1,721,004 research outputs found
Directed Self-Assembly of Spheres into a Two-Dimensional Colloidal Crystal by Viscoelastic Stresses
No full textOrdering induced by shear flow can be used to create specific microstructures in particle suspensions. They are determined by the balance between a range of forces, such as direct interparticle, Brownian and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, one dimensional string-like structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon never observed in Newtonian fluids at similar particle volume fractions.
Here we report for the first time on the formation of a Directed Self-Assembly (DSA), flow-induced two-dimensional planar crystals of spherical particles. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2-D crystalline patches along the flow direction. This is a bulk phenomenon, as 2-D crystals form throughout the whole gap between plates, the gap thickness being much larger than the particle size. Patches lay in planes parallel to the shearing surfaces
Steady state and transient rheological behavior of organoclay-polypropylene nanocomposites
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The effect of particle size and migration on the formation of flow-induced structures in viscoelastic suspensions
No full textFlow-induced structures in suspensions contain- ing spheres in viscoelastic suspending media were investi- gated by microscopy and rheo-optical methods. Suspensions of monodisperse polystyrene spheres of different diameters (1-3μ m) dispersed in aqueous solutions of hydroxypropy- lcellulose were studied in simple shear flows. Optical mi- croscopy observations as well as small angle light scattering (SALS) experiments were performed using a parallel plate geometry. In agreement with previous work, aligned strings of particles were observed when shearing faster then a criti- cal shear rate, which was found to be independent of particle size. In contrast to earlier work, however, the role of parti- cle migration was found to be of prime importance. Particles were shown to migrate towards the plates where the particles assembled and aligned in strings running in the flow direc- tion. For the smallest particles (1 μ m diameter), the align- ment of particle doublets or short strings along the vorticity direction was observed at low shear rates, which flipped to a an orientation into the flow direction of longer strings at higher shear rates. This phenomenon never detected before for spherical particles and maybe attributed to an interplay between the hydrodynamic and the attractive inter-particle forces. Finally,SALS experiments were used to quantify the degree of alignment and its dependence o particle size, shear rate and gap. For the system under investigation, the degree of alignment was found to increase with increasing shear rate and particle size and with decreasing gap. The present results suggest that, depending on the details of the suspend- ing medium and the size and nature of the suspending media, the formation of aligned structures is affected by the relative magnitude of the colloidal and hydrodynamic forces and the kinetics of string formation versus the kinetics of migratio
Steady state and transient rheological behavior of organo-clay polypropylene nanocomposites
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Rheology of viscoelastic suspensions
No full textThe rheology of suspensions of solid particles in viscoelastic fluids is important in many technological applicatoins as exemplified by the processinf of filled polymers. Consequently, considerable attentions in the literature is given to the rheology of suspesnions fo particles. Most studies are focused on highly filled systems with polydisperse partciles of irregular shapes. In contrat, few studies are conducted ont he rheology of dilute and semi-dilute suspensions of monodisperse spheres.
We elucidate the effect of viscoelasticity ont he bulk rheological properties. The hehaviour of model suspensions composed of non Bronwnian rigid spehres immmersed in Newtonian and viscoelastic matrices is investigated in the concentration range from 0 up to 10%. In parallel with the experiments, new simultaion techniques for suspesnions in Newtonian fluids under oscillatory shear flow is presented. The case of a single sphere and two particels are studied and discussed.
The flow induced microstructure of suspensions in viscoelastic fluids is studied by rheo-optical techiniques. More specifically,the flow-induced alignment of non-colloidal particles in viscoelastic fluiids is investigated systematically in an attempt to quantify the alignment of the particles and correlate it with the shear rate, size aof the particles and interactions with the wall
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
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