1,721,082 research outputs found
Analysis of UV Assisted Phase Transitions in Mixtures of Liquid Crystals with Photochromic Compounds by Photopyroelectric Calorimetry
No full textIn this work, an upgraded photopyroelectric setup has been used to investigate phase transitions in liquid crystals dispersed with photochromic molecules. In particular, we have studied the thermal diffusivity before, during, and after the photo-induced conformational changes of the dispersed photochromic molecules. Upon sample irradiation, an opposite shift of the phase transition temperatures were observed in the investigated 8CB–NP and 8CB–7AB mixtures due to the different changes in the shape undergone by the dispersed active molecules
Recent developments in photopyroelectric calorimetry for phase transition studies in liquid crystals
No full textThermophysical Parameters Evaluation by Pyroelectric Detection
No full textThe different configurations of the photopyroelectric calorimetry which can be adopted for the evaluations of the thermophysical parameters in liquids and in solids, and the advantages, with respect to other calorimetric techniques, are briefly described. A review of the recent results obtained by the photopyroelectric back-detection configuration for the determination of the different thermophysical properties in various materials is presented with particular attention to the cases where the temperature dependence of such parameters is studied. The results concerning the studies carried out by performing simultaneous calorimetric and imaging evaluations are also described. Finally, the possibility of performing “absolute” measurements of the thermal parameters, eliminating the influence of eventual coupling fluid, is discussed
Photothermal approach for cultural heritage research
No full textOver the last few years, there has been an increasing interest in the application of photothermal techniques to the investigation of cultural heritage. Thanks to their peculiar ability of depth-resolving the position of subsurface features, these techniques are in fact well suited for a nondestructive analysis of multi-layer structures such as the one typical of artwork. In addition, the recent availability of highly developed instrumentation makes it possible to effectively carry out in situ investigations into different kinds of artwork. Such circumstances have created new opportunities in both theoretical and experimental research whose development is still in progress. In this work, we report an overview of the results that have been obtained on several kinds of artwork and of further developments that can be used to improve the effectiveness of the application of photothermal techniques to this field
Mathematical models for infrared analysis applied to cultural heritage
No full textActive pulsed infrared thermography is an effective technique consisting in moderately heating the specimen by means of the absorption of a visible light pulse and, then, in detecting the transient variation in the emitted infrared radiation by an infrared camera. Inhomogeneities and buried features eventually located into the specimen volume can be revealed by the recorded infrared images. Such a technique has been successfully applied to the analysis of cultural heritage artifacts like ancient bronzes and manuscripts. The former belong to the category of optically opaque materials, whereas the second to the one of optically semi-transparent materials. For both the two considered categories, a mathematical model for the analysis of the thermographic signal is here presented, together with an implementation in Matlab environment using the finite element technique. The developed models are then used to analyse the experimental results and, hence, to obtain both qualitative and quantitative information about the investigated items
Time resolved calorimetric investigations of photoinduced isomeric molecular changes in the nematic phase of an azobenzene compound
No full textBy monitoring the time dependence of the thermal conductivity of a p,p’-diheptylazobenzene sample irradiated with UV light of various intensities, a quantitative study is presented on the effects played by the photo-induced trans to cis isomerization on the degree of the homeotropic alignment in its nematic phase. In particular, the kinetics of the changes eventually leading to the nematic–isotropic phase transformation has been investigated. Unlike the case of the thermal diffusivity analyzed in a recent study, the thermal conductivity analysis has enabled the quantitative evaluation of the trans–cis transformation rates since it is not affected by the specific heat critical behavior and, therefore, it is determined only by changes in the molecular structure induced by the photo-isomerization. The study has been possible thanks to the peculiarity of the adopted photopyroelectric calorimetric setup that enabled both the calorimetric evaluations, as well as the sample texture characterization, to be performed during the UV illumination of the sample
A note on the early thermographic approaches for the investigation of the Cultural Heritage
No full textIn this paper, we present an overview of some of the early applications of Infrared Thermography (IRT) to Cultural Heritage (CH) where Ermanno Grinzato, to whom the present issue is dedicated, was the leading scientist. In such a presentation, the works of arts have been divided into two main categories, namely movable and immovable CH. For each kind of CH, the good quality of the results that have been obtained despite the technical limitations of the adopted experimental equipment is highlighted, having such early results played a crucial role in the subsequent developments, thanks to which IRT has been nowadays established as a valuable technique for the CH surveys
Optically induced effects on the nematic-smecticA and smecticA-smecticC phase transitions critical behaviour in photochromic liquid crystal mixtures with different nematic range
No full textThe effect of the UV light irradiation on the characteristics of the nematic-smectic-A and smecticA-smecticC phase transitions is investigated by the photopyroelectric calorimetric technique in mixtures of photochromic naphtopyran molecules with the 8 ̄S5 liquid crystal and the results are compared with those previously obtained in mixtures with the 8CB liquid crystal having a smaller nematic range. In 8 ̄S5, where the transition shows 3D-XY-like characteristics, the photoisomerization of the naphtopyran molecules to a more planar and elongated “open” form did not affect significantly the critical behaviour over the nematic-smecticA transition, while in 8CB it shifted it from a non universal to a the 3D-XY-like one. The results are discussed in terms of the effects caused by the naphtopyran molecules in their “open” form being able to be hosted in the smectic layers, while remaining confined between the planes when in their “closed” form. Similar arguments are used to also justify the dramatic increase of the transition temperature induced at the smecticA-smecticC phase transition by the UV irradiation of the 8 ̄S5 sample. Results obtained also in 8 ̄S5 -silica nanoparticles mixtures are also reported for comparison and discussed
Photopyroelectric Investigation of the trans–cis Isomerization Effect on the Nematic- Isotropic Phase Transition of a Liquid Crystalline Azobenzene
Full text linkPhotopyroelectric calorimetry has been applied to the study of the effects of different concentration of photo-induced cis isomeric molecules on the characteristics of the nematic – isotropic phase transition in p,p’-diheptylazobenzene (7AB) samples. The investigations could be carried out by monitoring the behavior of the thermal diffusivity over the phase transition before and during the sample irradiation with UV light of varying intensity. Among other, it is shown that the monitoring of the time dependence of the thermal diffusivity at fixed sample temperature enabled the comparative study of the dynamics of the trans–cis and cis–trans isomeric transitions. It was shown that the increasing cis isomers concentration caused an increase in the fraction of material undergoing the phase transitions and increased the cis–trans conversion rate. Finally, polarization microscopy observations of the sample texture, which could be carried out together with the calorimetric evaluations proved useful for a more comprehensive analysis of the obtained results
Simultaneous specific heat, thermal conductivity and imaging evaluations in thin samples of 8CB liquid crystal dispersed with microemulsion of DDAB/water micelles
No full textIn this work we use a photopyroelectric calorimetric set up to analyze the transformations occurring in thin samples of liquid crystal-microemulsion system by combining the results of the simultaneously performed high temperature resolution polarization microscopy observations and the temperature dependence of the specific heat, and of the thermal conductivity. This enabled an accurate direct correlation of the observations concerning the different measured quantities for a most comprehensive interpretation of the obtained results. It is found that the I-N conversion progresses in the entire nematic range because of the micelles expelled from the nucleating nematic volumes remaining confined in the residual isotropic material. A hysteretic behavior is observed in the nematic range between cooling and heating run because of the different micelle content in the material during the cooling and heating runs. A moderate hysteresys also over the N-A transition indicated an additional material refinement from micelles during the transition. Finally in samples with different micelle concentration the same maximum micelle content is retained in the nematic and smectic phases
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