Flow Shear Rate Research Articles

Optical coherence tomography in biofilm research: A comprehensive review.

Imaging of biofilm systems is a prerequisite for a better understanding of both structure and its function. The review aims to critically discuss the use of optical coherence tomography (OCT) for the visualization of the biofilm structure as well as its dynamic behavior. A short overview on common and well-known, established imaging techniques for biofilms such as scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), Raman microscopy (RM), and magnetic resonance imaging (MRI) paves the way to imaging biofilms at the mesoscale, which is perfectly covered by means of OCT. Principle, resolution, imaging velocity, and limitations of OCT are subsequently presented and discussed in the context of biofilm applications. Examples are provided showing the strength of this technique with respect to the visualization of the mesoscopic biofilm structure as well as the estimation of flow profiles and shear rates. Common and new structural parameters derived from OCT datasets are presented. Additionally, the review shows the importance of OCT with respect to a better description of mechanical biofilm properties. Finally, the implementation of multi-dimensional OCT datasets in biofilm modelling is shown by several examples aiming on an improved understanding of mass transfer at the bulk-biofilm interface. Biotechnol. Bioeng. 2017;114: 1386-1402. © 2017 Wiley Periodicals, Inc.

Identification of key liquid metal flow features in the physical conditioning of molten aluminium alloy with high shear processing

Although treating molten alloy with high shear processing (HSP) can dramatically refine the microstructure of solidified aluminium alloys, it was also recently employed as part of an effective route to purification of contaminated aluminium alloy scrap. The key mechanisms of HSP include the dispersion of large aluminium oxide films and clusters into very fine oxide particles by the high shear rate, and the redistribution of bulk melt by the agitation. These fine oxides act as nucleation sites for iron-based intermetallic phases, the formation of which is a pre-cursor to purification of the alloy. Macroscopic flow features of HSP, such as flow rate and shear rate, influence its performance significantly. Simulation based on Computational Fluid Dynamics was used to predict key features of fluid flow during HSP in a static direct chill (DC) caster. It was found that the distribution of shear rate and mass flow rate is highly nonuniform in the caster, and only in the close vicinity of the mixing head is there a relatively high level of shear rate and effective melt agitation. Therefore, effective dispersion of oxide films and clusters, and resulting significant nucleation of the intermetallics and/or primary aluminium phase, can only occur near the mixing head, and not throughout the whole crucible. Confidence in the model validity was built, by comparison with post-solidification microstructures in a previous experiment with similar process parameters and geometry.

Shear Rate Determination from Pore-Scale Flow Fields

Aqueous solutions with polymer additives often used to improve the macroscopic sweep efficiency in oil recovery typically exhibit non-Newtonian rheology. In order to predict the Darcy-scale effective viscosity mu _{mathrm{eff}} required for practical applications often, semi-empirical correlations such as the Cannella or Blake–Kozeny correlation are employed. These correlations employ an empirical constant (“C-factor”) that varies over three orders of magnitude with explicit dependency on porosity, permeability, fluid rheology and other parameters. The exact reasons for this dependency are not very well understood. The semi-empirical correlations are derived under the assumption that the porous media can be approximated by a capillary bundle for which exact analytical solutions exist. The effective viscosity mu _{mathrm{eff}} (v_{mathrm{Darcy}} ) as a function of flow velocity is then approximated by a cross-sectional average of the local flow field resulting in a linear relationship between shear rate gamma and flow velocity. Only with such a linear relationship, the effective viscosity can be expressed as a function of an average flow rate instead of an average shear rate. The local flow field, however, does in general not exhibit such a linear relationship. Particularly for capillary tubes, the velocity is maximum at the center, while the shear rate is maximum at the tube wall indicating that shear rate and flow velocity are rather anti-correlated. The local flow field for a sphere pack is somewhat more compatible with a linear relationship. However, as hydrodynamic flow simulations (using Newtonian fluids for simplicity) performed directly on pore-scale resolved digital images suggest, flow fields for sandstone rock fall between the two limiting cases of capillary tubes and sphere packs and do in general not exhibit a linear relationship between shear rate and flow velocity. This indicates that some of the shortcomings of the semi-empirical correlations originate from the approximation of the shear rate by a linear relationship with the flow velocity which is not very well compatible with flow fields from direct hydrodynamic calculations. The study also indicates that flow fields in 3D rock are not very well represented by capillary tubes.

Unsteady heat transfer in non-axisymmetric Homann stagnation-point flows

An analysis is carried out to study the unsteady non-axisymmetric Homann’s stagnation-point flow and heat transfer of an incompressible viscous fluid over a rigid plate in the presence of time-dependent free stream. The temperature of the plate is assumed to be higher than the ambient fluid temperature. Using similarity variables, the governing partial differential equations are transformed into nonlinear ordinary differential equations. These equations are then solved numerically using fourth-order Runge–Kutta method with shooting technique. The effects of the shear-to-strain rate ratio parameter \(\gamma \) (\(\gamma =b/a\) where a and b are the strain rate and shear rate of the stagnation-point flow, respectively) and the unsteadiness parameter \(\lambda \) on wall shear stress parameters, dimensionless velocities, rate of heat transfer at the wall and dimensionless temperature are analysed. It is found that the large-\(\gamma \) asymptotes do not depend on the parameter \(\lambda \).

Study of adhered particle’s secondary migration on the slider air bearing surface

An entrapped particle on the slider’s air bearing damages the surface of the slider or the disk. The study of particle movement on and the particle adhesion mechanism onto the slider surface is critical to reduce entrapped particle-induced damage. In this paper, we proposed a dynamical model of the adhesive particle’s secondary migration on the slider surface. The model predicts whether the adhered particle will remain static by the action of the aerodynamic and adhesion forces. There are three migration movement styles which are rolling, sliding and lifting away from the original adhered location. Further, particle rolling migration behaviors on the slider air bearing surface (ABS) are analyzed considering the effects of flow shear rate, particle diameter and properties. Finally, the particle migration trajectory and velocity with time on the investigated slider are presented. It was found that most particles that adhere to the slider surface remain static for the investigated slider; pure rolling motion on the slider ABS is the main migration styles. The particle with a larger diameter and air shear rate speeds up to a higher velocity, and it takes more time to speed up to the constant velocity. A particle’s secondary migration motion can be observed for the particle that adheres to the slider recession region and trailing pad. Then the particle enters airflow blowing out of or rolls away from the head-disk interface.

A General Shear-Dependent Model for Thrombus Formation.

Modeling the transport, activation, and adhesion of platelets is crucial in predicting thrombus formation and growth following a thrombotic event in normal or pathological conditions. We propose a shear-dependent platelet adhesive model based on the Morse potential that is calibrated by existing in vivo and in vitro experimental data and can be used over a wide range of flow shear rates (). We introduce an Eulerian-Lagrangian model where hemodynamics is solved on a fixed Eulerian grid, while platelets are tracked using a Lagrangian framework. A force coupling method is introduced for bidirectional coupling of platelet motion with blood flow. Further, we couple the calibrated platelet aggregation model with a tissue-factor/contact pathway coagulation cascade, representing the relevant biology of thrombin generation and the subsequent fibrin deposition. The range of shear rates covered by the proposed model encompass venous and arterial thrombosis, ranging from low-shear-rate conditions in abdominal aortic aneurysms and thoracic aortic dissections to thrombosis in stenotic arteries following plaque rupture, where local shear rates are extremely high.

In Vitro and Ex Vivo Models to Study T Cell Migration Through the Human Liver Parenchyma.

The liver is the largest internal organ and filters around 3 pints of blood per minute. This continuous flux of blood should not be confused with rapid egress of lymphocytes through the liver; this organ has intricate corridors of specialized sinusoidal spaces, ensuring that immune cells decelerate to shear flow rates, and providing ample opportunities to interact with parenchymal cells. Migration has been intricately linked to T cell function; it is therefore important to study liver T cell biology into context within the liver microenvironment. Here we discuss the highly organized architecture of liver-resident cells (sinusoidal endothelia, Kupffer cells, stellate cells/myofibroblasts, and biliary and hepatic epithelia) and showcase basic, multicellular, and complex systems to model T cell migration through the human liver microenvironment in vitro and ex vivo.

High-Vorticity with Periodic Flow Enhances in Vitro Biogenesis of Healthy Platelets from iPSC-Derived-Megakaryocytes

Each transfusion requires 200-300 billion platelets in patients with thrombocytopenia. To continuously supply such a huge number of platelets by ex vivo generation, two distinct steps, megakaryopoiesis and platelet shedding, must be both considered. For the former, one approach is to increase the number of source cell, megakaryocytes. For example, the immortalized megakaryocyte cell line (imMKCL) system uses self-renewing megakaryocyte (MK) cell lines derived from induced pluripotent stem cells (iPSCs) (Nakamura et al., Cell Stem Cell, 2014). For the latter, there have been an idea of bioreactors whereby shedding of platelets from proplatelets could be promoted by flow-dependent shear force within the bone marrow in vivo (Junt et al., Science, 2007; Zhang et al., J Exp Med, 2012). Based upon this idea, we constructed a flow chamber type bioreactor recapitulating in vivo blood flow shear rate. However, this bioreactor failed to efficiently yield platelets, and moreover, the produced platelets had poor quality as indicated by high Annexin V levels (Exp Hematol, 2011 and unpublished result).Recently, we demonstrated two different kinetics of platelet biogenesis from bone marrow MKs, whereby either thrombopoietin (TPO) mostly regulates steady-state shedding of platelets from proplatelets, or interleukin-a (IL-1a) triggers inflammation-dependent rupture of MK cytoplasm contributing to a quick increase of platelet count at higher rate (Nishimura et al., J Cell Biol, 2015). However, the rupture type platelets revealed shorter half-life with relatively higher Annexin V levels. Therefore, to gain insights from platelet biogenesis in vivo, we focused on biophysical analysis of steady-state platelet biogenesis via proplatelets in bone marrow. Our observations strongly indicated that the presence of ’vorticity’ defined by vortex turbulence in addition to shear-dependent ’stress’ and ’strain’ correlates with the efficient shedding of competent platelets. From this new finding, we developed an alternative bioreactor system, which enabled generation of 100 billion platelets from imMKCL in a 16L-scale liquid culture condition without any adherent machinery using two 10L-bioreactors. Furthermore, platelets generated via new bioreactors showed low Annexin V levels (<10-15%) and shortened bleeding time post transfusion into NOG mice and rabbits with thrombocytopenia, comparable to human blood product platelets. Regarding the platelet production using WAVE bag system (GE healthcare, UK), the system is already clinically available for cord blood cell expansion in most countries, but lacks adequate levels of vorticity and shear strain/stress. Accordingly, the produced platelets had high Annexin V levels (i.e., 50-65%) as well as diminished yield efficiency (P<0.001).In conclusion, our study has uncovered the novel biophysical aspect of platelet biogenesis. The application of the new set of physical parameters in constructing large sized bioreactors shall facilitate the industrialization of platelet production. DisclosuresEto:Megakaryon Co. Ltd.: Research Funding.

Dynamic settling of particles in shear flows of shear-thinning fluids

Dynamic settling is the phenomenon whereby a relatively dense particle settles through a sheared flow of a non-Newtonian fluid at a speed that depends on the shear rate of the background flow. This means that due to the nonlinear rheology, the settling velocity may vary spatially and temporally as the background shear rate of the suspending fluid varies, an effect which does not occur in Newtonian fluids. In this contribution, the consequences of this dependency are explored for a dilute suspension of particles released uniformly from a source in a sustained and externally-driven flow of shear-thinning fluid. It is shown theoretically that the concentration field does not remain uniform, but evolves downstream, allowing calculation of the runout length, settling times and distribution of the deposited particles. Flows with a velocity maximum are demonstrated to affect the concentration field very strongly as they develop a ‘kinematic barrier’ over which settling times are considerably lengthened. Flows with bidisperse suspensions are shown to produce deposits that vary non-monotonically in thickness and composition with distance downstream, an effect which is solely due to dynamic settling. Finally flows of viscoplastic fluids which exhibit yielded and unyielded regions may accentuate the role and effects of the kinematic barrier to settling.

Nanofluidic flow assisted assembly of dispersed plasmonic nanostructures into shallow nanochannel sensors

The authors present a method for assembling plasmonic nanostructures into already-sealed shallow nanochannel-based nanofluidic sensor structures. This method is termed as nanofluidic-flow-assisted-assembly (NFAA). NFAA utilizes nanofluidic flows with large shear rate and stress to deposit high-areal-density, well-dispersed plasmonic nanoparticles (NPs) into shallow nanochannel sensing areas. In particular, in a NFAA process, the nano/microfluidic structures are first patterned into a Si or SiO2 substrate and permanently sealed with fused quartz coverslips using plasma sealing. Afterward, a colloidal solution of plasmonic NPs is driven into the shallow nanochannel structures. In the shallow nanochannel areas, the large shear rate and stress of the nanofluidic colloidal solution flow results in the deposition of well-dispersed NPs and effectively prevents undesirable aggregation of NPs. Using NFAA, the authors have demonstrated the deposition of well-dispersed Au NPs with various areal densities (102–104 μm−2) into shallow nanochannels. The light absorbance peak of NFAA-coated Au NPs exhibits the narrower full-width-at-half-maximum than that of the Au NPs directly deposited from a colloidal solution, further indicating that NFAA can result in the higher degree of dispersion of high-density NPs. The authors also demonstrated that the additional nanoscale anchoring structures prepatterned in a shallow nanochannel, in combination with NFAA processes, can enable selective deposition of functional nanoparticles around designated locations. This work provides a nanofabrication scheme for introducing functional nanostructures into already-sealed nanofluidic structures. This method could be further generalized to enable integration of various electrically/optically active nanoscale components into permanently sealed nano/microfluidic structures and therefore address the incompatibility among the fabrication routes of these device structures.

Computational analysis of magnetic effects on pulsatile flow of couple stress fluid through a bifurcated artery

Background and objectivesThe objective of the present study is to investigate the magnetic field effect on pulsatile flow of blood through a bifurcated artery with mild stenosis in its parent lumen by taking blood as couple stress fluid. MethodsThe equations governing the flow are made non-dimensional, and coordinate transformation is employed to convert the irregular boundary to a regular boundary. The resulting system of equations is solved numerically using the finite difference method. ResultsThe shear stress, flow rate and impedance near the apex with pertinent parameters are obtained numerically and analyzed graphically. ConclusionsIt is noticed that physical quantities are changing suddenly with all parameters on both sides of the apex. This occurs because of back flow of the streaming blood at the onset of the lateral junction and secondary flow near the apex in the daughter artery.

Numerical simulation of pulsatile flow of blood in a porous-saturated overlapping stenosed artery

Pulsatile flow of blood through a porous-saturated stenotic artery has been studied under the influence of periodic body acceleration. The constitutive equation of Cross model is considered to characterize the blood and also modified form of Darcy’s law applicable to Cross model is used in this study. The shape of the stenosis in the arterial lumen is chosen to be overlapping w-shape. The present analysis models the pathological situation in which blood flows through an artery filled with blood clots and fatty cholesterol. In order to obtain the numerical solution, the modeled partial differential equation are normalized and then solved by using finite difference method. Numerical results are calculated for thorough investigation of the effects of porous medium on velocity, impedance, wall shear stress and flow rate. Calculations reveal that velocity, flow rate and shear stress increase while resistance to flow decreases with greater permeability parameter. Furthermore, the size of trapped bolus of fluid is also found to be reduced for large values of the permeability parameter indicating that progressively more porous media avoid bolus growth.

Flow analysis of the low Reynolds number swimmerC. elegans

Swimming cells and microorganisms are a critical component of many biological processes. In order to better interpret experimental studies of low Reynolds number swimming, we combine experimental and numerical methods to perform an analysis of the flow-field around the swimming nematode Caenorhabditis elegans. We first use image processing and particle tracking velocimetry to extract the body shape, kinematics, and flow-fields around the nematode. We then construct a three-dimensional model using the experimental swimming kinematics and employ a boundary element method to simulate flow-fields, obtaining very good quantitative agreement with experiment. We use this numerical model to show that calculation of flow shear rates using purely planar data results in significant underestimates of the true three-dimensional value. Applying symmetry arguments, validated against numerics, we demonstrate that the out-of-plane contribution can be accounted for via incompressibility and therefore simply calculated from particle tracking velocimetry. Our results show how fundamental fluid mechanics considerations may be used to improve the accuracy of measurements in biofluiddynamics.

A PMMA microfluidic dielectric sensor for blood coagulation monitoring at the point-of-care.

This paper describes the design and construct of a fully biocompatible, microfluidic, dielectric sensor targeted at monitoring human whole blood coagulation at the point-of-care (POC). The sensor assembly procedure involves using sputtered electrodes in a microfluidic channel with a physiologically relevant height of 50μm to create a three-dimensional (3D), parallel-plate, capacitive sensing area. The sensor is constructed with biocompatible materials of polymethyl methacrylate (PMMA) for the substrate and titanium nitride (TiN) for the sensing and floating electrodes. The real part of the complex relative dielectric permittivity of human whole blood is measured from 10kHz to 100MHz using an impedance analyzer and under static conditions. The temporal variation in dielectric permittivity at 1MHz for human whole blood undergoing coagulation shows a peak in permittivity at 5 minutes, which closely matches our previously established results. This sensor can pave the way for monitoring blood coagulation under physiologically relevant shear flow rates in the future.

Dynamic settling of particles in shear flows of shear-thinning fluids

Dynamic settling is the phenomenon whereby a relatively dense particle settles through a sheared flow of a non-Newtonian fluid at a speed that depends on the shear rate of the background flow. This means that due to the nonlinear rheology, the settling velocity may vary spatially and temporally as the background shear rate of the suspending fluid varies, an effect which does not occur in Newtonian fluids. In this contribution, the consequences of this dependency are explored for a dilute suspension of particles released uniformly from a source in a sustained and externally-driven flow of shear-thinning fluid. It is shown theoretically that the concentration field does not remain uniform, but evolves downstream, allowing calculation of the runout length, settling times and distribution of the deposited particles. Flows with a velocity maximum are demonstrated to affect the concentration field very strongly as they develop a ‘kinematic barrier’ over which settling times are considerably lengthened. Flows with bi-disperse suspensions are shown to produce deposits that vary non-monotonically in thickness and composition with distance downstream, an effect which is solely due to dynamic settling. Finally flows of viscoplastic fluids which exhibit yielded and unyielded regions may accentuate the role and effects of the kinematic barrier to settling.

Simultaneous assessment of red blood cell aggregation and oxygen saturation under pulsatile flow using high-frequency photoacoustics.

We investigate the feasibility of photoacoustic (PA) imaging for assessing the correlation between red blood cell (RBC) aggregation and the oxygen saturation (sO2) in a simulated pulsatile blood flow system. For the 750 and 850 nm illuminations, the PA amplitude (PAA) increased and decreased as the mean blood flow velocity decreased and increased, respectively, at all beat rates (60, 120 and 180 bpm). The sO2 also cyclically varied, in phase with the PAA for all beat rates. However, the linear correlation between the sO2 and the PAA at 850 nm was stronger than that at 750 nm. These results suggest that the sO2 can be correlated with RBC aggregation induced by decreased mean shear rate in pulsatile flow, and that the correlation is dependent on the optical wavelength. The hemodynamic properties of blood flow assessed by PA imaging may be used to provide a new biomarker for simultaneous monitoring blood viscosity related to RBC aggregation, oxygen delivery related to the sO2 and their clinical correlation.

Flow and Thixotropic Parameters for Rheological Characterization of Hydrogels.

The goal of this paper was to design several sodium carboxymethylcellulose hydrogels containing a BCS class II model drug and to evaluate their flow and thixotropic properties. The rheological measurements were performed at two temperatures (23 °C and 37 °C), using a rotational viscometer. The hydrogels were stirred at different time intervals (10 s, 2, 5, 10 and 20 min at 23 °C, and 10 s, 2 and 5 min at 37 °C), with a maximum rotational speed of 60 rpm, and the corresponding forward and backward rheograms were recorded as shear stress vs. shear rate. For all hydrogels, the rheological data obtained at both temperatures showed a decrease of viscosity with the increase of the shear rate, highlighting a pseudoplastic behaviour. The flow profiles viscosity vs. shear rate were quantified through power law model, meanwhile the flow curves shear stress vs. shear rate were assessed by applying the Herschel-Bulkley model. The thixotropic character was evaluated through different descriptors: thixotropic area, thixotropic index, thixotropic constant and destructuration thixotropic coefficient. The gel-forming polymer concentration and the rheological experiments temperature significantly influence the flow and thixotropic parameters values of the designed hydrogels. The rheological characteristics described have an impact on the drug release microenvironment and determine the stasis time at the application site.

A joint computational-experimental study of intracranial aneurysms: Importance of the aspect ratio

Rupture of a cerebral aneurysm (abnormal swelling of blood vessel in the brain) will cause subarachnoid hemorrhage, and will result in an alarming rate of mortality and morbidity. A joint computational-experimental study is conducted to assess the importance of the aspect ratio in the dynamics of blood flow. The aspect ratio is defined here to be the ratio of the height of the aneurysm to the linear dimension of the neck. Idealized models of such aneurysms located near a bifurcation point were investigated. Numerical simulations for hemodynamic properties like shear stress and flow rate were performed. The computational results were verified experimentally with specially fabricated phantoms, blood mimicking fluid and Doppler ultrasound imaging. Excellent agreements were obtained. Two features are highlighted, providing information in the intensely debated link between rupture risk and geometric factors. On increasing the aspect ratios, firstly, a jet impinging on the distal part of the neck can be observed, and secondly, a region of positive shear stress gradient can be found there. Furthermore, computational analyses for four patient-specific models were conducted to correlate with the results of idealized models and to provide further clinical insight.

Mathematical model for blood flow through a bifurcated artery using couple stress fluid

In this article, the blood flow through a bifurcated artery with mild stenosis is investigated taking blood as couple stress fluid. The artery configuring bifurcation is assumed to be symmetric about the axis of the artery and straight cylinders of finite length. The governing equations are non-dimensionalized and coordinate transformation is used to convert the irregular boundary to a regular boundary. The resulting system of equations is solved numerically using the finite difference method. The variation of shear stress, flow rate and impedance near the apex with pertinent parameters are studied graphically. It has been noticed that shear stress, flow rate and impedance have been changing suddenly with all the parameters on both sides of the apex. This occurs because of the backflow of the streaming blood at the onset of the lateral junction and secondary flow near the apex in the daughter artery.

Rheological behavior of aqueous dispersions containing blends of rhamsan and welan polysaccharides with an eco-friendly surfactant

Small amplitude oscillatory shear and steady shear flow properties of rhamsan gum and welan gum dispersions containing an eco-friendly surfactant (a polyoxyethylene glycerol ester) formulated to mimic the continuous phase of O/W emulsions were studied using the surface response methodology. A second order polynomial equation fitted the influence of surfactant concentration, rhamsan/welan mass ratio and total concentration of polysaccharides. Systems containing blends of rhamsan and welan did not show synergism but thermodynamic incompatibility and made it possible to adjust the linear viscoelastic and low shear rate flow properties to achieve values in between those of systems containing either rhamsan or welan as the only polysaccharide.All the systems studied exhibited weak gel rheological properties as the mechanical spectra displayed the plateau or rubber-like relaxation zone, the linear viscoelastic range was rather narrow and flow curves presented shear thinning behavior, which fitted the power-law equation. While mechanical spectra of the systems studied demonstrated that they did not control the linear viscoelastic properties of the corresponding emulsions, the blend of rhamsan and welan gums was able to control the steady shear flow properties.