1,720,976 research outputs found
DEPOSITION OF MINERAL COLLOIDS ON ROUGH ROCK SURFACES
No full textDeposition of colloids on mineral and rock surfaces is an important mechanism to alter surface reactivity and to govern contaminant migration. Particle retention in aquifers occurs predominantly under electrostatically unfavorable conditions owing to the prevailing negative charge of both mineral colloids and rock surfaces. Mineral and rock surfaces show often an irregular surface topography and roughness variations over several orders of magnitude. This complicates the colloid-surface attachment predictability and results in poor understanding towards retention efficiency. Here we study the impact of submicron-scale morphology on the interaction between rock surfaces and mineral colloids. Colloid retention experiments using micrite surfaces were performed under electrostatically unfavorable conditions. Results showed a positive and linear correlation between adsorbed particle density and surface roughness (RMS roughness <100 nm). The existence of a minimum roughness range, required for initial colloid deposition was detected. Deposition occurred mainly at micrite grain boundaries that acted as surface steps. Linear deposition kinetics was found at constant flow rates. The site-specific impact of surface roughness was studied using nanostructured silicon wafer surfaces as a well-defined analog material. Experimental results from deposition on such surfaces suggest that the surface step density on collector surfaces is a critical parameter for quantitative prediction of colloid deposition. The importance of colloidal retention is highlighted for the diagenetic evolution of rocks, especially due to inhibition mechanisms that has consequences for cement mineral distribution and concentration as well as resulting reservoir quality. For further quantitative prediction and modeling of retention on rock surfaces, we suggest the application of an energy potential function that includes beside DLVO contribution the impact of particle kinetic energy (via fluid-flow velocity) as well as the impact of reactive site density (via surface roughness parameter data)
Deposition of Latex Colloids at Rough Mineral Surfaces: An Analogue Study Using Nanopatterned Surfaces
No full textDeposition of latex colloids on a structured silicon surface was investigated. The surface with well-defined roughness and topography pattern served as an analogue for rough mineral surfaces with half-pores in the submicrometer size. The silicon topography consists of a regular pit pattern (pit diameter = 400 nm, pit spacing = 400 nm, pit depth = 100 nm). Effects of hydrodynamics and colloidal interactions in transport and deposition dynamics of a colloidal suspension were investigated in a parallel plate flow chamber. The experiments were conducted at pH similar to 5.5 under both favorable and unfavorable adsorption conditions using carboxylate functionalized colloids to study the impact of surface topography on particle retention. Vertical scanning interferometry (VSI) was applied for both surface topography characterization and the quantification of colloidal retention over large fields of view. The influence of particle diameter variation (d = 0.3-2 mu m) on retention of monodisperse as well as polydisperse suspensions was studied as a function of flow velocity. Despite electrostatically unfavorable conditions, at all flow velocities, an increased retention of colloids was observed at the rough surface compared to a smooth surface without surface pattern. The impact of surface roughness on retention was found to be more significant for smaller colloids (d = 0.3, 0.43 vs. 1, 2 mu m). From smooth to rough surfaces, the deposition rate of 0.3 and 0.43 mu m colloids increased by a factor of similar to 2.7 compared to a factor of 1.2 or 1.8 for 1 and 2 mu m colloids, respectively. For a substrate herein, with constant surface topography, the ratio between substrate roughness and radius of colloid, Rq/r(c), determined the deposition efficiency. As Rq/r(c), increased, particle-substrate overall DLVO interaction energy decreased. Larger colloids (1 and 2 mu m) beyond a critical velocity (7 X 10(-5) and 3 X 10(-6) m/s) (when drag force exceeds adhesion force) tend to detach from the surface irrespective of the impact of roughness. For polydisperse solutions, an increase in the polydispersity and flow velocity resulted in a reduction of colloid deposition efficiency due to the resulting enhanced double-layer repulsion. Quantification of surface topography variations of two endmembers of natural grain surfaces showed that half-pore depths and roughness of sedimentary quartz grains are mainly in the micrometer range. Grains with diagenetically formed quartz overgrowths, however, show surface roughness mainly in the submicrometer range. Thus, surface topography features applied in the here presented analogue study and resulting variation in particle retention can serve as quantitative analogue for particle reactions in diagenetically altered quartz sands and sandstones. The reported impact of particle polydispersity can have an important application for quantitative prediction of retention of varying types of minerals, such as different clay minerals in the environment under prevailing unfavorable conditions
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
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