1. Twist grain boundary phases in proper ferroelectric liquid crystals realmDamian Pociecha, Jadwiga Szydłowska, Nataša Vaupotič, Katarzyna Kwiatkowska, Marijus Juodka, Julian Spiess, John M. D. Storey, Corrie T. Imrie, Rebecca Walker, Ewa Górecka, 2025, izvirni znanstveni članek Ključne besede: chirality, liquid crystals, proper ferroelectrics, twist-grain-boundary
Objavljeno v DKUM: 19.08.2025; Ogledov: 0; Prenosov: 7
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2. Multiple quartz crystals connected in parallel for high-resolution sensing of capacitance changesVojko Matko, 2022, izvirni znanstveni članek Opis: We present a new highly sensitive, low-value capacitance sensor method that uses multiple
quartz crystals connected in parallel inside the oscillator. In the experimental setup, the measured
(sensible) reactance (capacitance) is connected in parallel to the total shunt capacitance of the quartz
crystals, oscillating in the oscillator. Because AT-cut crystals have a certain nonlinear frequency–
temperature dependence, we use the switching mode method, by which we achieve a temperature
compensation of the AT-cut crystals’ frequency–temperature characteristics in the temperature range
between 0–50 ◦C. The oscillator switching method also compensates for any other influences on the
frequency of the oscillator, such as ageing of the crystals and oscillator elements, supply voltage
fluctuations, and other parasitic impedances in the oscillating circuit. Subsequently using two
50-ms-delayed switches between the measuring and reference capacitors, the experimental error in
measuring the capacitance is lowered for measurements under a dynamic temperature variation in
the range of 0–50 ◦C. The experimental results show that the switching method, which includes a
multiple quartz connection and high-temperature compensation improvement of the quartz crystals’
characteristics, enables a sub-aF resolution. It converts capacitance changes in the range 10 zF–200 fF
to frequencies in the range 4 kHz–100 kHz.
Ključne besede: multiple quartz crystals in parallel, quartz capacitive sensing method, temperature compensation, switching mode method
Objavljeno v DKUM: 01.04.2025; Ogledov: 0; Prenosov: 6
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3. Criticality controlling mechanisms in nematic liquid crystalsMaha Zid, George Cordoyiannis, Zdravko Kutnjak, Samo Kralj, 2024, izvirni znanstveni članek Opis: We theoretically study the generic mechanisms that could establish critical behavior in nematic liquid crystals (NLCs). The corresponding free energy density terms should exhibit linear coupling with the nematic order parameter and, via this coupling, enhance the nematic order. We consider both temperature- and pressure-driven, order–disorder phase transitions. We derive a scaled effective free energy expression that describes how qualitatively different mechanisms enforce critical behavior. Our main focus is on the impact of nanoparticles (NPs) in homogeneous NP-NLC mixtures. We illustrate that in the case of pressure-driven phase changes, lower concentrations are needed to impose critical point conditions in comparison with pure temperature variations.
Ključne besede: nematic liquid crystals, critical point, critical behavior, phase transition
Objavljeno v DKUM: 28.03.2025; Ogledov: 0; Prenosov: 13
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4. Dynamics and pretransitional effects in C60 fullerene nanoparticles and liquid crystalline dodecylcyanobiphenyl (12CB) hybrid systemSylwester Rzoska, Szymon Starzonek, Joanna M. Łoś, Aleksandra Drozd-Rzoska, Samo Kralj, 2020, izvirni znanstveni članek Opis: The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic–solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the “mixed” (“activated” and “critical”) relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau–de Gennes–Ginzburg mesoscopic approach.
Ključne besede: nanoparticles, liquid crystals, soft materials, fullerenes, dielectric spectroscopy, phase transitions, dynamics
Objavljeno v DKUM: 10.03.2025; Ogledov: 0; Prenosov: 6
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5. Annihilation of highly-charged topological defectsEva Klemenčič, Pavlo Kurioz, Milan Ambrožič, Charles Rosenblatt, Samo Kralj, 2020, izvirni znanstveni članek Opis: We studied numerically external stimuli enforced annihilation of a pair of daughter nematic topological defect (TD) assemblies bearing a relatively strong topological charge |m|=3/2. A Landau- de Gennes phenomenological approach in terms of tensor nematic order parameter was used in an effectively two-dimensional Cartesian coordinate system, where spatial variations along the z-axis were neglected. A pair of {m=3/2,m=−3/2} was enforced by an appropriate surface anchoring field, mimicking an experimental sample realization using the atomic force microscope (AFM) scribing method. Furthermore, defects were confined within a rectangular boundary that imposes strong tangential anchoring. This setup enabled complex and counter-intuitive annihilation processes on varying relevant parameters. We present two qualitatively different annihilation paths, where we either gradually reduced the relative surface anchoring field importance or increased an external in-plane spatially homogeneous electric field E. The creation and depinning of additional defect pairs {12,−12} mediated the annihilation in such a geometry. Furthermore, we illustrate the absorption of TDs by sharp edges of the confining boundary, accompanied by m=±1/4↔∓1/4 winding reversal of edge singularities, and also E-driven zero-dimensional to one-dimensional defect core transformation.
Ključne besede: liquid crystals, topological defects, annihilation, order reconstruction
Objavljeno v DKUM: 24.01.2025; Ogledov: 0; Prenosov: 15
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6. Microfluidic control over topological states in channel-confined nematic flowsSimon Čopar, Žiga Kos, Tadej Emeršič, Uroš Tkalec, 2020, izvirni znanstveni članek Opis: Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate.
Ključne besede: nematic liquid crystals, microfluidics, topological states, nematic flows
Objavljeno v DKUM: 15.01.2025; Ogledov: 0; Prenosov: 9
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7. Multiple twisted chiral nematic structures in cylindrical confinementMilan Ambrožič, Apparao Gudimalla, Charles Rosenblatt, Samo Kralj, 2020, izvirni znanstveni članek Opis: In this article, we theoretically and numerically study the chirality and saddle-splay elastic constant (K24)
-enabled stability of multiple twist-like nematic liquid crystal (LC) structures in cylindrical confinement. We focus on the so-called radially z-twisted (RZT) and radially twisted (RT) configurations, which simultaneously exhibit twists in different spatial directions. We express the free energies of the structures in terms of dimensionless wave vectors, which characterise the structures and play the roles of order parameters. The impact of different confinement anchoring conditions is explored. A simple Landau-type analysis provides an insight into how different model parameters influence the stability of structures. We determine conditions for which the structures are stable in chiral and also nonchiral LCs. In particular, we find that the RZT structure could exhibit macroscopic chirality inversion upon varying the relevant parameters. This phenomenon could be exploited for the measurement of K24.
Ključne besede: liquid crystals, chirality, confinement, saddle-splay elasticity, double twist deformations
Objavljeno v DKUM: 14.01.2025; Ogledov: 0; Prenosov: 7
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8. Nano and micro-structural complexity of nematic liquid crystal configurationsAndreja Jelen, Maha Zid, Kaushik Pal, Remya Rajan Renuka, Dejvid Črešnar, Samo Kralj, 2024, izvirni znanstveni članek Opis: Of our interest are frustration-driven pattern generating mechanisms in systems which in bulk equilibrium display spatially homogeneous long-range orientational order in absence of perturbations. As testbed material, we select thermotropic nematic liquid crystals. In bulk, they exhibit weakly discontinuous order-disorder phase transformation on varying temperature where the ordered nematic phase features spatially uniform axial order along an arbitrary symmetry breaking direction. However, due to continuous symmetry breaking (CSB) the established order is extremely susceptible to various perturbations which are in real systems in general always present. We theoretically illustrate how diverse complex patterns could be excited. Particularly intriguing configurations could appear if topological defects are present that could be generated via CSB. Our analysis is based on a relatively simple Lebwohl-Lasher-type model in which we could get analytical insight into phenomena of our interest. Using it we illustrate history dependent early stage isotropic-nematic phase evolution and final patterns in presence of "impurities" (e.g., nanoparticles). We show how characteristic effective interaction characteristics predict qualitatively different emerging patterns. Our analysis is based on CSB which is ubiquitous in nature. Consequently, demonstrated mechanisms are expected to manifest also in other condensed matter systems whose ordered phase is formed via CSB. We illustrate how kinetics and impurities could impact key structural properties of the systems of our interest.
Ključne besede: continuous symmetry breaking, patterns, topological defects, nematic liquid crystals
Objavljeno v DKUM: 05.12.2024; Ogledov: 0; Prenosov: 9
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9. Interpretation of dielectric spectroscopy measurements of ferroelectric nematic liquid crystalsVojko Matko, Ewa Górecka, Damian Pociecha, Joanna Matraszek, Nataša Vaupotič, 2024, izvirni znanstveni članek Opis: The magnitude of the relative permittivity of the ferroelectric nematic phase (NF ) has been the subject of lively
scientific discussion since the phase was recently discovered. Dielectric spectroscopy measurements (DSMs)
give a huge value of relative permittivity, which depends on the cell thickness, but this is argued to result from
a misinterpretation of the DSM results. We have conducted DSM using a set of cells differing in thickness of
the NF layer, type of electrodes, and presence/absence of nanoscale-thick surface polymer layers. To model the
DSM results, cells are presented by an equivalent electric circuit that includes a capacitor due to the NF layer with
frequency dependent complex relative permittivity, capacitors due to surface layers, and a resistor describing the
limited conductivity of electrodes. DSM results for different cells with the same liquid crystal in the NF phase
are semiquantitatively reproduced by the same set of physical parameters if a huge relative permittivity of the
NF , which is even orders of magnitude larger than the measured apparent values, is assumed. We show that
the capacitance of surface layers should also be considered in cells with no polymer alignment layer on the
electrodes.
Ključne besede: liquid crystals, statistical physics
Objavljeno v DKUM: 27.11.2024; Ogledov: 0; Prenosov: 17
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