Nematic smectic-A Research Articles

Effects of quantum dots on the nematic to smectic-A phase transition

ABSTRACT Experiments on the composites of liquid crystals and quantum dots showed the significant role of quantum dots on the properties of the nematic–smectic-A phase transition. The effects of quantum dots on the nematic-smectic-A phase transition have been discussed using Flory- Huggins theory of isotropic mixing and Landau-de Gennes theory. The impact of quantum dots on the smectic-A order parameters, nematic–smectic-A transition temperature, splay and bend elastic constants has been discussed. The smectic-A order parameters, transition temperature and elastic constants decrease with the increase of volume fraction of quantum dots.

Investigation on the critical behaviour in the vicinity of the smectic-A to re-entrant nematic-phase transition

ABSTRACT The study focused on the binary system 6OCB and 8OCB, which is well-suited for investigating stable re-entrant nematic-phase transitions. High-resolution measurements of heat capacity and birefringence are used to analyse critical behaviours near the nematic – smectic-A (N-SmA) and the smectic-A – re-entrant nematic (SmA-) phase transitions. The critical behaviour of these transitions has been explored with the aid of a differential quotient extracted from the birefringencevalues. Simultaneously, critical exponents extracted from the specific heat capacity anomaly are also used to analyse the critical behaviour of the phase transition. The results indicate that the SmA to phase transition exhibits non-universal behaviour, with effective exponents falling between the tricritical (TCP) and 3D-XY values, similar to the N-SmA phase transition. The study reveals the existence of two tricritical points: one associated with the N-SmA transition and the other corresponding to the SmA- transition. This is supported by plotting the McMillan ratio against the critical exponents obtained from both birefringence and specific heat capacity anomaly. Again, with an increase in the SmA range, the SmA- transition tends towards the tricritical point (TCP), suggesting a relationship between the SmA range and the transitional behaviour.

Critical behaviour near the nematic-smectic-C and nematic-smectic-A phase transitions of a binary system: a comparative study from calorimetric, optical and dielectric measurements

ABSTRACT Detailed heat capacity, birefringence and dielectric anisotropy measurements have been carried out near the nematic – smectic-C (NC) and the nematic – smectic-A (NA) phase transitions of a mesogenic binary system. Important feature is the existence of two tricritical points one for the NC transition and other for the NA transition. The dependence of critical exponent (α) is well explained in terms of temperature ratios (TNC/TNI and TNA/TNI) for the NC and NA phase transitions. Both transitions exhibit non-universal behaviour and showing effective critical exponents in between tricritical and 3D-XY values. However, the nematic range necessary for the NC tricritical point is quite large (>20°C). The enthalpy, phase shift and imaginary heat capacity are also analogous to the nature of the critical exponent (α). Moreover, within a very small concentration range, the smectic-A – smectic-C (AC) transition line appears. Although the smectic-A width above the AC transition is quite small~2K, heat capacity and birefringence data clearly confirm second-order nature. Additionally, there exists a region where the NC, NA and AC line segments converge, i.e. a NAC multicritical point. At this point, the latent heat for the NC, NA and AC transition lines disappears, indicating an obvious second-order nature of the NAC multicritical point.

High-resolution dielectric anisotropy investigation of carbon nanotube-smectic-A liquid crystal dispersions: An upper limit of the latent heat and the pretransitional anomaly at the nematic-smectic-A transition.

Within the entire mesomorphic range, high-precision dielectric anisotropy data with the high-temperature resolution is presented for a highly polar smectic-A liquid crystal 8CB (octylcyanobiphenyl) as well as 8CB nanocomposites doped with both pristine multi-walled carbon nanotubes (p-MWCNTs) and carboxyl group (-COOH) functionalized MWCNTs (f-MWCNTs). The temperature variation of the nematic order parameter across both the nematic-isotropic (N-I) and the nematic-smectic-A (N-Sm-A) phase transitions of the neat 8CB and 8CB+MWCNT nanocomposites has then been derived from the dielectric anisotropy data within the framework of the Maier-Meier theory. With the inclusion of MWCNTs, both the N-I and the N-Sm-A transition temperatures have been noted to shift to lower temperatures as compared to the 8CB host. Also, for all 8CB+MWCNT nanocomposites, regardless of the surface functionalization, it has been well documented that the N-I transition is weakly first order, whereas the N-Sm-A transition remains continuous within the experimental resolution. For all investigated samples, the temperature dependence of the nematic order parameter has been shown to be quasitricritical, within the experimental resolution. From the attentive inspection of the dielectric anisotropy data in the vicinity of the N-Sm-A transition, the upper limits for a possible latent heat ΔH_{NA} for the 8CB host and all 8CB+MWCNT composites, for the first time, have been derived. The so-derived ΔH_{NA} values for all investigated samples have also been compared with those extracted from the optical birefringence data, and an excellent consistency has then been noted. The N-Sm-A pretransitional anomaly has been investigated and the effective specific heat capacity exponent α values have been yielded from the power-law analysis of our high-resolution Δɛ(T) data across the N-Sm-A transition for all investigated samples. It has been well documented that the incorporation of MWCNTs to the 8CB host leaves the N-Sm-A transition essentially bulklike. We discussed that, to some extent, the compliance of MWCNTs does not drive the N-Sm-A transition to 3D-XY-like behavior, that is the strength of de Gennes coupling will remain the same for all nanocomposites. This issue can be ascribed to the weaker nature of MWCNT disorder. In this work, for the first time, high-resolution Δɛ(T) data has been shown to be very adequate and offer an easy way to investigate the N-Sm-A transition as compared to calorimetric methods.

Peculiarities in the orientational dynamics caused by a thermomechanical force in a nematic microfluidic channel

Abstract We have described a numerical study of the hydrodynamic flow in a hybrid-aligned nematic (HAN) microfluidic channel excited by a thermomechanical force, caused by coupling between gradients of the director field ∇ n ^ and the vertically applied temperature gradient ∇T. Calculations show that due to the thermomechanical force, the microsized nematic layer being initially in the rest, if heated both from below or above, starts moving in the horizontal direction. It has been shown that the magnitude and direction of the hydrodynamic flow, excited by the vertically applied heat flux q, are influenced both by the magnitude and direction of q, whereas the character of the preferred anchoring of the LC molecules to the restricted surfaces is almost invisible. The role the pretransitional SmA fluctuations in the peculiarities in the relaxation processes, when the temperature on the bounding surfaces is reduced towards TNA, e.g., at a few tens of mK above a nematic-smectic-A (NA) phase transition temperature TNA, for hydrodynamic presmectic regime in 4 − n − octyl − 4′ − cyanobiphenyl HAN microfluidic channel also has been investigated.

Nematic to smectic-A phase transition in mixture of liquid crystal and nonmesogenic impurities: existence of tricritical point

ABSTRACTExperiments on the mixture of liquid crystals and nonmesogenic impurities showed the significant role of nonmesogenic impurities on the nematic–smectic-A phase transition. Using both Flory–Huggins theory of isotropic mixing and Landau–de Gennes theory, we present a phenomenological theory that discusses the role of such impurities on the nematic–smectic-A phase transition in a mixture of smectic liquid crystal and nonmesogenic impurities. We discuss the impact of nonmesogenic impurities on the order parameters, Frank elastic constants (splay and bend) and transition temperature of the nematic–smectic-A phase transition. Our theoretical results show that there exists a tricritical point for which the second-order nematic–smectic-A phase transition becomes first order at a tricritical point by increasing the concentration of nonmesogenic solute. We find a remarkable agreement between theoretical and experimental results.

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer.

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod–coil amphiphilic monolayer at the aqueous–LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic–smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic–isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

Birefringence in the vicinity of the smectic-Ato smectic-Cphase transition: Crossover fromXYcritical to tricritical behavior

High-resolution birefringence (Δn) measurements are carried out to probe the critical behavior at the smectic-A-smectic-C (Sm-A-Sm-C) phase transition in a binary system. The critical behavior of this transition is explored with the aid of a differential quotient extracted from the Δn values. The results obtained reveal that the Sm-A-Sm-C and nematic-smectic-A (N-Sm-A) transitions exhibit nonuniversal behaviors with effective exponents lying between the tricritical and three-dimensional XY values and follow two distinctly different curves with decreasing width of the Sm-A and N phases, respectively. The origin of such critical behavior is a unique feature for the respective phase transitions.

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

ABSTRACTWe investigated the temperature dependence of the splay elastic constant (K11) and rotational viscosity (γ1) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.

Effect of the hockey-shaped mesogen as an additive on the critical behaviour at the smectic A to nematic phase transition

We present systematic experimental data on high resolution measurements of temperature dependence of optical birefringence (Δn) in a binary liquid crystalline system comprising calamitic 4-cyano-4′-octyloxybiphenyl (8OCB) and a hockey stick-shaped compound, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). The hockey stick-shaped mesogen shows a smectic A (SmA) phase in addition to two polymorphic tilted smectic phases. From an analysis of the Δn data, a quotient Q(T) has been derived which exhibits power law divergences both above and below the smectic A-nematic (SmA-N) phase transition. A noticeable shrinkage in width of the nematic range has been observed for a small increase in concentration of the angular mesogenic dopant. In the vicinity of the SmA-N phase transition, the values of effective critical exponent (α′), characterizing the critical fluctuation at this transition, have appeared to be in between those predicted by the 3D-XY and tricritical hypothesis. Furthermore, with increasing hockey stick-shaped molecular concentration, the SmA-N transition shows a tendency to be driven towards a first order nature.

Strong Orientational Coupling of Block Copolymer Microdomains to Smectic Layering Revealed by Magnetic Field Alignment.

We elucidate the roles of the isotropic-nematic (I-N) and nematic-smectic A (N-SmA) transitions in the magnetic field directed self-assembly of a liquid crystalline block copolymer (BCP), using in situ X-ray scattering. Cooling into the nematic from the disordered melt yields poorly ordered and weakly aligned BCP microdomains. Continued cooling into the SmA, however, results in an abrupt increase in BCP orientational order with microdomain alignment tightly coupled to the translational order parameter of the smectic layers. These results underscore the significance of the N-SmA transition in generating highly aligned states under magnetic fields in these hierarchically ordered materials.

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

ABSTRACTAs a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4ʹ-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities αe and αo are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. αe and αo calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (ne-no) and this value of S is compared with those obtained from field models.

Critical behavior at the isotropic to nematic, nematic to smectic-A and smectic-A to smectic-C phase transitions in a pyrimidine liquid crystal compound

High-resolution optical birefringence (∆n) measurement of a pyrimidine liquid crystal compound having nematic, smectic-A and smectic-C phases are reported. The high-resolution ∆n data are rather successful in assessing the critical anomaly at different phase transitions in the investigated compound with a reasonably good accuracy. The critical exponent β, describing the limiting behavior of the nematic order parameter close to the isotropic–nematic (I–N) phase transition, is found to be in good agreement with the tricritical hypothesis. The critical behavior at the nematic–smectic-A (N–Sm-A) and the smectic-A–smectic-C (Sm-A–Sm-C) phase transitions has been explored with the aid of a differential quotient extracted from the ∆n values. The yielded effective critical exponent α′ is appeared to be nearly tricritical in nature for the N–Sm-A phase transition. For the Sm-A–Sm-C phase transition, α′ exhibits a weak dependence on the fit range and assumes tricritical value for large temperature range considered, which again is found to be diminished slightly with reduction in the temperature range. Related critical amplitude quotient and corrections-to-scaling quotient are found to display deviations from the theoretical models. Such behavior signals the appearance of a non-Landau character for the orthogonal to tilted smectic phase transition in the investigated compound.

Effect of hockey-stick-shaped molecules on the critical behavior at the nematic to isotropic and smectic-Ato nematic phase transitions in octylcyanobiphenyl

In the field of soft matter research, the characteristic behavior of both nematic-isotropic (N-I) and smectic-A nematic(Sm-A N) phase transitions has gained considerable attention due to their several attractive features. In this work, a high-resolution measurement of optical birefringence (Δn) has been performed to probe the critical behavior at the N-I and Sm-A N phase transitions in a binary system comprising the rodlike octylcyanobiphenyl and a laterally methyl substituted hockey-stick-shaped mesogen, 4-(3-n-decyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-dodecyloxycinnamate. For the investigated mixtures, the critical exponent β related to the limiting behavior of the nematic order parameter close to the N-I phase transition has come out to be in good conformity with the tricritical hypothesis. Moreover, the yielded effective critical exponents (α', β', γ') characterizing the critical fluctuation near the Sm-A N phase transition have appeared to be nonuniversal in nature. With increasing hockey-stick-shaped dopant concentration, the Sm-A N phase transition demonstrates a strong tendency to be driven towards a first-order nature. Such a behavior has been accounted for by considering a modification of the effective intermolecular interactions and hence the related coupling between the nematic and smectic order parameters, caused by the introduction of the angular mesogenic molecules.

Anomaly of Pretransitional Behavior at the Nematic–Smectic-A Phase Transition of Amphiphilic Liquid Crystals with a Hydrophilic Group

In order to clarify the origin of the X-ray diffraction peak corresponding to the smectic-like layer ordering appearing even in the nematic phase over a wide temperature range above the nematic-smectic A (NA) phase transition in liquid crystal (LC) molecules with a hydroxy group, we investigated the critical behavior of bend elastic constants and the correlation length of the smectic-like ordering in the N phase. It is found that cybotactic clusters with the transient layer ordering grow up extremely even far above the NA phase transition, and the critical exponent of the correlation length of the cybotactic clusters is estimated anomalously larger than that of conventional LC materials. Furthermore, we measured diffusion constants parallel and perpendicular to the director in the N phase, and concluded that cybotactic clusters with the smectic layer ordering create a finite potential barrier to prevent diffusion of the molecules parallel to the director as well as the true smectic A layer structure.

Critical behavior of a nonpolar smectogen from high-resolution birefringence measurements

We report high-sensitivity and high-temperature resolution experimental data for the temperature dependence of the optical birefringence of a nonpolar monolayer smectogen 4-butyloxyphenyl-4'-decyloxybenzoate (10[over ¯].O.4[over ¯]) liquid crystal by using a rotating-analyzer technique. The birefringence data cover nematic and smectic-A phases of the 10[over ¯].O.4[over ¯] compound. The birefringence data are used to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions. For the N-I transition, from the data sufficiently far away from the smectic-A phase, the average value of the critical exponent β describing the limiting behavior of S(T) is found to be 0.2507±0.0010, which is in accordance with the so-called tricritical hypothesis, which predicts β=0.25 and excludes higher theoretical values. The critical behavior of S(T) at the N-I transition is discussed in detail by comparing our results with the latest reports in the literature and we conclude that by comparing with the previously reported results, the isotropic internal field assumption by the Vuks-Chandrasekhar-Madhusudana model is adequate to extract the critical behavior of S(T) from the optical birefringence data. We observe that there is no discontinuous behavior in the optical birefringence, signaling the second-order nature of the N-SmA transition. The effect of the coupling between the nematic and smectic-A order parameters on the optical birefringence near the N-SmA transition is also discussed. In a temperature range of about 4K above and below the N-SmA transition, the pretransitional evidence for the N-SmA coupling have been detected. From the analysis of the optical birefringence data above and below the N-SmA transition by means of various fitting expressions we test the validity of the scaling relation λ=1-α between the critical exponent λ describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent α. We then show that the temperature derivative of the nematic order parameter S(T) near T(NA) exhibits the same power-law divergence as the specific heat capacity with an effective critical exponent of 0.2303±0.0035.

Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.

ASYMPTOTICS OF MINIMIZERS OF VARIATIONAL PROBLEMS IN A MULTI-CONNECTED DOMAIN ASSOCIATED WITH THE THEORY OF LIQUID CRYSTALS

We consider the asymptotic behavior of minimizers of the Ericksen functional with the Dirichlet boundary condition in a general domain. We examine the asymptotic behavior of minimizers of the functional as some of the elastic coefficients tend to the infinity, according to the de Gennes predictions with respect to nematic smectic-A phase transition.

Phase Transition, Birefringence and Orientational Order Parameter Studies in Four Members of Benzylidene Aniline Liquid Crystals

The nature of the phase transitions across isotropic–nematic (IN) and nematic–smectic-A (N-SmA) are carried out using density measurements in N-(p-n-ethoxy benzylidene)-p-n-pentyl aniline, 2O.5, N-(p-n-methoxy benzylidene)-p-n-pentyloxy aniline, 1O.O5, N-(p-n-ethyl benzylidene)-p-n-dodecyl aniline, 2.12 and N-(p-n-butyl benzylidene)-p-n-dodecyl aniline, 4.12. The compounds exhibit IN, IN, IN and INSmA phase sequences respectively with varying thermal ranges suitable for the study of birefringence. The birefringence studies are carried out using Newton’s rings and wedge techniques which provide δn = (ne- no) (the birefringence) ne, and norespectively. The orientational order parameter, S is evaluated (i) directly from δn and Δn (the birefringence in perfect order), independent of the local field that the nematic molecule experiences and (ii) from molecular polarizabilities estimated from ne, noand density values assuming isotropic as well as anisotropic field that the nematic molecule experiences. The estimated order parameter from the different techniques is compared with one another and with the literature data.

A single-site anisotropic soft-core model for the study of phase behavior of soft rodlike particles

A novel mesoscopic simulation model is proposed to study the liquid crystal phase behavior of the anisotropic rodlike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. The influences of the repulsion strength and the particle shape on the phase behavior of soft rodlike particles are examined. In the simulations, we observe the formation of the nematic phase and smectic-A phase from the initially isotropic phase. Moreover, we find that shorter soft rodlike particles with anisotropic repulsive interactions can form a stable smectic-B phase. Our results demonstrate that the soft anisotropic purely-repulsive potential between the rodlike particles can reflect the interaction nature between soft rodlike particles in a simple way and is sufficient to produce a range of ordered LC-like mesophases.