Velocity contrasts enhancement for shear thinning solutions flowing in a rough fracture

Flow and transport are studied in transparent model fractures with rough complementary self-affine walls with a relative shear displacement $\vec{u}$. The aperture field is shown to display long range correlations perpendicular to $\vec{u}$: for flow in that direction, the width and geometry of the front of a dyed shear-thinning polymer solution displacing a transparent one have been studied as a function of the fluid rheology and flow rate. The front width increases linearly with distance indicating a convection of the fluids with a low transverse mixing between the flow paths. The width also increases with the flow-rate as the fluid rheology shifts from Newtonian at low shear rates $\dot \gamma$ towards a shear thinning behaviour at higher $\dot \gamma$ values. The width also increases with the polymer concentration at high flow-rates. These results demonstrate the enhancement of the flow velocity contrasts between different flow channels for shear thinning fluids. The relative widths at low and high $\dot \gamma$ values for different polymer concentrations are well predicted by an analytical model considering the fracture as a set of parallel ducts of constant hydraulic apertures. The overall geometry of the experimental front geometry is also predicted by the theoretical model from the aperture map

Characterization of fracture aperture field heterogeneity by electrical resistance measurement

We use electrical resistance measurements to characterize the aperture field in a rough fracture. This is done by performing displacement xperiments using two miscible fluids of different electrical resistivity and monitoring the time variation of the overall fracture resistance. Two fractures have been used: their complementary rough walls are identical but have different relative shear displacements which create “channel” or “barrier” structures in the aperture field, respectively parallel or perpendicular to the mean flow velocity →U. In the “channel” geometry, the resistance displays an initial linear variation followed by a tail part which reflects the velocity contrast between slow and fast flow channels. In the “barrier” geometry, a change in the slope between two linear zones suggests the existence of domains of different characteristic aperture along the fracture. These variations are well reproduced analytically and numerically using simple flow models. For each geometry, we present then a data inversion procedure that allows one to extract the key features of the heterogeneity from the resistance measurement.Fil: Boschan, Alejandro. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chertcoff, Ricardo Héctor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Hulin, J. P.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Auradou, H.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; Franci

E coli Accumulation behind an Obstacle

This paper describes our findings regarding the accumulation of motile bacte-ria at the rear of a confined obstacle and the physical description of the me-chanisms at play. We found that the modification of flow due to the presence of the obstacle produces vorticity that favor the diffusion of bacteria towards the downstream stagnation point. By testing different flow rates, we deter-mined the range in which bacteria accumulate. More interestingly, we observe that hydrodynamic interaction between the bacteria and the top and bottom surface of the microfluidic chip maintain the bacteria in the region where the flow velocity is lower than their own velocity. In the case of non-motile bacte-ria, this effect is not observed because bacteria follow the streamlines as pas-sive tracers do.Fil: Miño, Gastón Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Entre Ríos. Universidad Nacional de Entre Ríos. Centro de Investigaciones y Transferencia de Entre Ríos; ArgentinaFil: Baabour, Magali Denise. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Gutkind, Gabriel Osvaldo. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay; ArgentinaFil: Clément, Eric. Centre National de la Recherche Scientifique; FranciaFil: Auradou, Harold. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Prevalence and etiology of community-acquired pneumonia in immunocompromised patients

Background. The correct management of immunocompromised patients with pneumonia is debated. We evaluated the prevalence, risk factors, and characteristics of immunocompromised patients coming from the community with pneumonia. Methods. We conducted a secondary analysis of an international, multicenter study enrolling adult patients coming from the community with pneumonia and hospitalized in 222 hospitals in 54 countries worldwide. Risk factors for immunocompromise included AIDS, aplastic anemia, asplenia, hematological cancer, chemotherapy, neutropenia, biological drug use, lung transplantation, chronic steroid use, and solid tumor. Results. At least 1 risk factor for immunocompromise was recorded in 18% of the 3702 patients enrolled. The prevalences of risk factors significantly differed across continents and countries, with chronic steroid use (45%), hematological cancer (25%), and chemotherapy (22%) the most common. Among immunocompromised patients, community-acquired pneumonia (CAP) pathogens were the most frequently identified, and prevalences did not differ from those in immunocompetent patients. Risk factors for immunocompromise were independently associated with neither Pseudomonas aeruginosa nor non\u2013community-acquired bacteria. Specific risk factors were independently associated with fungal infections (odds ratio for AIDS and hematological cancer, 15.10 and 4.65, respectively; both P = .001), mycobacterial infections (AIDS; P = .006), and viral infections other than influenza (hematological cancer, 5.49; P < .001). Conclusions. Our findings could be considered by clinicians in prescribing empiric antibiotic therapy for CAP in immunocompromised patients. Patients with AIDS and hematological cancer admitted with CAP may have higher prevalences of fungi, mycobacteria, and noninfluenza viruses