1,720,954 research outputs found
Broadband boundary effects on Brownian motion
Get PDFBrownian motion of particles in confined fluids is important for many applications, yet the effects of the boundary over a wide range of time scales are still not well understood. We report high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately flat wall. At short distances we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of a particle in bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes flat at low frequencies. This detailed understanding of boundary effects on Brownian motion opens a door to developing a 3D microscope using particles as remote sensors.Sid W. Richardson FoundationR. A. Welch Foundation F-1258Physic
Recommended from our members
Brownian motion in liquids : theory and experiment
Get PDFSince the theoretical work of Einstein [1905] and von Smoluckowski [1906], and the experiments of Perrin [1909], Brownian motion at long time-scales has been extensively studied for over a century. Short time-scale aspects of Brownian motion are however becoming increasingly relevant, as technology attempts to make smaller and faster devices. The subject matter of this dissertation is the study of short time-scale (typically ~µs) aspects of Brownian motion of microscopic particles in liquids, where the dynamics of the fluid medium are significant.
We detail two recent experiments probing this regime: an experiment [Kheifets et. al., 2014] that measured the hydrodynamic instantaneous velocity of a dielectric particle in liquid medium and confirmed theories of Brownian motion based on hydrodynamics up to sub-microsecond time-scales, and a subsequent experiment [Mo et. al., 2015a] that verified the Maxwell-Boltzmann distribution of velocities well into the tails.
In a liquid medium, the presence of a boundary near a particle has a significant impact on the characteristics of its Brownian motion, owing to the hydrodynamic coupling between the bounding walls and the particle. However, exact solutions to the hydrodynamic equations are not known even for the common situation of a flat wall. An approximate theory was developed by Felderhof [2005], using a point-particle approximation. Despite agreement with previous experiments, that work results in a drag coefficient with a spurious dependence on the particle's density. In this work, we describe a modification [Simha et. al., 2017] to the point-particle approximation that resolves this inconsistency. Moreover, Felderhof's approximation scheme neglects the size of the particle not only in comparison to the distance of the bounding wall, but also to the skin-depth of rotational flow it generates in the fluid. Since this skin-depth depends on the time-scale of the motion, it is not obvious that such an approximation scheme works at all time-scales. We use the formalism of boundary integral equations to set up a perturbative framework, and obtain the point-particle framework through a series of systematic approximations. This derivation explains why the theory works so well at all time-scales. An alternative calculation for a simple case of a no-slip sphere near a full-slip wall is presented, with results indicating that the point-particle approximation may not capture all non-perturbative terms.
We then discuss an experiment [Mo et. al., 2015b] that probed the effects of a boundary on Brownian motion at short time-scales. The experiment agrees very well with the point-particle theory, demonstrating that the boundary significantly impacts Brownian motion down to a certain time-scale, and that the effects are diminished at shorter time-scales. Such effects allude to the possibility of using Brownian motion as a probe of the local environment.Physic
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
- …
