A forward-backward view of some primal-dual optimization methods in image recovery

A wide array of image recovery problems can be abstracted into the problem of minimizing a sum of composite convex functions in a Hilbert space. To solve such problems, primal-dual proximal approaches have been developed which provide efficient solutions to large-scale optimization problems. The objective of this paper is to show that a number of existing algorithms can be derived from a general form of the forward-backward algorithm applied in a suitable product space. Our approach also allows us to develop useful extensions of existing algorithms by introducing a variable metric. An illustration to image restoration is provided

Corrosive environment in the annulus of flexible pipes: pH measurements in confined conditions and under high pressure

International audienceAnnulus of flexible pipelines represents a specific corrosive environment for the high strength carbon steel wires, mainly due to the low V/S ratio between the volume of electrolyte and the exposed steel surface. Corrosion experiments with continuous in situ pH measurements were performed under CO 2 partial pressure from 0.01 bar and up to 10 bar, and with a V/S ratio of 0.3 mL/cm². After only a few days immersion, supersaturated pH was measured at levels close to +1 pH unit above the saturated pH calculated corresponding to iron carbonate solubility. Good correlation was found between experiments and a pH and corrosion model specially developed for confined environment. This model predicts that the corrosion rate in super-saturated conditions is considerably reduced in comparison with bulk situation. Long term behavior was also examined. It appears that the evolution of super-saturated pH with time passes by a maximum value, then decreases slowly to a new equilibrium. It was postulated that the maximum corresponds to a fully active surface. Then, as precipitation occurs, corrosion rate is decreased by corrosion scale. The decrease of corrosion rate could be quantified from the final pH value: value as low as a few µm/year was found. Even with such low corrosion rates, high confinement still induced super-saturation effect, with final pH values at +0.5 pH units above saturated pH

Phonon Transmission Rate, Fluctuations, and Localization in Random Semiconductor Superlattices: Green's Function Approach

We analytically study phonon transmission and localization in random superlattices by using a Green's function approach. We derive expressions for the average transmission rate and localization length, or Lyapunov exponent, in terms of the superlattice structure factor. This is done by considering the backscattering of phonons, due to the complex mass density fluctuations, which incorporates all of the forward scattering processes. These analytical results are applied to two types of random superlattices and compared with numerical simulations based on the transfer matrix method. Our analytical results show excellent agreement with the numerical data. A universal relation for the transmission fluctuations versus the average transmission is derived explicitly, and independently confirmed by numerical simulations. The transient of the distribution of transmission to the log-normal distribution for the localized phonons is also studied.Comment: 36 pages, Late

Disrupting the wall accumulation of human sperm cells by artificial corrugation

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very different from that observed in the bulk. Swimming along walls is beneficial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells $60 \mu m$ long, strongly confined to $25 \mu m$ shallow chambers. We investigate the nature of the cell trajectories between the confining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confirmed by depth resolved confocal microscopy studies. We show that the introduction of artificial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an effect which we hypothesize may be valuable for microfluidic applications in biomedicine.Comment: 9 pages, latex. In accepted version on April 14, v2: abstract modified, information added to Sec. II.A and experiments added to Sec. III.A and Fig.3. Sec. III.C was deleted. Requested references adde

Geometrical guidance and trapping transition of human sperm cells

The guidance of human sperm cells under confinement in quasi-2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with diluted sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy.publishedVersionFil: Guidobaldi, Héctor Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Biológicas y Tecnológicas; Argentina.Fil: Guidobaldi, Héctor Alejandro. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Biología Celular y Molecular; Argentina.Fil: Jeyaram, Y. Katholieke Universiteit Leuven. Institute for Nanoscale Physics and Chemistry; Bélgica.Fil: Berdakin, Ivan. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Berdakin, Ivan. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Moshchalkov, V. V. Katholieke Universiteit Leuven. Institute for Nanoscale Physics and Chemistry; Bélgica.Fil: Condat, Carlos Alberto. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Condat, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Marconi, Verónica Iris. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Marconi, Verónica Iris. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Giojalas, Laura Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Biológicas y Tecnológicas; Argentina.Fil: Giojalas, Laura Cecilia. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Biología Celular y Molecular ; Argentina.Fil: Silhanek, Alejandro V. Université de Liège. Faculté des Sciences. Departement de Physique; Bélgica.Biofísic

Portal vein thrombosis; risk factors, clinical presentation and treatment

<p>Abstract</p> <p>Background</p> <p>Portal vein thrombosis (PVT) is increasingly frequently being diagnosed, but systematic descriptions of the natural history and clinical handling of the condition are sparse. The aim of this retrospective study was to describe risk factors, clinical presentation, complications and treatment of portal vein thrombosis in a single-centre.</p> <p>Methods</p> <p>Sixty-seven patients were identified in the electronic records from 1992 to 2005. All data were obtained from the patient records.</p> <p>Results</p> <p>One or more risk factors (e.g. prothrombotic disorder or abdominal inflammation) were present in 87%. Symptoms were abdominalia, splenomegaly, fever, ascites, haematemesis, and weight loss. Abdominalia and fever occurred more frequently in patients with acute PVT. Frequent complications were splenomegaly, oesophageal- and gastric varices with or without bleeding, portal hypertensive gastropathy and ascites. Varices and bleeding were more frequent in patients with chronic PVT. Patients who received anticoagulant therapy more frequently achieved partial/complete recanalization. Patients with varices who were treated endoscopically in combination with β-blockade had regression of the varices. The overall mortality was 13% in one year, and was dependent on underlying causes.</p> <p>Conclusion</p> <p>Most patients had a combination of local and systemic risk factors for PVT. We observed that partial/complete recanalization was more frequent in patients treated with anticoagulation therapy, and that regression of varices was more pronounced in patients who where treated with active endoscopy combined with pharmacological treatment.</p

Defect structures in sine-Gordon-like models

We investigate several models described by real scalar fields, searching for topological defects. Some models are described by a single field, and support one or two topological sectors, and others are two-field models, which support several topological sectors. Almost all the defect structures that we find are stable and finite energy solutions of first-order differential equations that solve the corresponding equations of motion. In particular, for the double sine-Gordon model we show how to find small and large BPS solutions as deformations of the BPS solution of the $\phi^4$ model. And also, for most of the two field models we find the corresponding integrating factors, which lead to the complete set of BPS solutions, nicely unveiling how they bifurcate among the several topological sectors.Comment: RevTex, 18 pages, 17 figures; Version to appear in Physica