High performance microfluidic rectifiers for viscoelastic fluid flow

The flow of Newtonian and non-Newtonian fluids within microfluidic rectifiers with a hyperbolic shape was investigated to assess the effect of the bounding walls on the diodicity of the microfluidic device and achieve high flow anisotropy. Three microchannels were used, with different depths and the same geometrical configuration, which creates a strong extensional flow and generates high anisotropic flow resistance between the two flow directions. The Newtonian fluid, de-ionized water, was used as a reference fluid. The viscoelastic fluid used was an aqueous solution of polyethylene oxide (0.1% w/w) with high molecular weight. The flow patterns were visualized using streak photography and the velocity field was investigated using micro-particle image velocimetry. Moreover, pressure drop measurements were performed in order to compare the diodicity achieved in the microfluidic rectifiers. For the Newtonian fluid flow, the experimental results are compared with numerical predictions obtained using a finite-volume method and good agreement was found between both approaches. For the viscoelastic fluid, significant anisotropic flow resistance can be achieved. The effect of the bounding walls was analysed and found to be qualitatively similar for all microchannels. Nevertheless, in quantitative terms, the diodicity is enhanced when the wall effect is reduced, i.e. when the channels are deeper. A maximum diodicity above six was found for the deeper channel, a value well beyond those previously reported

Laminar flow in three-dimensional square-square expansions

In this work we investigate the three-dimensional laminar flow of Newtonian and viscoelastic fluids through square–square expansions. The experimental results obtained in this simple geometry provide useful data for benchmarking purposes in complex three-dimensional flows. Visualizations of the flow patterns were performed using streak photography, the velocity field of the flow was measured in detail using particle image velocimetry and additionally, pressure drop measurements were carried out. The Newtonian fluid flow was investigated for the expansion ratios of 1:2.4, 1:4 and 1:8 and the experimental results were compared with numerical predictions. For all expansion ratios studied, a corner vortex is observed downstream of the expansion and an increase of the flow inertia leads to an enhancement of the vortex size. Good agreement is found between experimental and numerical results. The flow of the two non-Newtonian fluids was investigated experimentally for expansion ratios of 1:2.4, 1:4, 1:8 and 1:12, and compared with numerical simulations using the Oldroyd-B, FENE-MCR and sPTT constitutive equations. For both the Boger and shear-thinning viscoelastic fluids, a corner vortex appears downstream of the expansion, which decreases in size and strength when the elasticity of the flow is increased. For all fluids and expansion ratios studied, the recirculations that are formed downstream of the square–square expansion exhibit a three-dimensional structure evidenced by a helical flow, which is also predicted in the numerical simulations

Three-dimensional flow of Newtonian and Boger fluids in square-square contractions

The flow of a Newtonian fluid and a Boger fluid through sudden square–square contractions was investigated experimentally aiming to characterize the flow and provide quantitative data for benchmarking in a complex three-dimensional flow. Visualizations of the flow patterns were undertaken using streakline photography, detailed velocity field measurementswere conducted using particle image velocimetry (PIV) and pressure drop measurements were performed in various geometries with different contraction ratios. For the Newtonian fluid, the experimental results are compared with numerical simulations performed using a finite volume method, and excellent agreement is found for the range of Reynolds number tested (Re2 ≤23). For the viscoelastic case, recirculations are still present upstream of the contraction but we also observe other complex flow patterns that are dependent on contraction ratio (CR) and Deborah number (De2) for the range of conditions studied: CR = 2.4, 4, 8, 12 and De2 ≤150. For low contraction ratios strong divergent flow is observed upstream of the contraction, whereas for high contraction ratios there is no upstream divergent flow, except in the vicinity of the re-entrant corner where a localized a typical divergent flow is observed. For all contraction ratios studied, at sufficiently high Deborah numbers, strong elastic vortex enhancement upstream of the contraction is observed, which leads to the onset of a periodic complex flow at higher flow rates. The vortices observed under steady flow are not closed, and fluid elasticity was found to modify the flow direction within the recirculations as compared to that found for Newtonian fluids. The entry pressure drop, quantified using a Couette correction, was found to increase with the Deborah number for the higher contraction ratios

Effect of the contraction ratio upon viscoelastic fluid flow in three-dimensional square-square contractions

In this work we investigate the laminar flow through square–square sudden contractions with various contraction ratios (CR¼2.4, 4,8and12), using a Newtonian fluid and a shear-thinning viscoelastic fluid. Visualizations of the flow patterns were carried out using streakline photography and detailed velocity field measurements were performed using particle image velocimetry. The experimental results are compared with numerical predictions obtained using a finite-volume method. For the Newtonian fluid, a corner vortex is found upstream of the contraction and increasing flow inertia leads to a reduction of the vortex size. Good agreement is observed between experiments and numerical simulations. For the shear-thinning fluid flow a corner vortex is also observed upstream of the contraction independently of the contraction ratio. Increasing the elasticity of the flow, while still maintaining low inertia flow conditions, leads to a strong increase of the vortex size, until an elastic instability sets in and the flow becomes time-dependent at DeE200, 300, 70 and 450 for CR¼2.4, 4, 8 and 12, respectively. At low contraction ratios, viscoelasticity brings out an anomalous divergent flow upstream of the contraction. For both fluids studied the flow presents a complex three-dimensional helical vortex structure which is well predicted by numerical simulations. However, for the viscoelastic fluid flow the maximum Deborah number achieved in the numerical simulations is about one order of magnitude lower than the critical Deborah number for the onset of the elastic instability found in the experiments

Flow of a blood analogue solution through microfabricated hyperbolic contractions

The flow of a blood analogue solution past a microfabricated hyperbolic contraction followed by an abrupt expansion was investigated experimentally. The shape of the contraction was designed in order to impose a nearly constant strain rate to the fluid along the centerline of the microgeometry. The flow patterns of the blood analogue solution and of a Newtonian reference fluid (deionized water), captured using streak line imaging, are quite distinct and illustrate the complex behavior of the blood analogue solution flowing through the microgeometry. The flow of the blood analogue solution shows elastic-driven effects with vortical structures emerging upstream of the contraction, which are absent in Newtonian fluid flow. In both cases the flow also develops instabilities downstream of the expansion but these are inertia driven. Therefore, for the blood analogue solution at high flow rates the competing effects of inertia and elasticity lead to complex flow patterns and unstable flow develops

Shear viscosity and nonlinear behaviour of whole blood under large amplitude oscillatory shear

We investigated experimentally the rheological behavior of whole human blood subjected to large amplitude oscillatory shear under strain control to assess its nonlinear viscoelastic response. In these rheological tests, the shear stress response presented higher harmonic contributions, revealing the nonlinear behavior of human blood that is associated with changes in its internal microstructure. For the rheological conditions investigated, intra-cycle strain-stiffening and intra-cycle shear-thinning behavior of the human blood samples were observed and quantified based on the Lissajous–Bowditch plots. The results demonstrated that the dissipative nature of whole blood is more intense than its elastic component. We also assessed the effect of adding EDTA anticoagulant on the shear viscosity of whole blood subjected to steady shear flow. We found that the use of anticoagulant in appropriate concentrations did not influence the shear viscosity and that blood samples without anticoagulant preserved their rheological characteristics approximately for up to 8 minutes before coagulation became significant

Finite-Size Scaling and Damage Spreading in Ising Systems with Multispin Interactions

We investigate two-dimensional Ising systems with multisping interactions of three- (m=3) and four-body terms (m=4). The application of a new type of finite-size algorithm of de Oliveira allow us to clearly distinguish a first-order transition (in the m=4 case) from a continuous one (in the m=3 one). We also study the damage spreading in these systems. In this study, a dynamical phenomenon is observed to occur at a critical point separating a chaotic phase from a frozen one. However, the width of the interval where this transition happens does not yield a conclusive evidence about the order of the phase transition.Comment: 18 pages, 6 figure

Non-adiabatic collapse of a quasi-spherical radiating star

A model is proposed of a collapsing quasi-spherical radiating star with matter content as shear-free isotropic fluid undergoing radial heat-flow with outgoing radiation. To describe the radiation of the system, we have considered both plane symmetric and spherical Vaidya solutions. Physical conditions and thermodynamical relations are studied using local conservation of momentum and surface red-shift. We have found that for existence of radiation on the boundary, pressure on the boundary is not necessary.Comment: 8 Latex pages, No figures, Revtex styl

Study for pediatric protocol optimization in chest CT scan

Radiological exams are increasingly used in clinic for diagnostic analysis of different types of pathologies. These exams are associated with a dose that is received by the patient. In addition, the risks in exposure to ionizing radiation are different according to the group which the individual belongs. According to age, the group of children is more radiosensitive than adults. In this work we have obtained values of the air-weighted kerma index for chest scans studies in a General Electric Computed Tomography (CT) scanner model Discovery with 64-channels. Using an adult protocol, two scans have been performed, one using a cylinder standard PMMA phantom while the second one has used an oblong chest phantom designed for a two year old pediatric patient. Furthermore, other protocols have been selected with a constant voltage but changing the X-ray tube current and maintaining the image quality in order to obtain a reduction in the received dose by the pediatric patient. The use of the adult protocol in the child phantom has an air-weighted kerma index of 89.5% greater than the kerma index using the adult phantom. Due small patients receive higher doses; the use of specific protocols for children is important for the dose reduction in CT tests. An optimized pediatric chest protocol is presented, obtaining as a result dose reduction compared with the adult protocol of 62.2%. Because of different CT scanners characteristics and in order to optimize protocols regard to dose and diagnostic image quality, it is necessary to use pediatric phantoms in health centers