Collective waves in dense and confined microfluidic droplet arrays

Excitation mechanisms for collective waves in confined dense one-dimensional microfluidic droplet arrays are investigated by experiments and computer simulations. We demonstrate that distinct modes can be excited by creating specific `defect' patterns in flowing droplet trains. Excited longitudinal modes exhibit a short-lived cascade of pairs of laterally displacing droplets. Transversely excited modes obey the dispersion relation of microfluidic phonons and induce a coupling between longitudinal and transverse modes, whose origin is the hydrodynamic interaction of the droplets with the confining walls. Moreover, we investigate the long-time behaviour of the oscillations and discuss possible mechanisms for the onset of instabilities. Our findings demonstrate that the collective dynamics of microfluidic droplet ensembles can be studied particularly well in dense and confined systems. Experimentally, the ability to control microfluidic droplets may allow to modulate the refractive index of optofluidic crystals which is a promising approach for the production of dynamically programmable metamaterials.Comment: 13 pages, 17 figure

A Gaussian Mixture MRF for Model-Based Iterative Reconstruction with Applications to Low-Dose X-ray CT

Markov random fields (MRFs) have been widely used as prior models in various inverse problems such as tomographic reconstruction. While MRFs provide a simple and often effective way to model the spatial dependencies in images, they suffer from the fact that parameter estimation is difficult. In practice, this means that MRFs typically have very simple structure that cannot completely capture the subtle characteristics of complex images. In this paper, we present a novel Gaussian mixture Markov random field model (GM-MRF) that can be used as a very expressive prior model for inverse problems such as denoising and reconstruction. The GM-MRF forms a global image model by merging together individual Gaussian-mixture models (GMMs) for image patches. In addition, we present a novel analytical framework for computing MAP estimates using the GM-MRF prior model through the construction of surrogate functions that result in a sequence of quadratic optimizations. We also introduce a simple but effective method to adjust the GM-MRF so as to control the sharpness in low- and high-contrast regions of the reconstruction separately. We demonstrate the value of the model with experiments including image denoising and low-dose CT reconstruction.Comment: accepted by IEEE Transactions on Computed Imagin

On a fourth order nonlinear Helmholtz equation

In this paper, we study the mixed dispersion fourth order nonlinear Helmholtz equation $\Delta^2 u -\beta \Delta u + \alpha u= \Gamma|u|^{p-2} u$ in $\mathbb R^N$ for positive, bounded and $\mathbb Z^N$-periodic functions $\Gamma$. Using the dual method of Evequoz and Weth, we find solutions to this equation and establish some of their qualitative properties

The distribution of helium 3 in the deep Western and Southern Indian Ocean

Almost a decade after the Geochemical Ocean Sections Study Indian Expedition, the new deep 3He data from the INDIGO program give a further insight into the distribution of this tracer in the Indian Ocean. This distribution exhibits some major features related on one hand to a hydrothermal 3He input in the Gulf of Aden and on the Mid-Indian Ocean Ridge, and on the other to the origin of the water masses and to the characteristics of the deep circulation. (D'après résumé d'auteur

A network approach to studying research programmes : mobilizing and coordinating public responses to HIV/AIDS

Based on the analysis of the Medical and Public Health Research programme (Commission of the European Communities), the paper shows how new scientific communities are created in response to the HIV/AIDS problem. We analyze how actors are mobilized (three mobilization modes : public impetus, scientists'initiative, scientists' initiative with public networking) and how their work is coordinated. We defend the hypothesis that these new scientific communities are flexible cooperation networks. In the case of AIDS research, there are only a limited number of network types (the data collection structure, the forum, the thematic partition with harmonization of research practices, the starred around a central facility). The coordination of these scientific cooperative networks passes through fixed and circulating intermediaries. The management of these intermediaries is a major activity for involved actors. thus, our aim is not to study the wider social context, but to analyze networking in response to policy initiatives. (Résumé d'auteur

Activating social change together: A qualitative synthesis of collaborative change research, evaluation and design literature

Researchers, evaluators and designers from an array of academic disciplines and industry sectors are turning to participatory approaches as they seek to understand and address complex social problems. We refer to participatory approaches that collaboratively engage/partner with stakeholders in knowledge creation/problem solving for action/social change outcomes as collaborative change research, evaluation and design (CCRED). We further frame CCRED practitioners by their desire to move beyond knowledge creation for its own sake to implementation of new knowledge as a tool for social change. In March and May of 2018, we conducted a literature search of multiple discipline-specific databases seeking collaborative, change-oriented scholarly publications. The search was limited to include peer-reviewed journal articles, with English language abstracts available, published in the last five years. The search resulted in 526 citations, 236 of which met inclusion criteria. Though the search was limited to English abstracts, all major geographic regions (North America, Europe, Latin America/Caribbean, APAC, Africa and the Middle East) were represented within the results, although many articles did not state a specific region. Of those identified, most studies were located in North America, with the Middle East having only one identified study. We followed a qualitative thematic synthesis process to examine the abstracts of peer-reviewed articles to identify practices that transcend individual disciplines, sectors and contexts to achieve collaborative change. We surveyed the terminology used to describe CCRED, setting, content/topic of study, type of collaboration, and related benefits/outcomes in order to discern the words used to designate collaboration, the frameworks, tools and methods employed, and the presence of action, evaluation or outcomes. Forty-three percent of the reviewed articles fell broadly within the social sciences, followed by 26 percent in education and 25 percent in health/medicine. In terms of participants and/or collaborators in the articles reviewed, the vast majority of the 236 articles (86%) described participants, that is, those who the research was about or from whom data was collected. In contrast to participants, partners/collaborators (n=32; 14%) were individuals or groups who participated in the design or implementation of the collaborative change effort described. In terms of the goal for collaboration and/or for doing the work, the most frequently used terminology related to some aspect of engagement and empowerment. Common descriptors for the work itself were ‘social change’ (n=74; 31%), ‘action’ (n=33; 14%), ‘collaborative or participatory research/practice’ (n=13; 6%), ‘transformation’ (n=13; 6%) and ‘community engagement’ (n=10; 4%). Of the 236 articles that mentioned a specific framework or approach, the three most common were some variation of Participatory Action Research (n=30; 50%), Action Research (n=40; 16.9%) or Community-Based Participatory Research (n=17; 7.2%). Approximately a third of the 236 articles did not mention a specific method or tool in the abstract. The most commonly cited method/tool (n=30; 12.7%) was some variation of an arts-based method followed by interviews (n=18; 7.6%), case study (n=16; 6.7%), or an ethnographic-related method (n=14; 5.9%). While some articles implied action or change, only 14 of the 236 articles (6%) stated a specific action or outcome. Most often, the changes described were: the creation or modification of a model, method, process, framework or protocol (n=9; 4%), quality improvement, policy change and social change (n=8; 3%), or modifications to education/training methods and materials (n=5; 2%). The infrequent use of collaboration as a descriptor of partner engagement, coupled with few reported findings of measurable change, raises questions about the nature of CCRED. It appears that conducting CCRED is as complex an undertaking as the problems that the work is attempting to address

MYSTIC: Michigan Young STar Imager at CHARA

We present the design for MYSTIC, the Michigan Young STar Imager at CHARA. MYSTIC will be a K-band, cryogenic, 6-beam combiner for the Georgia State University CHARA telescope array. The design follows the image-plane combination scheme of the MIRC instrument where single-mode fibers bring starlight into a non-redundant fringe pattern to feed a spectrograph. Beams will be injected in polarization-maintaining fibers outside the cryogenic dewar and then be transported through a vacuum feedthrough into the ~220K cold volume where combination is achieved and the light is dispersed. We will use a C-RED One camera (First Light Imaging) based on the eAPD SAPHIRA detector to allow for near-photon-counting performance. We also intend to support a 4-telescope mode using a leftover integrated optics component designed for the VLTI-GRAVITY experiment, allowing better sensitivity for the faintest targets. Our primary science driver motivation is to image disks around young stars in order to better understand planet formation and how forming planets might influence disk structures.Comment: Presented at the 2018 SPIE Astronomical Telescopes + Instrumentation, Austin, Texas, US

Gemini Planet Imager Observational Calibrations III: Empirical Measurement Methods and Applications of High-Resolution Microlens PSFs

The newly commissioned Gemini Planet Imager (GPI) combines extreme adaptive optics, an advanced coronagraph, precision wavefront control and a lenslet-based integral field spectrograph (IFS) to measure the spectra of young extrasolar giant planets between 0.9-2.5 um. Each GPI detector image, when in spectral model, consists of ~37,000 microspectra which are under or critically sampled in the spatial direction. This paper demonstrates how to obtain high-resolution microlens PSFs and discusses their use in enhancing the wavelength calibration, flexure compensation and spectral extraction. This method is generally applicable to any lenslet-based integral field spectrograph including proposed future instrument concepts for space missions.Comment: 10 pages, 6 figures. Proceedings of the SPIE, 9147-282 v2: reference adde

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let