1. DIFFRACTION GRATINGS FORMED BY BENT-CORE LIQUID CRYSTALS IN THE TWIST – BEND NEMATIC PHASEMuhammad Ali, 2021, doctoral dissertation Abstract: In this thesis, we study the structure and optical transmission properties of the twist-bend nematic liquid crystalline phase, made of bent dimers, confined in thin planar cells. Confinement leads to the formation of a periodic modulated structure, the formation of which is explained as follows. The twist-bend nematic phase is characterized by a heliconical modulation of the molecular long axes. Due to a short pitch of modulation (approximately 10 nm), the twist-bend nematic phase behaves as a pseudo-layered medium. At temperatures below the nematic – twist-bend nematic phase transition, the heliconical pitch and thus the thickness of the pseudo-layers reduces, which leads to a two-dimensional undulation of pseudo-layers in the direction perpendicular to the cell surfaces and along the surfaces. The undulated structure is responsible for a stripe texture observed under a polarizing microscope and acts as a diffraction grating.
We constructed theoretical models to predict the pseudo-layer structure of a confined twist-bend nematic phase and to describe the properties of light diffracted on such cells. The free energy of the two-dimensional pseudo-layer structure of the twist-bend nematic phase is expressed in terms of the nematic director field, by which we describe the direction of the heliconical axis, and a complex smectic order parameter, the gradient of which gives the direction of the layer normal. At first, we assume that pseudo-layers are perpendicular to the surfaces (bookshelf geometry) and find a stable structure by assuming an ansatz for the pseudo-layer displacement from the bookshelf geometry and then minimizing the free energy at a very strong and very weak surface anchoring. In this way a threshold condition for the onset of the modulated structure is obtained, as well as the amplitude and period of modulation. Next, we assume that, at the onset of the twist-bend nematic phase, pseudo-layers are formed at some angle (pre-tilt) with respect to the surface. We find that in both cases, the bookshelf and pre-tilted one, the calculated period of modulation far from the phase transition is always approximately twice the cell thickness, which agrees with experimental observations.
The properties of light diffracted by the spontaneously formed grating were studied both experimentally and theoretically. We measured the intensity and polarization properties of the first two orders of the diffracted light and the temperature dependence of the polarization of the second order diffraction peaks. To predict the observed properties of the diffracted light and to simplify the description of such gratings, we consider different preliminary models of a one-dimensional spatial variation of the optic axis, the direction of which is given by two angles. A transfer matrix method is used and a good agreement between the experimental and theoretical results is obtained. In a more comprehensive approach, we determine the spatial variation of the optic axis direction from the modeled structure. The electric field in the diffracted light is obtained by using the transfer matrix method and beam propagation method. In the case of a pre-tilt of the pseudo-layers and very strong surface anchoring both methods give good qualitative agreement with experimental results, only in the case of the temperature dependence of the second order diffraction peaks, a more complex beam propagation method is superior to the transfer matrix method.
The thesis is divided into three parts. In the first part, we focus on the physical properties of the twist-bend nematic phase and its structure in thin planar cells. In the second part, a continuum model is proposed and finally, the properties of diffracted light are discussed and theoretically predicted by using the beam propagation method and transfer matrix method.
Keywords: Bent-dimer liquid crystals, twist-bend nematic phase, undulation of pseudo-layers, polarization, diffraction grating, beam propagation method, transfer matrix method.
Published in DKUM: 21.10.2021; Views: 1359; Downloads: 90
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2. Suitability of the double Langevin function for description of anhysteretic magnetization curves in NO and GO electrical steel gradesSimon Steentjes, Martin Petrun, G. Glehn, Drago Dolinar, Kay Hameyer, 2017, original scientific article Abstract: This paper compares the match obtained using the classical Langevin function, the tanh function as well as a recently by the authors proposed double Langevin function with the measured anhysteretic magnetization curve of three different non-oriented electrical steel grades and one grain-oriented grade. Two standard non-oriented grades and a high-silicon grade (Si content of 6.5%) made by CVD are analyzed. An excellent match is obtained using the double Langevin function, whereas the classical solutions are less appropriate. Thereby, problems such as those due to propagation of approximation errors observed in hysteresis modeling can be bypassed.
Keywords: polarization, magnetization measurement, magnetic materials, hysteresis models, Langevin function, electrical steel, saturation
Published in DKUM: 03.08.2017; Views: 1246; Downloads: 430
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3. Fiber-optic sensors for measurements of torsion, twist and rotation : a reviewVedran Budinski, Denis Đonlagić, 2017, review article Abstract: Optical measurement of mechanical parameters is gaining significant commercial interest in different industry sectors. Torsion, twist and rotation are among the very frequently measured mechanical parameters. Recently, twist/torsion/rotation sensors have become a topic of intense fiber-optic sensor research. Various sensing concepts have been reported. Many of those have different properties and performances, and many of them still need to be proven in out-of-the laboratory use. This paper provides an overview of basic approaches and a review of current state-of-the-art in fiber optic sensors for measurements of torsion, twist and/or rotation.
Keywords: fiber optic sensors, twist sensors, rotation sensors, circular birefringence, linear birefringence, FBG, polarization, optical fibers, Fiber Bragg Gratings
Published in DKUM: 22.06.2017; Views: 1673; Downloads: 416
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4. Ferroelectric smectic-A phase made of bent-core liquid crystals: Structure and Dielectric response in thin cellsKristina Leskovar, 2016, doctoral dissertation Abstract: In the thesis we study the structure and response of bent-core liquid crystals in the orthogonal ferroelectric Smectic-A (SmAP$_F$) phase in thin planar cells. We construct a phenomenological continuum model to study the structure in thin planar cells. A set of molecules within a small volume is presented by the director ($vec{n}$), which defines the average direction of the long molecular axes within this volume, and by the polar director ($vec{p}$), which points in the direction of local polarization. We choose a geometry in which the director in the smectic layer is constant and the polar director varies across the cell. The polar director structure inside the cell is determined by a competition among the torques due to the bulk elasticity, electrostatic effects and surface anchoring. The equilibrium profile structure of the polar director is obtained by minimization of the free energy. We find the polar director profile in a cell as a function of the type and strength of the surface anchoring, bulk elastic constants and cell thickness.
The effect of the external electric bias field on the structure in the cell is studied, as well. Bent-core molecules have a permanent electric dipole moment. In the external electric field the electric torque tends to rotate the dipoles in the direction of the external field. The polar director profile in the cell in external bias field thus depends on the competition among three effects: anchoring at the surfaces, the elastic properties of the bulk and the influence of the external electric field. By the rotation of molecules in the external electric field, optical properties of the cell are changed, therefore the SmAP$_F$ phase is a promising phase for use in displays with high response time, high contrast, continuous gray level and wide viewing angle.
The response of the SmAP$_F$ phase to an alternating external electric field (the dielectric response) is also considered. The dielectric response of the SmAP$_F$ phase consists of two modes: the phase and amplitude mode. The phase mode is due to fluctuations in the orientation of the local direction of the spontaneous polarization and the amplitude mode is due to the change in the magnitude of spontaneous polarization. The frequency of the phase and amplitude mode and the dielectric permittivity are calculated numerically as a function of the bias external DC electric field, cell thickness, the type and strength of surface anchoring and the ratio between the bend and splay elastic constants. Analytical solution for a very specific case of chosen parameters is also obtained. Theoretically obtained dependencies are in agreement with the reported experimental measurements.
In the thesis we study the effect of different types of surface anchoring and for this purpose three different types of cells are defined. The polar director profile and the dielectric response is calculated in all three types of cells. The type I cell has polar surface anchoring of equal strengths at both surfaces. The type II cell has, in addition to the polar surface anchoring of equal strengths at both surfaces, a nonpolar anchoring at the bottom surface. The type III cell has polar surface anchoring at both surfaces but not of equal strengths. We predict that by comparison of the dielectric response of the SmAP$_F$ phase in all three types of cells the type of the polarization splay in bent-core liquid crystals can be determined.
The thesis is divided into three parts. In the first part the basic physical properties of ferroelectric liquid crystals, focusing on the bent-core liquid crystals, are discussed. In the second part phenomenological theoretical model is developed. In the third part of the thesis a dielectric response in external bias field is studied.
Keywords: Bent-core liquid crystals, ferroelectric smectic-A phase, dielectric response, confined geometry, phenomenological continuum model, surface anchoring, polarization splay
Published in DKUM: 06.05.2016; Views: 1989; Downloads: 166
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5. The removal of reactive dye printing compounds using nanofiltrationIrena Petrinić, Niels Peder Raj Andersen, Sonja Šostar-Turk, Alenka Majcen Le Marechal, 2007, original scientific article Abstract: A synthetically prepared reactive dye print wastewater, mimicking real wastewater obtained from a local textile mill, was treated by nanofiltration using an NFT-50 membrane in a plate and frame module configuration at different cross-flow velocities (0.4, 0.6 and 0.8 m/s) and pressures (2-15 bar). The nanofiltration membrane was evaluated for membrane fouling, permeate flux and its suitability for removing colour, conductivity, Na+ ions and COD as a function of operating pressure and feed concentration. The permeate separation efficiency was monitored by measuring the removal efficiency of colour, conductivity, Na+ ions and COD retention. The membrane achieved high dye retention for each of the four dyes (from 99.4 to 99.9%) and electrolytes used (63-73%). The retention of organic substances varied between 20 and 50%, depending on the pressure used; higher retentions were achieved at higher pressure and by using higher cross-flow velocities.
Keywords: nanofiltration, nanofitration membranes, wastewater after reactive printing, reactive dye, concentration polarization
Published in DKUM: 01.06.2012; Views: 1997; Downloads: 100
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6. Effects of plasma treatment on water sorption in viscose fibresMiha Devetak, Nejc Skoporc, Martin Rigler, Zdenka Peršin Fratnik, Irena Drevenšek Olenik, Martin Čopič, Karin Stana-Kleinschek, 2012, original scientific article Abstract: We investigated water sorption in viscose nonwoven fibres manufactured by Tosama d.d. with the surface density of 175 g/m2. A comparison between untreated fibres and by oxygen plasma treated fibres was made using optical polarization microscopy. Plasma treatment was done for 10 minutes at pressure of 75 Pa at current of 250 mA at the power of 500 W. Swelling was characterized by measurements of fibre diameter. Modifications of intensity of the polarized light transmitted through the fibre were measured as a function of time of exposure to water. Characteristic swelling and intensity modification times were resolved for untreated and oxygen plasma treated fibres. The swelling time of oxygen plasma in comparison to untreated plasma is reduced by the factor of 0.54 and intensity change time by the factor of 0.4. From the characteristic swelling and intensity change times it was concluded that oxygen plasma treatment of viscose increases the speed of water sorption.
Keywords: plasma treatment, viscose, optical polarization microscopy
Published in DKUM: 01.06.2012; Views: 1972; Downloads: 88
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7. All-fiber quasi-distributed polarimetric temperature sensorDenis Đonlagić, Miran Lešič, 2006, original scientific article Abstract: This paper presents an all-fiber design of a quasi-distributed polarimetric temperature sensor array that utilizes commercially available single polarization and high birefringence fibers. The modulation depth of temperature induced loss and the operational temperature range of individual sensors in the network are set by the rotational alignment of fibers before fusion splicing and through fine adjustment of the sensing fiber lengths. A practical sensor network was built with sensors that operated in the temperature range from 0 to 100 C. Individual sensors in the network generatedtemperature dependent loss that changed proportionally from 0.9 to 1.8 dB. With current standard telecommunication OTDRs, more than 20 prototype sensors could be interrogated.
Keywords: fiber optics, sensors, temperature measurements, polarization, alignement, birefringence, polarization maintaining
Published in DKUM: 31.05.2012; Views: 2384; Downloads: 115
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8. An impedance study of two types of stainless steel in Ringer physiological solution containing complexing agentsMojca Slemnik, Ingrid Milošev, 2006, original scientific article Abstract: The effect of complexing agents EDTA and citric acid on the electrochemical behaviour of AISI 304 and orthopaedic stainless steels in Ringer physiological solution was investigated by potentiodynamic polarization method and electrochemical impedance spectroscopy (EIS). The content of Mo has a pronounced effect on the corrosion resistance, as evident by the broader passive range of the orthopaedic stainless steel containing Mo. The addition of complexing agents induces significant changes in polarization and impedance characteristics, i.e., the shift of corrosion and breakdown potentials in a more negative direction, an increase in current density, and a significant decrease in charge transfer resistance. The results were interpreted by the formation of soluble complexes of metal ions with chelating agents, especially EDTA, which suppressed the formation of the outer Fe(III) layer of the passive film. The impact of complexing agent on the electrochemical parameters was found to be related to its concentration in electrolyte and the stability constant of the complex formed with the related metal ion.
Keywords: resistance, corrosion resistance, stainless steel, orthopedy, potentiodynamic polarization emthod, electrochemical impedance spectroscopy
Published in DKUM: 31.05.2012; Views: 2038; Downloads: 119
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