Sheathless hydrodynamic positioning of buoyant drops and bubbles inside microchannels

Particles, bubbles, and drops carried by a fluid in a confined environment such as a pipe can be subjected to hydrodynamic lift forces, i.e., forces that are perpendicular to the direction of the flow. We investigated the positioning effect of lift forces acting on buoyant drops and bubbles suspended in a carrier fluid and flowing in a horizontal microchannel. We report experiments on drops of water in fluorocarbon liquid, and on bubbles of nitrogen in hydrocarbon liquid and silicone oil, inside microchannels with widths on the order of 0.1–1 mm. Despite their buoyancy, drops and bubbles could travel without contacting with the walls of channels; the most important parameters for reaching this flow regime in our experiments were the viscosity and the velocity of the carrier fluid, and the sizes of drops and bubbles. The dependencies of the transverse position of drops and bubbles on these parameters were investigated. At steady state, the trajectories of drops and bubbles approached the center of the channel for drops and bubbles almost as large as the channel, carried by rapidly flowing viscous liquids; among our experiments, these flow conditions were characterized by larger capillary numbers and smaller Reynolds numbers. Analytical models of lift forces developed for the flow of drops much smaller than the width of the channel failed to predict their transverse position, while computational fluid dynamic simulations of the experiments agreed better with the experimental measurements. The degrees of success of these predictions indicate the importance of confinement on generating strong hydrodynamic lift forces. We conclude that, inside microfluidic channels, it is possible to support and position buoyant drops and bubbles simply by flowing a single-stream (i.e., “sheathless”) carrier liquid that has appropriate velocity and hydrodynamic properties.Chemistry and Chemical Biolog

The problematic backreaction of SUSY-breaking branes

In this paper we investigate the localisation of SUSY-breaking branes which, in the smeared approximation, support specific non-BPS vacua. We show, for a wide class of boundary conditions, that there is no flux vacuum when the branes are described by a genuine delta-function. Even more, we find that the smeared solution is the unique solution with a regular brane profile. Our setup consists of a non-BPS AdS_7 solution in massive IIA supergravity with smeared anti-D6-branes and fluxes T-dual to ISD fluxes in IIB supergravity.Comment: 27 pages, Latex2e, 5 figure

(Anti-)Brane backreaction beyond perturbation theory

We improve on the understanding of the backreaction of anti-D6-branes in a flux background that is mutually BPS with D6-branes. This setup is analogous to the study of the backreaction of anti-D3-branes inserted in the KS throat, but does not require us to smear the anti-branes or do a perturbative analysis around the BPS background. We solve the full equations of motion near the anti-D6-branes and show that only two boundary conditions are consistent with the equations of motion. Upon invoking a topological argument we eliminate the boundary condition with regular H flux since it cannot lead to a solution that approaches the right kind of flux away from the anti-D6-brane. This leaves us with a boundary condition which has singular, but integrable, H flux energy density.Comment: 12 pages + appendices, 1 figure; v2: minor changes, version published in JHE

New Examples of Flux Vacua

Type IIB toroidal orientifolds are among the earliest examples of flux vacua. By applying T-duality, we construct the first examples of massive IIA flux vacua with Minkowski space-times, along with new examples of type IIA flux vacua. The backgrounds are surprisingly simple with no four-form flux at all. They serve as illustrations of the ingredients needed to build type IIA and massive IIA solutions with scale separation. To check that these backgrounds are actually solutions, we formulate the complete set of type II supergravity equations of motion in a very useful form that treats the R-R fields democratically.Comment: 38 pages, LaTeX; references updated; additional minor comments added; published versio

D-brane Moduli Spaces and Superpotentials in a Two-Parameter Model

We study D2-branes on the K3-fibration P^4_(11222)[8] using matrix factorizations at the Landau-Ginzburg point and analyze their moduli space and superpotentials in detail. We find that the open string moduli space consists of various intersecting branches of different dimensions. Families of D2-branes wrapping rational curves of degree one intersect with bound state branches. The influence of non-toric complex structure deformations is investigated in the Landau-Ginzburg framework, where these deformations arise as bulk moduli from the twisted sectors.Comment: 35 pages, 2 figures, reference adde

On Cosmological Constants from alpha'-Corrections

We examine to what extent perturbative alpha'-corrections can generate a small cosmological constant in warped string compactifications. Focusing on the heterotic string at lowest order in the string loop expansion, we show that, for a maximally symmetric spacetime, the alpha'-corrected 4D scalar potential has no effect on the cosmological constant. The only relevant terms are instead higher order products of 4D Riemann tensors, which, however, are found to vanish in the usual perturbative regime of the alpha'-expansion. The heterotic string therefore only allows for 4D Minkowski vacua to all orders in alpha', unless one also introduces string loop and/or nonperturbative corrections or allows for curvatures or field strengths that are large in string units. In particular, we find that perturbative alpha'-effects cannot induce weakly curved AdS_4 solutions.Comment: 18 pages, no figures. v2: minor modifications, references adde

Successful lung volume reduction surgery in combined pulmonary emphysema and fibrosis without body-plethysmographic hyperinflation-a case report

Surgical and bronchoscopic lung volume reduction (LVR) have been demonstrated to improve lung function, dyspnea and quality of life in patients with severe pulmonary emphysema. The most important functional prerequisite for a successful LVR is hyperinflation measured by body plethysmography. A residual volume (RV) of more than 180% predicted and a RV/total lung capacity (TLC) ratio of more than 0.58 were inclusion criteria in major LVR trials. Here we report a successful LVR in a 68-year-old man with advanced, heterogeneous emphysema without plethysmographic evidence of severe hyperinflation (RV/TLC 0.45). Computed tomography (CT) revealed severe, partly bullous upper lobe emphysema and subtle fibrotic changes with volume loss of lower lobes. Since lower lobes appeared compressed by upper lobe emphysema, these target areas were removed by thoracoscopic LVR. Four months later, the patient reported major improvements of dyspnea, FEV (by 1.27 L) and 6-minute walking distance (by 150 meters). LVR reduced total lung volume measured by CT-volumetry by 0.5 L and upper lobe volume by 1.85 L while lower lobe volume increased by +1.34 L. Low density volume (-950 HU) reflecting emphysema was reduced by 1.73 L. We conclude that the opposing effects of emphysema and fibrosis resulted in a barely increase in total lung volume that was only slightly reduced by LVR. Nevertheless, resection of emphysematous target areas identified by quantitative CT analysis provided major clinical and physiologic improvements related to decompression of low-compliance lower lobe areas retracted by early fibrosis. Therefore, in the combined presence of severe, heterogeneously distributed emphysema and fibrosis, LVR may improve respiratory mechanics even if RV/TLC, an established body-plethysmographic predictor of LVR success is not severely elevated

Lung volume reduction surgery as salvage procedure after previous use of endobronchial valves

OBJECTIVES Lung volume reduction (LVR) is an efficient and approved treatment for selected emphysema patients. There is some evidence that repeated LVR surgery (LVRS) might be beneficial, but there are no current data on LVRS after unsuccessful bronchoscopic LVR (BLVR) with endobronchial valves (EBVs). We hypothesize good outcome of LVRS after BLVR with valves. METHODS In this study, we retrospectively investigated all patients who underwent LVRS between 2015 and 2019 at 2 centres after previous unsuccessful EBV treatment. They were further divided into subgroups with patients who never achieved the intended improvement after BLVR (primary failure) and patients whose benefit was fading over time due to the natural development of emphysema (secondary failure). Patients with severe air leak after BLVR and immediate concomitant LVRS and fistula closure thereafter were analysed separately. RESULTS A total of 38 patients were included. Of these, 19 patients had primary failure, 15 secondary failure and 4 were treated as an emergency due to severe air leak. At 3 months after LVRS, forced expiratory volume in 1 s had improved significantly by 12.5% (P = 0.011) and there was no 90-day mortality. Considering subgroups, patients with primary failure after BLVR seem to profit more than those with secondary failure. Patients with severe air leak after BLVR did not profit from fistula closure with concomitant LVRS. CONCLUSIONS LVRS after previous BLVR with EBVs can provide significant clinical improvement with low morbidity, although results might not be as good as after primary LVRS

Outcome After Lung Volume Reduction Surgery in Patients With Severely Impaired Diffusion Capacity

BACKGROUND Lung volume reduction surgery (LVRS) has been proven to be a successful procedure and can be performed with low mortality when defined selection criteria are met. We hypothesized good outcome and low mortality after LVRS for selected patients with severe hyperinflation and nonhomogeneous morphology even when diffusion capacity of the lung for carbon monoxide (Dlco) is less than 20%. METHODS The study included all patients scheduled for LVRS between March 2005 and May 2014 with a preoperative Dlco of less than 20%. Postoperative 90-day mortality was the primary end point. Secondary end points were postoperative lung function and surgical morbidity at 3, 6, and 12 months. RESULTS Included were 33 patients with a median forced expiratory volume in 1 second of 23% (interquartile range, 19% to 28%), a median diffusion capacity of 15% (interquartile range, 13% to 18%), and a median hyperinflation of 76% (residual volume-to-total lung capacity ratio of 70% to 76%). Mean follow-up was 44.8 months (range, 10 to 141 months). Heterogeneous emphysema was present in 26 patients, and 7 showed intermediately heterogeneous morphology. Sixteen procedures were bilateral, and 31 were performed by video-assisted thoracoscopic surgery. The 90-day mortality was 0%. Median forced expiratory volume in 1 second percentage predicted at 3 months increased from 23% to 29% (p < 0.001). Median Dlco increased from 15% to 24% (p < 0.001), and median hyperinflation decreased from 76% to 63% (p < 0.001). A prolonged air leak exceeding 7 days occurred in 16 patients (48.5%), and 6 required reoperation for fistula closure. The 7 patients with intermediately heterogeneous emphysema showed a median increase in forced expiratory volume in 1 second from 20% preoperatively to 28% postoperatively (p = 0.028). CONCLUSIONS Selected patients with severely impaired Dlco of less than 20% can cautiously be considered as potential candidates if hyperinflation is severe and the lungs show areas with advanced destruction as targets for resection