1,720,982 research outputs found
Differential dynamic microscopy of fluctuating liquid crystals
No full textFluctuations of the director orientation in liquid crystalline samples can reveal precious information about their viscoelasticity. Laser light scattering (LLS) is a well established tool for extracting such information and has been extensively used for a long time [1]. Although these fluctuations can be easily seen in depolarized microscopy with the naked eye, only in a few isolated cases a quantitative study was attempted [2-4]. We present here experimental results obtained with the recently introduced Differential Dynamic Microscopy (DDM) [5,6] on thin layers of nematic liquid crystals (LC). DDM allows to obtain scattering information from the study of microscopy images. We show that depolarized DDM is perfectly suitable to determine the viscoelastic properties of thin layers of nematic LC, providing direct access to the intermediate scattering function at small scattering wavevectors, which are precluded to ordinary LLS. The differential nature of the technique allows also relaxing the strict cleanliness requirements typically needed in LLS experiments. With a single experiment less than 4 s long, all the three viscoelastic ratios can be measured in a LC sample with suitable alignment, thereby demonstrating a very powerful tool for the rapid characterization of LC. Our results, in agreement with literature values, suggest a routine use of microscopes for the determination of the viscoelastic properties of thermotropic and lyotropic LCs in harsh conditions and for the characterization of various optically anisotropic fluids. References
[1] H. F. Gleeson in Handbook of Liquid Crystals, edited by D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess (Wiley-VCH, Halle, 2008), pp. 699-718.
[2] Y. Galerne, I. Poinsot, and D. Schaegis, Appl. Phys. Lett. 71, 222 (1997)
[3] H. Orihara, A. Sakai, and T. Nagaya, Mol. Cryst. Liq. Cryst. 366, 143 (2001)
[4] A. Yethiraj, R. Mukhopadhyay, and J. Bechhoefer Phys. Rev. E 65, 021702 (2002)
[5] R. Cerbino, and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)
[6] F. Giavazzi, D. Brogioli, V. Trappe, T. Bellini, and R. Cerbino, Phys. Rev. E 80, 031403 (2009
Scattering information obtained by optical microscopy: Differential dynamic microscopy and beyond
No full textWe describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for (a) the three-dimensional nature of the sample, (b) the arbitrary coherence properties of the light source, and (c) the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique [R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)]. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy
Differential dynamic microscopy: probing wave vector dependent dynamics with a microscope
No full textWe demonstrate the use of an ordinary white-light microscope for the study of the q-dependent dynamics of colloidal dispersions. Time series of digital video images are acquired in bright field with a fast camera, and image differences are Fourier analyzed as a function of the time delay between them. This allows for the characterization of the particle dynamics independent of whether or not they can be resolved individually. The characteristic times are measured in a wide range of wave vectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium
Reti di fibre colloidali sotto sforzo : deformazione di taglio e carico gravitazionale
No full textWe describe our investigation of the yield behavior of a network of hydrophobic fibers in an aqueous surfactant solution, when submitted either to a macroscopic shear stress or to the gravitational load of density-mismatched particles. In both cases, the gel undergoes an initial, slow deformation followed by delayed yielding or respectively collapse. However, the time scales for the yielding process strongly differ from each other both on an absolute scale and in their dependence on the effective stress applied. We discuss the analogies, the differences and the interplay between these destabilization modes
Colloidal fiber networks under stress : shear versus gravitational load
No full textColloidal gels are widely used in applications to modify the mechanical properties of fluids; in particular, they can provide yield stress, i.e. the ability to display solid-like properties at rest, while flowing upon application of a mechanical load. In this contribution we describe the yield behavior of a colloidal fiber network when it is subjected either to a macroscopic shear stress, or to a gravitational load obtained by inserting density-mismatched particles into the network. Delayed yielding is observed in both experiments. However, the time scales for the yielding process strongly differ from each other both on an absolute scale and in their dependence on the effective stress applied. We discuss how delayed yielding depends on background viscosity, intrinsic network relaxations, and on network porosity and elasticity
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
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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