl-2yloxy) carbonyl) phenyl (D16) and 1,6 hexanediol diacrylate as a monomer was prepared by in situ photopolymerization. The selected antiferroelectric liquid crystal contains a fast- switching smectic C*alpha phase (SmC*alpha),and the influence of the polymer network on the thermodynamic stability of this phase and its switching behavior under applying time-dependent electric field were studied. Using dielectric spectroscopy and polarizing microscopy, the liquid crystal materials were characterized, and subsequently with the use of the reversal current method (RCM) the current response, especially from the SmC*alpha phase was carefully analyzed. The current response is complex and also depends on the neighboring liquid crystal phases. In the liquid crystal-polymer system, as well as in the liquid crystal-monomer mixture, a significant shift of the temperature range of the SmC*alpha phase toward lower temperatures was observed; however, the thermodynamic instability related to the transformation to the crystalline phase was also noted and characterized. Because of the fuzzy phase transitions detected in the liquid crystal-polymer system by dielectric spectroscopy and also because of the lack of the characteristic dielectric signature of SmC*alpha after polymerization, we proposed the use of the RCM, as a complementary one, to identify the SmC*alpha phase even in such complex materials.

Measurements of the non-linear electro-optical effects for the well-known prototype liquid crystal material (MHPOBC) are presented. The method to identify liquid crystalline phases and to determine temperatures of phase transitions based on the analysis of the second harmonic component of electro-optical response spectra is used. Applying that method, the values of the frequency (f(ext)) at which the second harmonic electro-optic response (EOR) possesses an extremum are determined for each smectic phase. We suggest that this characteristic frequency correspond to the phase-type mode processes. Furthermore, we show that the usually neglected results on heating can be useful in discussions of dynamical behaviour of second harmonic EOR in case of smectic phases.

Nonlinear dynamics induced in surface stabilized ferroelectric liquid crystals by strong alternating external electric fields is studied both theoretically and experimentally. As has already been shown, molecular reorientations induced by sufficiently strong fields of high-enough frequencies can reveal a long transient behavior that has a weakly chaotic character. The resulting complex dynamics of ferroelectric liquid crystals can be considered not only as a consequence of irregular motions of particular molecules but also as a repercussion of a surface-enforced partial decorrelation of nonlinear molecular motions within smectic layers. To achieve more insight into the nature of this phenomenon and to show that the underlying complex field-induced behavior of smectic liquid crystals is not exceptional, ranges of system parameters for which the chaotic behavior occurs are determined. It is proved that there exists a large enough set of initial phase trajectory points, for which weakly chaotic long-time transitory phenomena occur, and, thereby, it is demonstrated that such a chaotic behavior can be regarded as being typical for strongly field-driven thin liquid crystal systems. Additionally, the influence of low-amplitude random noise on the duration of the transient processes is numerically studied. The strongly nonlinear contribution to the electro-optic response, experimentally determined for liquid crystal samples at frequencies lower than the actual field frequency, is also analyzed for long-time signal sequences. Using a statistical approach to distinguish numerically response signals of samples from noise generated by measuring devices, it is shown that the distribution of sample signals distinctly differs from the device noise. This evidently corroborates the occurrence of the nonlinear low-frequency effect, found earlier for different surface stabilized liquid crystal samples.

The chiral smectic phases of two liquid crystals (LCs) were investigated, one of them is a high tilted, long pitch, near ortoconic LC mixture and the second exhibits broad temperature of the SmC alpha* phase. For both materials second-harmonic electro-optic (EO) spectra were studied. The particular LC phases were characterised by a specific temperature dependence of the frequency at which the real part of the second-order electro-optic response (EOR) possesses an extremum (f(ext)). The obtained values of f(ext) (correlated with the phase-type mode processes) were used to identify LC phases and temperature of phase transitions. The sensitivity of the used method allowed to identify the SmC gamma* and SmC alpha* phases for the studied materials, not detected before by DSC or dielectric measurements.[Graphics].

In this paper, we report on how flexoelectric and piezoelectric polarization components can be determined by a method based on simultaneous studies of dielectric and electrooptic properties of the chiral smectic liquid crystal in the regime of weak electric fields. As a rule, the measurements of spontaneous polarization are performed using switching experiments. The polarization measured in this way is not complete—it contains the piezoelectric component only. However, the knowledge of the entire local polarization of a single smectic layer is of great importance—it is necessary for correct determination of some material parameters, for instance elastic constants. Our experiments performed in a helical smectic mixture demonstrated that flexoelectric contribution to the local spontaneous polarization is significant in both ferroelectric and antiferroelectric phases. In the antiferroelectric phase, the flexoelectric polarization is less due to higher helical pitch.

Dielectric properties of chiral smectic liquid crystals characterised by the occurrence of the C(α)(*) phase were investigated in the frequency range 10 Hz-1 MHz. In the range of existence of this phase the observed relaxation spectrum is composed of two kinds of mode, and not of a single one, as commonly thought. Phase modes of the Goldstone type coexist in it with an amplitude type soft mode. The share of the soft mode in the global value of electric permittivity ε can be dominant and attain 90%. A possible explanation for that effect is sought in the similarity to chiral phases of the de Vries type.

Electro-optical measurements of the second, third and fourth harmonic frequencies have been performed in prototype antiferroelectric liquid crystal materials: 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methyl-heptyloxycarbonyl) phenyl 4-(4′-octyloxyphenylcarbonyloxy) benzoate (MHPOPB). These two substances possess a very rich sequence of phase transitions between various helical smectic subphases. The analysis of the observed anomalous behavior of the real and imaginary parts of the second, third and fourth harmonics of the electro-optical response spectra can serve for identification of various liquid crystalline phases. The anomalies of the nonlinear electro-optical response of the investigated liquid crystals have been compared with the linear electro-optical measurements.

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript

We employed fluorescence confocal polarizing microscopy (FCPM) to visualize three-dimensional patterns of the director field in cholesteric and nematic liquid crystals (LCs). Two types of textures were investigated: fingerprint and schlieren, by both FCPM imaging and conventional polarizing optical microscopy (POM). Some kinds of spontaneous periodic distribution of the nematic director were presented.

Molecular reorientation processes induced in thin ferroelectric liquid crystal systems by strong alternating external electric fields are studied both by solving numerically the equation of reorientation motion of molecules and by measuring the electro-optic response of thin samples. It is shown that the occurrence of a wide band in nonlinear response spectra above the Goldstone-mode frequency is a consequence of complex partially uncorrelated molecular reorientations enforced within smectic layers by sufficiently high fields of high enough frequencies. Such nonlinear reorientational motions of molecules are argued to have a character of weakly chaotic long-lasting transients, related to almost periodic modulations of the amplitude of rotational oscillations performed by molecules with the field frequency. These modulations have been numerically proved to proceed with lower frequencies than the field frequency and with space-dependent depths of temporal changes. The occurrence of the modulations has experimentally been confirmed by registering distinct contributions to electro-optic response spectra at frequencies less than the running frequency of the applied electric field.

We present the results of the study of three fluorinated mesogens with three aromatic rings and different length of alkyl chain. The investigated substances might be of interest for using in technical devices. Calorimetric, dielectric and electro-optical properties were examined. Special attention was paid to the dependence of the first and second harmonics of the electrooptic response on temperature. It is shown that the analysis of the light modulation depth of the first and second harmonics prove to be useful for the identification of phase transition.

The knowledge of the material parameters related to the microscopic structure of liquid crystals is one of the main problems in the construction of the liquid crystal devices. In the case of ferroelectric liquid crystals, the viscoelastic properties are very important as they determine the switching speed and the threshold voltage in displays. There exist several experimental methods for measurements of viscosity and elasticity constants in tilted smectic phases which exploit various phenomena for deformation detection, e.g., light transmission, polarization current, light modulation, dielectric constant, and helix deformation or helix unwinding. In this article, we compared the results of measurements obtained for the same material using various methods. The experiments proved that the correct bulk values of the mentioned material constants can be determined using thick, homeotropically aligned samples only.

Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry.

The viscoelastic properties belong to the most important properties of chiral smectic liquid crystals. In this paper different methods for determination of viscoelastic properties of chiral smectic liquid crystals are compared. All of them belong to small deformation methods. The importance of the director alignment in the sample for determination of the bulk elasticity and viscosity coefficients is described. Using a typical ferroelectric in the chiral smectic C* phase we demonstrated that the correct bulk values of mentioned material constants can be determined using thick, homeotropically aligned samples only.

Propagation of solitary waves activated in thin ferroelectric liquid crystal cells under external, sinusoidally alternating electric fields is investigated using the electro-optic technique. It is shown that solitons give contributions only to the loss component of the response spectrum, within rather narrow ranges of frequencies and in sufficiently strong fields. The limit frequency, at which the amplitude of the velocity of the solitary waves is greatest, is found to be related to material constants of liquid crystals. Measuring this threshold frequency provides the capability to determine the elastic constant of surface stabilized liquid crystalline materials in the bookshelf or chevron layer geometries.

An electro-optic method for determining the twist elastic coupling between smectic layers in antiferroelectric liquid crystals with a helical superstructure has been introduced. This method is based on a calibration procedure which enables a modulation of light intensity under an alternating applied electric field to relate to a respective modulation caused by mechanical oscillations of a sample. The elastic constant of antiferroelectric liquid-crystalline materials has been obtained by applying the method to a liquid crystal which displays a direct transition from antiferroelectric C*(a) phase to the smectic A phase.