1,720,959 research outputs found
Effective interactions and phase behavior of colloidal dispersions.
Get PDFThe thesis deals with a number topics concerning the equilibrium properties and the phase behavior of colloidal suspensions, a class of physical systems that have attracted a broad interest during the last few decades, serving both as a valuable test bench for ideas in condensed matter physics and as building blocks for novel materials. An emphasis is placed on the notion of effective interaction, which enables the construction of simple models that are studied by use of theory and computer simulation. The questions addressed include the thermodynamics of particles interacting through a short-ranged attraction, fluctuation-induced (critical Casimir) forces, and the effect of polydispersity on the freezing transition of charged colloids
Enhancement of depletion forces by electrostatic depletant repulsion
No full textA large variety of engaging phenomena stems from the occurrence of short-ranged attractive depletion forces. Yet, so far, most experimental studies have been interpreted on the basis of the simple Asakura–Oosawa model, where the depletion agent can be regarded as ideal. Here, conversely, we focus on a system where strong electrostatic coupling is present in the suspension. Specifically, from measurements of equilibrium sedimentation profiles, we obtain an equation of states for a colloidal system where depletion forces are tuned by the addition of a surfactant. At fixed colloid volume fraction, colloidal aggregation takes place when the surfactant concentration reaches a critical value which rises for increasing ionic strength. Screening repulsive electrostatic interactions inhibits the depletion mechanism and weakens the effective colloid-colloid attraction. The metastable coexistence curve displays the universal scaling behavior predicted for short-ranged potentials. The experimental data are compared with the theoretical predictions of a simple model which includes only electrostatic interactions. The effective depletion force on the colloids is evaluated by using the hypernetted-chain equation of liquid state theory at different salt concentrations. This model provides a convincing interpretation of the observed enhancement of the depletion mechanism by Coulomb repulsion
An investigation of SCOZA for narrow square-well potentials and in the sticky limit
No full textWe present a study of the self-consistent Ornstein-Zernike approximation (SCOZA) for square-well (SW) potentials of narrow width . The main purpose of this investigation is to elucidate whether, in the limit 0, the SCOZA predicts a finite value for the second virial coefficient at the critical temperature B2(Tc), and whether this theory can lead to an improvement of the approximate Percus-Yevick solution of the sticky hard-sphere (SHS) model due to Baxter [J. Chem. Phys. 49, 2770 (1968)]. For the SW of non-vanishing , the difficulties due to the influence of the boundary condition at high density, already encountered in an earlier investigation by Scholl-Paschinger et al. [J. Chem. Phys. 123, 234513 (2005)], prevented us from obtaining reliable results for 0.1. In the sticky limit, this difficulty can be circumvented, but then the SCOZA fails to predict a liquid-vapor transition. The picture that emerges from this study is that, for 0, the SCOZA does not fulfill the expected prediction of a constant B2(Tc) [J. Chem. Phys. 113, 2941 (2000)], and that, for thermodynamic consistency to be usefully exploited in this regime, one should probably go beyond the Ornstein-Zernike ansatz
Critical depletion
No full textDepletion interactions and the critical Casimir effect are usually regarded as distinct phenomena in colloidal suspensions. By experimentally investigating how the Asakura-Oosawa picture, appropriate for a weakly correlated depletant, is modified when critical correlations develop within the depletion agent, we conversely show that the former merges continuously into the latter, leading to a distinctive scaling behavior solely dictated by the depletant correlation length. A model based on density functional theory provides a microscopic understanding of the phenomenon and properly accounts for the observed trends
When depletion goes critical
No full textDepletion interactions in correlated fluids are investigated both theoretically and experimentally. A formally exact derivation of a general expression for depletion interactions is presented and then specialized to the case of critical correlations in the depletant by employing a long wavelength approximate analysis. A scaling expression is obtained in the critical region, suggesting a close connection to the critical Casimir effect. As a result we are able to compute the full scaling function of the critical Casimir effect in terms of the known scaling form of the depletant equation of state. These predictions are experimentally tested in a colloidal suspension with a micellar solution as depletion agent. Colloids are seen to aggregate reversibly when the micellar concentration exceeds a temperature dependent value which becomes remarkably small as the temperature approaches the lower consolution point of the micellar suspension. Continuity between the standard depletion picture at low temperature and the Casimir effect in the critical region is demonstrated by identifying several approximate scaling laws which compare favorably with the theoretical analysis. The transition line is seen to lie close to the curve of maximum susceptibility of the depletant. A model, analyzed within mean field approximation, is shown to reproduce the main qualitative features of the phenomenon
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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