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Electric-field driven director oscillations in nematic liquid crystals
No full textWe have used deuterium NMR spectroscopy to investigate the director dynamics and equilibrium behaviour in nematic liquid crystals (4-pentyl and 4-octyl-4?-cyanobiphenyl (5CB and 8CB) both specifically deuteriated) when subject to magnetic and ac electric fields. The angle between the magnetic and electric fields can be varied between 0 and 90° and the most common geometry we have used is for an angle of about 45°. For 5CB and 8CB (with positive and ) the director orientation was measured using time-resolved NMR both when the electric field is applied and when it is turned off. In all cases it was found that the director alignment was uniform and the director relaxation follows closely the predictions of the torque-balance equation given by the Leslie-Ericksen theory. In all these experiments we have employed a 10kHz electric field; at such a relatively high frequency the director experiences an effectively constant value of the electric field.
We have now investigated the behaviour of the nematic director for the two liquid crystals at much lower frequencies of the electric fields: several Hz to about 1000Hz. As before, the director orientation was measured using time-resolved deuterium NMR spectroscopy. We have employed two geometries. In one, the electric and magnetic fields were inclined at ~ 50° We found that the director oscillates between two extreme orientations (determined by the frequency and the field strength) in a plane formed by the magnetic and electric fields. The oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The director was found to remain uniformly aligned. The two extreme director orientations can also be determined from the NMR spectrum time-averaged over many thousands of cycles of oscillations. At low frequencies (several Hz) these limiting angles are essentially independent of frequency but as the frequency increases so the two angles approach each other and become equal at high frequencies.
More recently, we have used a geometry with the angle between the fields of ~90º. A threshold behaviour is observed in this geometry for the director orientation as a function of the applied voltage. The time-averaged spectra at low frequencies and at certain voltages showed unusual powder-like features. Time-resolved NMR measurements at 40Hz and different voltages near the threshold value were carried out to understand the oscillatory behaviour which was also simulated. Turn-on and turn-off dynamics at high frequency were conducted revealing intriguing differences between the two pathways for the field-induced relaxation. These results will be discussed and interpreted in terms of the torque-balance equation with a time dependent electric field
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Electric field-induced alignment of the directors in the smectic A phase of 4-octyl-4'-cyanobiphenyl. A deuterium NMR study
No full textDeuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-?,?-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment was monitored by recording the deuterium NMR spectra as a function of time after the electric field was switched on. The results reveal a complex pattern of electric field-induced director alignment. At high electric field strengths a rapid process is observed in which the director switches from an orientation parallel to the magnetic field to one in which it is parallel to the electric field. An induction period is also observed in which no apparent change in director orientation occurs. This induction period becomes longer (hours in magnitude) as the electric field strength is lowered. Other, intermediate, director orientations are observed as the electric field strength is lowered further. The role of defects is invoked in trying to interpret some of the observed processes underlying the mechanism of the director alignment in the smectic A phase
The alignment of the smectic a phase of 4-octyl-4'-cyanobiphenyl induced by an electric field. A time-resolved deuterium NMR study
No full textStudies of the field-induced alignment of the SmA phase using deuterium NMR spectroscopy have revealed a complex pattern of behaviour when the director is initially orthogonal to the aligning field. Here we report the electric field-induced alignment of the SmA director using time-resolved deuterium NMR when the aligning electric field E is at an angle with the magnetic field B of the spectrometer which is considerably smaller than 90degrees; here the director is initially aligned parallel to the magnetic field. The dynamics of the electric field-induced alignment of the director for the smectic phase of 4-alpha,alpha-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)) was investigated at two angles between B and E of roughly 45degrees and 54.5degrees the so-called magic angle, at different electric field strengths and also at two temperatures, 305.1 K and 302.6 K The dynamics of the SmA director alignment of 8CB-d(2) was monitored by measuring the deuterium NMR spectrum as a function of time. The results for the 45degrees and 54.5degrees geometries revealed, in contrast to the complex dynamics of alignment of the SmA director observed for the 90degrees geometry, a much simpler pattern of relaxation. Here the director appeared to be aligned almost as a monodomain. In general, for either of the two geometries employed here, the ultimate angle of alignment the director achieves relative to E depends on the electric field strength. Lowering the temperature by just 2.5degreesC from 305.1 K to 302.6 K has a dramatic effect on the rate of director relaxation presumably because of the large increase in the combined rotational viscosity and the elastic energy effects of the SmA phase with decreasing temperature. Furthermore for the 45 geometry at 302.6 K, the SmA sample separates on relaxation ultimately into two domains with different alignment angles
Field-induced director dynamics in the nematic phase of 4-octyl-4'-cyanobiphenyl. A deuterium NMR investigation
No full textThe response times of liquid crystal display devices are determined by a range of factors but the most important of these is usually the rotational viscosity coefficient, gamma(1). In order to understand the relationship between molecular structure and viscosity it is of considerable interest to measure this viscosity coefficient for a variety of nematogens. Here we report the determination of gamma(1) for 4-octyl-4'-cyanobiphenyl-d(2) at two temperatures using deuterium NMR spectroscopy. In these experiments the time taken for the alignment of the director can be changed by a field, either magnetic or electric. To do this the liquid crystal film was enclosed in a cell which allows the application of an electric field within the NMR spectrometer. The rate of director relaxation was followed by recording the deuterium NMR spectrum as a function of time during the process of turning the electric field on or off. We have carried out the experiments for a geometry in which the director orientation with respect to the magnetic field of the spectrometer does not exceed 45degrees. The alignment of the director throughout the relaxation process was observed to be uniform. The director relaxation was found to follow closely the predictions of the torque-balance equation given by the Leslie-Eriksen theory. The relaxation times for the turn-on and turn-off processes were determined from this equation and found to be of the order of 1-2 ms. A knowledge of the anisotropic electric and magnetic susceptibilities then allows the determination of the rotational viscosity coefficient
A deuterium nuclear magnetic resonance investigation of the director distribution in a thin nematic liquid crystal slab
No full textThe director distribution in a thin nematic liquid crystal (NLC), 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the ex-position of the pentyl chain, confined between two glass plates, with untreated and treated anchoring conditions, has been investigated using a deuterium nuclear magnetic resonance (NMR). The NMR spectra have been measured as a function of the applied electric field. In the absence of surface forces it is found that the director aligns parallel to the magnetic field at relatively low values of the electric field as Delta is positive for 5CB. In the presence of surface forces with increasing electric field the quadrupolar splitting decreases, passes through zero and then increases again to a value which is essentially half of that at zero electric field. That is, the director orientation changes more or less continuously from being parallel to the magnetic field to being orthogonal to it, as the electric field grows
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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