Nematic-Smectic- A Tricritical Point in Alkylcyanobiphenyl Liquid Crystals
TL;DR: In this paper, it was shown that the specific heat exponent depends on the width of the $N$ range for continuous $\mathrm{AN}$ transitions, where AN is a tricritical point in the phase diagram.
Abstract: From adiabatic-scanning calorimetric results it is demonstrated that in mixtures of 9CB and 10CB, two compounds of the alkylcyanobiphenyl ($n\mathrm{CB}$) series, the nematic ($N$) to smectic-$A$ ($A$) transition becomes first order for narrow $N$ ranges. From the latent heats a tricritical point is located in the phase diagram. Measurements for 7CB+ 8CB mixtures show that the specific-heat exponent $\ensuremath{\alpha}$ depends on the width of the $N$ range for continuous $\mathrm{AN}$ transitions.
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TL;DR: In this paper, a review focusing on nematic liquid crystals is presented, where three main kinds of effects can be distinguished: the perturbation of the liquid crystalline structure close to the surface, the bulk liquid crystal structure is recovered with an orientation which is fixed by the surface and critical adsorption or wetting can occur at surfaces.
Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.
754 citations
TL;DR: The use of specular reflection of X-rays to study the structure of the liquid/vapour interfaces along the direction normal to the surface is described in this article, where specular reflectivity measurements of surface roughness by studying diffuse scattering at small angles off of the specular condition is illustrated with results from the H2O surface.
Abstract: The use of specular reflection of X-rays to study the structure of the liquid/vapour interfaces along the direction normal to the surface is described. If RF(θ) is the theoretical Fresnel reflection law for X-rays incident on an ideal flat surface at an angle θ, and R(θ) is the measured reflectivity from the true surface, the ratio R(θ)/RF(θ) is a measure of the electron density along the surface normal; i.e.R(θ)//RF(θ)≈|1//p∞∫∂〈p(z)〉//∂z exp (iQzz) dz|2. where p∞ is the electron density far from the surface, ∂〈p(z)〉/∂z is the gradient of the average electron density along the surface normal and Qz=(4π/λ) sin (θ). For simple liquids p–1∞∂〈p〉/∂z≈[1/√(2πσ2)] exp (–z2/2σ2), and R(θ)/RF(θ)≈ exp (–Q2σ2), where σ2 is dominated by the mean-square average of thermally excited fluctuations in the height of the surface. For liquid crystals and for lyotropic miceller systems temperature-dependent structure in R(θ) is due to surface-induced layering in 〈p(z)〉. Other experimental results from thin layers of liquid 4He and monolayers, of amphiphathic molecules on the surface of H2O will be described. The possibility of complementing specular reflectivity measurements of surface roughness by studying diffuse scattering at small angles off of the specular condition will also be illustrated with results from the H2O surface.
102 citations
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TL;DR: In this paper, the polymorphism associated with one-dimensional ordering has been studied and critical phenomena characteristic of the several phase transitions encountered in these systems, together with their very special dynamical properties.
Abstract: The smectic state has been known for over a century but is currently being very actively investigated. We review the polymorphism associated with one-dimensional ordering, critical phenomena characteristic of the several phase transitions encountered in these systems, together with their very special dynamical properties.
99 citations
TL;DR: All three of the blue phases of the liquid-crystal cholesteryl nonanoate are thermodynamically stable and that all the phase transitions observed are first order.
Abstract: An adiabatic scanning calorimeter has been used to study the thermal behavior of the liquid-crystal cholesteryl nonanoate in a temperature range covering all phase transitions involving the three different blue phases occurring in this substance. Results for the temperature dependence of the enthalpy and the heat capacity are reported. It is found that all three of the blue phases are thermodynamically stable and that all the phase transitions observed are first order.
78 citations
TL;DR: In this article, an adiabatic scanning calorimeter was used to investigate the temperature dependence of the enthalpy and the heat capacity near the smectic A to nematic (N) phase transition in mixtures of alkylcyanobiphenyl (nCB) liquid crystals.
Abstract: An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near the smectic A (A) to nematic (N) phase transition in mixtures of alkylcyanobiphenyl (nCB) liquid crystals. Mixtures of 8CB + 10 CB with small N-ranges undergo first-order transitions, but mixtures with large N-ranges exhibit continuous ones. A tricritical point could be located for the 10CB mole fraction of 0.3135. For a nearly tricritical mixture, a critical exponent α = 0.50 ± 0.02 has been observed. For mixtures with continuous AN transitions it is seen that this specific heat exponent α decreases with the width of the nematic range.
77 citations