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CaBER vs ROJER - Different time scales for the thinning of a weakly elastic jet
Get PDFIn this paper, we demonstrate that the capillary thinning dynamics of a weakly viscoelastic jet follow a different timescale than a liquid bridge of the same fluid between two stationary surfaces for similar geometrical scales. The thinning in the latter case observed with capillary breakup extensional rheometry (or CaBER) follows a well established scaling of the radius with time for an elasto-capillary (EC) balance of R ∼ exp (- t / 3 λ). However, for the thinning of the filaments between droplets in a jet, it was so far just assumed that the same scaling law holds. In this paper, we experimentally demonstrate that the jet thinning in a Rayleigh-Ohnesorge jetting extensional rheometer (or ROJER) follows a different scaling of R ∼ exp (- t / 2 λ). This is demonstrated by a direct comparison of the thinning dynamics of weakly viscoelastic (O h < 0.01) aqueous solutions of polyethylene oxide in the two experimental setups, covering a wide range of jetting velocities or Weber numbers of 1-70. We demonstrate outgoing from a momentum balance that includes inertia and elasticity that this difference in scaling is due to a constant axial tension in the jet arising from the constant creation rate of new surface at the nozzle. Numerical simulations using the FENE-P model support this theory and demonstrate that in the exponential thinning regime of the jet the elastic stresses are indeed balanced by the axial tension (rather than by capillary pressure as in the EC balance regime of the CaBER experiment). It is readily shown from the reduced stress balance that this axial-elastic balance regime in the ROJER experiment leads to a faster exponential thinning, following the new scaling of R ∼ exp (- t / 2 λ) that was experimentally observed. Furthermore, we observe both in experiment and simulation that a jet thinning does not exhibit a self-similar structure of the corner region where the thinning filament connects to the drop as it is generally observed for a filament with an axial tension decaying with the filament radius as in the CaBER. The resulting difference of 50% in extensional relaxation time λ extracted from ROJER experiments might require one to revisit previously reported ROJER experiments and is required for the correct evaluation of future jetting rheometry experiments
Filament stabilisatie tijdens vrije-oppervlaktestromingen van complexe vloeistoffen
No full textThis doctoral dissertation discusses the capillary-driven breakup of various low viscous non-Newtonian liquid filaments. A liquid filament is destabilised by surface tension that drives the free surface to minimise its surface energy by forming spherical droplets. These surface tension-driven flows are encountered in the form of liquid jets, dripping faucets and stretched filaments between two solid surfaces. The thinning dynamics during this free-surface flow are determined bynbsp;dominant resisting stress, which depends on the relative magnitude of fluid properties such as density, viscosity and elasticity.
Since a homogeneous uniaxial extensional flow field isnbsp;during the pinching process, free-surface flows are used as a characterisation method for the extensional rheological properties of low viscous liquids. We have enhanced the performance of the commercial Capillary Breakup Extensional Rheometer (CaBER) that creates an unstable liquid bridge by rapidly stretching a droplet of fluid between two plates and monitors the filament radius. The spatial and temporal resolution of the setup is improved by using a high-speed camera equipped with a custom-made microscopic tube lens to follow the filament thinning. Moreover, the image processing routines are substantially improved by employing advanced edge detection techniques to locate the position of the filament edges with sub-pixel accuracy. We are now able to quantify relaxation times in complex fluids of O(100 µs), which are the smallest relaxation times that have been reliably obtained.
The CaBER setup is further used to study two model complex fluids that display an opposite stabilisation effect on a filament. First, we examine the breakup dynamics of long stable jets of weak viscoelastic fluids. Capillary instabilities arise in a stable zone after the nozzle at wave numbers that are in line with predictions from linear stability analysis. This zone is followed by a short inertia-capillary regime where the flexible polymer chains are efficiently unraveled, resulting in strong non-linear elastic stresses that stabilise the filament during the important elasto-capillary regime. In this regime, the minimal filament radius decreases exponentially in time with a time scale that is proportional to the relaxation time of the fluid. By comparing the thinning dynamics during jetting and static CaBER experiments we observe a new time scale of twice the relaxation time, which implies that jets breakup faster than static liquid bridges of the same elastic fluid.nbsp;new scaling is theoretically explained with a stress balance over the filament, where the creation of surface below the nozzle generates a constant tensile force in the jet. Numerical simulations of the jet breakupnbsp;a non-linear dumbbell model (FENE) are in agreement with the experimental observation by selecting a proper value for the initial deformation of the polymer molecules.
The drop formation of non-colloidal suspensions in a Newtonian matrix is examined as a second model system, in which thenbsp;is locally destabilised during the pinching process. The breakup dynamics are explained by dividing the process into four stages. Initially, the suspension behaves as a homogeneous fluid and the suspensionnbsp;dominates thenbsp;resulting in a linear decrease of the filament radius in time. As the filament thins, local particle density fluctuations are amplified and the filament becomes heterogeneous with diluted zones that exhibit faster thinning rates due to the local decrease in viscosity. The onset of this concentration fluctuation regime is defined with a transition radius and a scaling for this radius is proposed based on the standard deviation of the mean inter-particle distance and the relative dependence of the viscosity to small changes the volume fraction. Third regime starts when a particle-free section is formed between two particle clusters. The breakupnbsp;a small-scale filament stretchingnbsp;and the thinning rate reaches a maximum. Finally, a deceleration is detected before breakup, where the depleted filament becomes slender again and the thinning rate approaches that of the pure medium fluid.status: Publishe
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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