1,721,390 research outputs found
The role of configurational entropy in chemical vitrification
No full textGlasses can be formed in numerous ways, involving very different microscopic processes. This article reviews some recent results on chemical vitrification, a process where the slowdown in the dynamics of a liquid is controlled by the irreversible formation of chemical bonds. Making a connection between the reduction in configurational entropy and the number of chemical bonds, the dynamics of vitrification in chemically reactive systems is explained in terms of their configurational restrictions in the same manner as in stable glass-forming liquids under cooling or compression
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
A simple analysis of Brillouin spectra from opaque liquids and its application to aqueous suspensions of poly-N-isopropylacrylamide microgel particles
No full textBrillouin spectroscopy is a powerful technique to probe the viscoelastic properties of materials. However, the phenomenon of multiple scattering makes getting information from opaque liquids quite difficult, thus limiting the use of this spectroscopy. In this paper we present a new method that greatly simplifies the problem of analyzing Brillouin spectra affected by multiple scattering from samples of moderate opacity. Our approach is based on the observation that multiple-scattered contributions broaden the spectrum acquired in external backscattering geometry, while preserving in the external side the information related to internally backscattered light. The new strategy avoids unnecessary approximations and requires minimum numerical effort to extract physical information. Here, we show the results of two Brillouin light scattering experiments performed on prototypical hard and soft colloidal systems. First, measurements on latex suspensions as a function of depth are used to validate the method and to derive new relations between the back-scattered and multiple-scattered components of the Brillouin spectrum. Second, measurements on poly-N-isopropylacrylamide (PNIPAM) microgels in water as a function of temperature are used as a testing ground to demonstrate the method's capabilities. Our analysis confirms that sound waves are extremely sensitive to the volume-phase transition of thermoresponsive particles. The presented approach, however, shows that a marked increase of attenuation is accompanied by only a moderate decrease of sound velocity. The study revises the viscoelastic properties of PNIPAM suspensions; more generally, it provides a new guideline in the characterization of moderately opaque media and fosters new theoretical investigations
Configurational and excess entropy in a fragile glass former and their relation with the structural relaxation
No full textA method to derive experimentally the configurational entropy that governs the structural relaxation process in supercooled (supercompressed) liquids is presented. Light scattering, dielectric, calorimetric and dilatometric measurements are used to test the method in o-terphenyl. The combined analysis of relaxation data as a function of temperature and pressure is found to be essential to quantify a realistic value of the non-structural thermal expansion and the configurational entropy of the system. The latter is estimated as a fraction of the excess entropy of the liquid over its stable crystalline phase. Our results indicate that the number of configurations connected to the structural relaxation contributes approximately 70% to the excess entropy
Can experiments select the configurational component of excess entropy?
No full textConfigurational entropy is frequently used to rationalize the structural dynamics of glass-forming liquids. The main problem with this concept is that it is not directly accessible to experiments. We introduce a procedure to estimate the configurational component of the excess entropy of a liquid --specifically, the configurational-entropy contribution from the structural relaxation process-- through a combined investigation of dynamic and thermodynamic properties as functions of temperature and pressure. We test our method on orthoterphenyl, salol, and glycerol, and find that the fraction of excess entropy that arises from structural configurations is about 70% for all three materials
Comment on “Decrease in the configurational entropy during a melt's polymerization” [Chem. Phys. 305 (2004) 231]
No full textComment on "Decrease in the configurational entropy during a melt's polymerization" [Chem. Phys. 305 (2004) 231]
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