1,720,997 research outputs found
3D microstructures for active and soft matter studies
No full textMicrofabrication techniques have opened up new ways to study the dynamics of microsystems expanding the range of applications in microengineering and cell biology. Among three-dimensional microfabrication techniques, two-photon polymerization enjoys a unique set of characteristics that make it appealing for designing complex structures of arbitrary form. During the last decades, two-photon polymerization has evolved from the first structure fabricated with this technique, a coil with a diameter of 7 μm and a total length of approximately 34 μm (by Maruo et al.), to generate sophisticated systems like remotely driven micromachines. In the present thesis, we address two main applications of microfabrication.
On the first line of research, the design, and fabrication of efficient and self-powered micro-robots have been a very active research topic. Motile micro-organisms like E. coli may provide an optimal solution to generate propulsion in artificial microsystems. It has been demonstrated that microstructures can be transported when released on a layer of swarming bacteria, suspended in a bacterial bath, or covered by surface adhering bacteria. Although it is possible to obtain a net movement in the mentioned cases, the displacement is stochastic and self-propulsion characteristics are hard to reproduce. In this thesis, we investigate possible design strategies for bio-hybrid micro shuttles having a defined number of propelling units that self-assemble onto precisely defined locations. One of the biggest issues involved in the optimization design process of the microshuttles is an irreversible adhesion of structures in the substrate, which often is caused by Van der Waals attraction. To overcome this problem we use different stabilization methods with unsuccessful results. Looking for a less invasive and biocompatible strategy we investigate the possibility of changing the sign of Van der Walls forces turning them from attractive to repulsive. To this aim, we develop a method that demonstrates to reduce the adhesion observed before. So, the final design aims at minimizing friction and adhesion with the substrate while optimizing propulsion speed and self-assembly efficiency. Finally, using a mutated strain of E. coli the microshuttle can be remotely controlled by dynamic structured light patterns for reaching an optimal control of the motion of the structures.
In a different direction of microfabrication applications, 3D microstructures can also offer new opportunities to address more fundamental problems in the soft matter dynamic. On this second line of research, we have designed and used complex 3D microstructures to investigate the Brownian dynamics and hydrodynamics of propeller shaped particles, as well as to probe effective interactions in colloidal systems, like critical Casimir forces.
In the dynamics of microhelices we use optical tweezers to study the mechanic and hydrodynamic properties of micro-fabricated helices suspended in a fluid. For the case of rigid helices, we track Brownian fluctuations around mean values with a high precision and over a long observation time. Through the statistical analysis of fluctuations in translational and rotational coordinates, we recover the full mobility matrix of the micro-helix including the off diagonal terms related with roto-translational coupling. Exploiting the high degree of spatial control provided by optical trapping, we can systematically study the effect of a nearby wall on the roto-translational coupling, and conclude that a rotating helical propeller moves faster near a no-slip boundary. We also study the relaxation dynamics of deformable micro-helices stretched by optical traps. We find that hydrodynamic drag only weakly depends on elongation resulting in an exponential relaxation to equilibrium.
In connection with the versatility of microfabrication by two-photon polymerization, we find the study of interaction in colloidal systems. At macroscopic scales, thermal fluctuations of a physical property on a system are typically negligible, but at the micrometer and nanometer scales instead, fluctuations become generally relevant and they give rise to novel and intriguing phenomena such as critical Casimir effect. Critical Casimir forces are induced between colloidal objects suspended in a critical binary mixture undergoing strong thermal fluctuations. So far, most of the experiments and proposed models consider the interaction between simple geometrical objects such as two spheres, or a single sphere and a plate. In the last part of this thesis, we propose a novel 3D printed microprobes consisting of the main body and two handles that can be optically trapped to directly measure effective forces and torques between colloidal objects with non spherical shapes.
The organization of this thesis is as follows. Chapter 1 gives a general introduction to the physical phenomenon behind the 3D microfabrication technique employed in our experiments, two-photon polymerization. We describe the differences between two phenomena: single-photon absorption and two-photon absorption, and explain the effectiveness of using two-photon polymerization for reaching a resolution of 100 nanometers in microfabrication. Then we present an experimental characterization of the voxel size of our custom-built two-photon polymerization set-up. We explain the sample preparation steps for microfabrication as well as the development of an innovative low-refractive index layer for eliminating irreversible adhesion of SU-8 microstructures.
Chapter 2 provides a general introduction to E. coli motility, the propulsion mechanism of these bacteria, and the circular trajectory developed by the microorganism when swimming near a rigid boundary. Besides, we briefly explain the possibility of using synthetic biology to obtain light-driven strains of E. coli by the expression of Proteorhodopsin on the bacteria membrane.
In Chapter 3 we combine two-photon polymerization technique and genetically modified bacteria to create a biohybrid microshuttle. We start with a basic microshuttle design whose propulsion is obtained from four E. coli bacteria. After integrating ramps in another microshuttle model for minimizing the circular trajectory showed in the microstructure trajectory, we make major changes in the distribution of microchambers inside the last model named catamaran microshuttle. Exploiting the ability of a mutated strain of E. coli expressing proteorhodopsin, we successfully control the microrobot steering by illuminating our sample with green light patterns.
In Chapter 4 we design and use 3D microhelices from two different materials to investigate, through optical tweezers, Brownian dynamics, and hydrodynamics of this kind of chiral particles. Through the statistical analysis of fluctuations in translational and rotational coordinates, we study the roto-translational coupling element from the mobility matrix of the micro-helix. Besides, we conclude that a rotating helical propeller moves faster near a no-slip boundary. For the case of a deformable micro=helix, we find that hydrodynamic drag only depends on its elongation.
Finally, Chapter 5 presents the design for a microprobe to measure critical Casimir forces using holographic optical tweezers. We show a characterization experiment for a micro-cube with two handles, concluding that a third handle will improve the stability of a microprobe inside the sample
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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