Passive Fluid Research Articles

Compartmental and Data-Based Modeling of Cerebral Hemodynamics: Nonlinear Analysis.

As an extension to our study comparing a putative compartmental and data-based model of linear dynamic cerebral autoregulation (CA) and CO2-vasomotor reactivity (VR), we study the CA-VR process in a nonlinear context. We use the concept of principal dynamic modes (PDM) in order to obtain a compact and more easily interpretable input-output model. This in silico study permits the use of input data with a dynamic range large enough to simulate the classic homeostatic CA and VR curves using a putative structural model of the regulatory control of the cerebral circulation. The PDM model obtained using theoretical and experimental data are compared. It was found that the PDM model was able to reflect accurately both the simulated static CA and VR curves in the associated nonlinear functions (ANFs). Similar to experimental observations, the PDM model essentially separates the pressure-flow relationship into a linear component with fast dynamics and nonlinear components with slow dynamics. In addition, we found good qualitative agreement between the PDMs representing the dynamic theoretical and experimental CO2-flow relationship. Under the modeling assumption and in light of other experimental findings, we hypothesize that PDMs obtained from experimental data correspond with passive fluid dynamical and active regulatory mechanisms. Both hypothesis-based and data-based modeling approaches can be combined to offer some insight into the physiological basis of PDM model obtained from human experimental data. The PDM modeling approach potentially offers a practical way to quantify the status of specific regulatory mechanisms in the CA-VR process.

Continuous synthesis of drug-loaded nanoparticles using microchannel emulsification and numerical modeling: effect of passive mixing.

By using interdigital microfluidic reactors, monodisperse poly(d,l lactic-co-glycolic acid) nanoparticles (NPs) can be produced in a continuous manner and at a large scale (~10 g/h). An optimized synthesis protocol was obtained by selecting the appropriated passive mixer and fluid flow conditions to produce monodisperse NPs. A reduced NP polydispersity was obtained when using the microfluidic platform compared with the one obtained with NPs produced in a conventional discontinuous batch reactor. Cyclosporin, an immunosuppressant drug, was used as a model to validate the efficiency of the microfluidic platform to produce drug-loaded monodisperse poly(d,l lactic-co-glycolic acid) NPs. The influence of the mixer geometries and temperatures were analyzed, and the experimental results were corroborated by using computational fluid dynamic three-dimensional simulations. Flow patterns, mixing times, and mixing efficiencies were calculated, and the model supported with experimental results. The progress of mixing in the interdigital mixer was quantified by using the volume fractions of the organic and aqueous phases used during the emulsification–evaporation process. The developed model and methods were applied to determine the required time for achieving a complete mixing in each microreactor at different fluid flow conditions, temperatures, and mixing rates.

Hydrodynamic Theories for Flows of Active Liquid Crystals and the Generalized Onsager Principle

We articulate and apply the generalized Onsager principle to derive transport equations for active liquid crystals in a fixed domain as well as in a free surface domain adjacent to a passive fluid matrix. The Onsager principle ensures fundamental variational structure of the models as well as dissipative properties of the passive component in the models, irrespective of the choice of scale (kinetic to continuum) and of the physical potentials. Many popular models for passive and active liquid crystals in a fixed domain subject to consistent boundary conditions at solid walls, as well as active liquid crystals in a free surface domain with consistent transport equations along the free boundaries, can be systematically derived from the generalized Onsager principle. The dynamical boundary conditions are shown to reduce to the static boundary conditions for passive liquid crystals used previously.

Monitoring fluid substitution in rock samples from noise correlation

An alternative laboratory technique to measure the elastic constants of solid samples, based on the analysis of the cross-correlation spectra of the vibratory response of randomly excited short solid cylinders, has been recently proposed. The aim of this paper is to check the ability of the technique called passive ultrasonic interferometry to monitor fluid substitution in different rock samples. Velocity variations due to fluid substitution are easily measured if the wave attenuation in the fluid-saturated rock is not too large (typically in rocks with few cracks or microfractures). The experimental results are in agreement with the predictions of Biot–Gassmann poroelastic theory. The effect of substituting water with a stiffer saturating fluid, such as ethylene glycol, is to increase the overall bulk modulus of the rock, without any substantial effect on shear modulus. Furthermore, the experimental results compare well with those obtained independently with conventional pulse-transmission technique using ultrasonic transducers. However, the measured pulse-transmission bulk moduli are slightly larger than the corresponding measured ultrasonic interferometry moduli, with the deviation increasing with increasing fluid viscosity. This can be explained by dispersion due to wave-induced flow of the viscous fluid since pulsetransmission experiments involve higher frequencies than ultrasonic interferometry experiments.

Monitoring the correlation between passive leg-raising maneuver and fluid challenge in pediatric cardiac surgery patients using impedance cardiography

Aim The aim of the study was to highlight the effectiveness of passive leg-raising maneuver as a predictor of fluid responsiveness in pediatric patients following cardiac surgery and to determine the parameter that we can depend on to assess its responsiveness. Patients and methods This prospective randomized study was performed in the ICU. Forty pediatric patients aged 2-7 years with a noncyanotic cardiac defect scheduled for elective corrective cardiac surgery under cardiopulmonary bypass (CPB) support were included if they needed fluid challenge (FC) in the early postoperative period (10 ml/kg normal saline infusion). Hemodynamic parameters [heart rate, mean blood pressure, stroke volume (SV), and cardiac index (CI)] were assessed at baseline, after passive leg raising (PLR), at baseline again, and after FC. Statistical analysis A comparison of pre-PLR and post-PLR hemodynamic parameters and those before and after FC was made using the paired Student's t-test, whereas a comparison after passive leg raising (PLR) and FC was made using the unpaired Student's t-test. The hemodynamic parameters after PLR and its relation to the responses to fluid administration were analyzed using diagnostic validity tests and the χ2 -test. Results The increase in SV and CI with PLR is significantly correlated with the response to fluid administration. An increase in CI by 10% or more due to PLR predicted preload-dependent status with a sensitivity of 65% and a specificity of 75%, whereas an increase in SV by more than 10% due to PLR predicted preload-dependent status with a sensitivity of 42.1% and a specificity of 85.7%. Conclusion The PLR maneuver is a reliable noninvasive method that can predict volume responsiveness in post-cardiac-surgery pediatric patients. Both SV and CI can be used as predictors of fluid responsiveness, although CI is a more accurate.

Effect of the vertical component of diffusion on passive scalar transport in an isolated vortex model.

On the basis of the ellipsoidal vortex model and a Monte-Carlo-type diffusion simulation, we examine the flux and ensuing distribution of passive fluid particles through the boundary of an idealized geophysical vortex. Our focus is on features that the horizontal and vertical diffusion components introduce into the fluid particle transport. We examine the concurrent effect of both components, and we compare it with the only horizontal diffusion impact. We analyze the ellipsoid vortex model in two cases: (i) the steady state when the ellipsoid is motionless, i.e., there is no variation in its axes' lengths, and consequently the exterior fluid is not being stirred; (ii) the perturbed case when the ellipsoid rotates periodically, varying it axes' lengths, which results in the appearance of stirred fluid outside the ellipsoid. Influenced by diffusion, a fluid particle is now permitted to move to another vertical horizon, thus there is an increased possibility that the particle will eventually be located in the exterior stirred region rather than in the ellipsoid vortex with the regular dynamics. This is because the area of the horizontal section of the ellipsoid vortex decreases with depth, but the region of stirred exterior fluid extends significantly deeper. Numerical calculations show that factoring in the vertical component of diffusion significantly affects scalar spreading in the horizontal plane in the perturbed case, while in the steady state the vertical component of diffusion only induces dispersion linear growth according to a Gaussian distribution.

Role of interfacial friction for flow instabilities in a thin polar-ordered active fluid layer.

We construct a generic coarse-grained dynamics of a thin inflexible planar layer of polar-ordered suspension of active particles that is frictionally coupled to an embedding isotropic passive fluid medium with a friction coefficient Γ. Being controlled by Γ, our model provides a unified framework to describe the long-wavelength behavior of a variety of thin polar-ordered systems, ranging from wet to dry active matter and free-standing active films. Investigations of the linear instabilities around a chosen orientationally ordered uniform reference state reveal generic moving and static instabilities in the system that can depend sensitively on Γ. Based on our results, we discuss estimation of bounds on Γ in experimentally accessible systems.

Mesoscale air transport at a midlatitude squall line in Europe – a numerical analysis

This study examines a fast‐propagating squall line over Western and Central Europe. A high‐resolution regional weather prediction model is used to analyze the three‐dimensional wind field and the corresponding air mass transport induced by the mesoscale convective system. The squall line can be divided into a bow‐shaped strong part with a continuous line of heavy convective precipitation, and a weaker part exhibiting several isolated multi‐cell storms. In the strong part, a deep and intense rear inflow jet occurs with relative wind speeds of 12–18 m s−1. This part is also characterized by moderate to strong wind shear, in contrast to the weaker part of the squall line where a shallow cold pool surges under the potentially unstable presquall air.To investigate the air mass transport along the squall line, in the model simulations passive fluid tracers are initialized in several vertical layers with different thermodynamic conditions. At the strong part of the squall line a very efficient vertical tracer transport occurs. Potentially warm low‐level air is lifted from the lowermost 2 km to the upper troposphere where an organized horizontal outflow is observed. The potentially coldest presquall air descends just in front of the updraught region and partly penetrates the convective precipitation area. In the weaker part of the squall line, the potential instability is not significantly reduced because the vertical air mass transport is less effective there. Additionally, passive tracers are used to analyze the composition of the cold pool which turns out to be a very heterogeneous mixture of descending post‐frontal cold air and presquall air originating at different heights.

A pump-free membrane-controlled perfusion microfluidic platform.

In this article, we present a microfluidic platform for passive fluid pumping for pump-free perfusion cell culture, cell-based assay, and chemical applications. By adapting the passive membrane-controlled pumping principle from the previously developed perfusion microplate, which utilizes a combination of hydrostatic pressure generated by different liquid levels in the wells and fluid wicking through narrow strips of a porous membrane connecting the wells to generate fluid flow, a series of pump-free membrane-controlled perfusion microfluidic devices was developed and their use for pump-free perfusion cell culture and cell-based assays was demonstrated. Each pump-free membrane-controlled perfusion microfluidic device comprises at least three basic components: an open well for generating fluid flow, a micron-sized deep chamber/channel for cell culture or for fluid connection, and a wettable porous membrane for controlling the fluid flow. Each component is fluidically connected either by the porous membrane or by the micron-sized deep chamber/channel. By adapting and incorporating the passive membrane-controlled pumping principle into microfluidic devices, all the benefits of microfluidic technologies, such as small sample volumes, fast and efficient fluid exchanges, and fluid properties at the micro-scale, can be fully taken advantage of with this pump-free membrane-controlled perfusion microfluidic platform.

Bolus intravenous 0.9% saline, but not 4% albumin or 5% glucose, causes interstitial pulmonary edema in healthy subjects.

Rapid intravenous (iv) infusion of 0.9% saline alters respiratory mechanics in healthy subjects. However, the relative cardiovascular and respiratory effects of bolus iv crystalloid vs. colloid are unknown. Six healthy male volunteers were given 30 ml/kg iv 0.9% saline, 4% albumin, and 5% glucose at a rate of 100 ml/min on 3 separate days in a double-blinded, randomized crossover study. Impulse oscillometry, spirometry, lung volumes, diffusing capacity (DLCO), and blood samples were measured before and after fluid administration. Lung ultrasound B-line score (indicating interstitial pulmonary edema) and Doppler echocardiography indices of cardiac preload were measured before, midway, immediately after, and 1 h after fluid administration. Infusion of 0.9% saline increased small airway resistance at 5 Hz (P = 0.04) and lung ultrasound B-line score (P = 0.01) without changes in Doppler echocardiography measures of preload. In contrast, 4% albumin increased DLCO, decreased lung volumes, and increased the Doppler echocardiography mitral E velocity (P = 0.001) and E-to-lateral/septal e' ratio, estimated blood volume, and N-terminal pro B-type natriuretic peptide (P = 0.01) but not lung ultrasound B-line score, consistent with increased pulmonary blood volume without interstitial pulmonary edema. There were no significant changes with 5% glucose. Plasma angiopoietin-2 concentration increased only after 0.9% saline (P = 0.001), suggesting an inflammatory mechanism associated with edema formation. In healthy subjects, 0.9% saline and 4% albumin have differential pulmonary effects not attributable to passive fluid filtration. This may reflect either different effects of these fluids on active signaling in the pulmonary circulation or a protective effect of albumin.

Shifting and breakup instabilities of squeezed elliptic jets

Liquid jets are inherently unstable and breakup into drops due to capillary instability driven by surface tension. Elliptic jets have been studied and reported to be more unstable than circular jets. The reason for their increased instability has been often ascribed to the axis-switching of elliptic jets. We theoretically elucidate, for the first time, that even in the absence of axis-switching, an elliptic jet exhibits greater instability compared to circular jets solely due to interfacial curvature effects. For this purpose, we analyse the stability of a jet with elliptic cross-section subject to a non-uniform ambient pressure. This azimuthally dependent pressure distribution in the ambient fluid helps to prescribe a static base state for the elliptic jet and maintain the azimuthally varying curvature of the interface. Physically, this mathematical treatment implies that the jet is squeezed into an elliptic configuration by a non-uniform pressure distribution in the passive ambient fluid and therefore cannot exhibit axis-switching. The jet is assumed to be incompressible and inviscid. The stability of such a jet is analyzed using Rayleigh’s Work Principle (RWP) and linear stability theory. It is also shown that in addition to capillary breakup, analogous to that of classical circular jets, squeezed elliptic jets are found to exhibit a shifting mode instability, in which the entire jet translates parallel to its major axis. Both capillary breakup and shifting modes are rendered more unstable by an increase in ellipticity. For low ellipticity, the breakup mode exhibits higher growth rates as compared to the shifting mode. We show that there exists a critical ellipticity beyond which the shifting mode starts to dominate the capillary breakup mode. Predictions from both RWP and linear stability theory converge to Rayleigh’s classic results for a circular cylindrical jet.

Retraction Note to: Generation of topological chaos in the stokes flow in a rectangular cavity

We have investigated the two-dimensional periodic flow of a viscous incompressible fluid in the Stokes approximation inside a rectangular cavity, which is driven by motion of the top and bottom boundaries. We have obtained the analytical solution of this problem by the improved reduction method for a given distribution of the velocities of motion of the boundaries. We have detected the existence of three periodic points in the flow that generate the chaotic motion of a selected passive fluid. The origin of topological chaos in the Stokes flow without stirrers placed inside the cavity has been shown.

Capillary instability of axisymmetric, active liquid crystal jets.

We study linear stability of an infinitely long, axisymmetric, cylindrical active liquid crystal (ALC) jet in a passive isotropic fluid matrix using a polar active liquid crystal (ALC) model. We identify three possible unstable modes (or mechanisms) as the result of the interaction between the flow and the active (or self-propelled) molecular motion. The first unstable mode is related to the polarity vector instability when coupled to the flow field in the presence of the molecular activity. It can be traced back to the inherent polarity vector instability in a bulk active liquid crystal flow. However, it can be grossly amplified in the ALC jet to encompass up to infinitely many unstable growth rates when the long range distortional elastic interaction is weak in certain parameter regimes; it can also be suppressed in other parameter regimes completely. The second unstable mode is related to the classical capillary or Rayleigh instability, which exists in a finite wave interval [0, k(cutoff)]. The new feature for this instability lies in the dependence of the cutoff wave number (k(cutoff)) on the activity of the active matter system. For ALC jets with sufficiently strong contractile activity, the instability can be completely suppressed though. The third unstable mode is due to the active viscous stress. This unstable mode can emerge in the intermediate wave number regime at a sufficiently strong active viscosity and even expand all the way to the zero wave number limit when the Rayleigh unstable mode is absent. It can also be suppressed in the regime of weak active viscous stress. At any given values of the model parameters, the three types of instabilities can show up either individually or in a certain combination, or be completely suppressed altogether. In this paper, we discuss the positive growth rates associated with the instabilities, windows of instability and their dependence on model parameters through extensive numerical computations aided by asymptotic analyses.

Microfluidic Solution Isolated Pumping (μSIP)

One of the core requirements for the further maturity in microfluidics is to develop a fluid actuation method in bubble-free and power-efficient manner. Although kinds of active and passive fluid control techniques have been described over the past several decades, complex microfluidic systems still require bulky or energy-inefficient actuation components and simpler systems lack the functionality required for a simple field diagnostic test. Here a novel microfluidic pumping strategy is reported which utilizes the high air permeability of silicone materials to actuate fluid flow, named as Microfluidic Sample Isolated Pumping (μSIP). The key elements of μSIP are two channel networks, the fluidic channel and the degas channel, isolated by selective barriers and located in close proximity to each other. The selective barriers allow gas to pass through while being inaccessible for aqueous liquids due to low surface energy of the porous material. An air concentration gradient across the two networks generates an in-situ diffusive flux out of the fluidic channel. It results in fluidic channel pressure reduction which drives the liquid solution into the fluidic channel while the solution being kept isolated from the surrounding degas channels by the selective barrier. To understand the physics behind μSIP, theoretical analysis and experimental characterizations were performed, the experimental data could be well illustrated and predicted by the developed theories and also average flow velocities in the range of 0.7 to 7 mm/s were observed with the current designs. The μSIP provides tunable, reproducible, and bubble-free microfluidic pumping without any auxiliary equipment or device pre-treatment, providing a powerful liquid handling tool for a broad range of applications.

Passive micromixer using by convection and surface tension effects with air-liquid interface.

This article describes a passive micromixer that utilizes an air-liquid interface and surface tension effects to enhance fluid mixing via convection and Marangoni effects. Performance of the microfluidic component is tested within a passive-pumping-based device that consists of three microchannels connected in succession using passive micro-mixers. Mixing was quantified at 5 key points along the length of the device using microscope images of patterned streams of Alexa 488 fluorescent-dyed water and pure DI water flowing through the device. The passive micro-mixer mixed fluid 15-20 times more effectively than diffusion between laminar flow streams alone and is a novel micro-mixer embodiment that provides an additional strategy for removing external components from microscale devices for simpler, autonomous operation.

Analysis of Capillary Filling in Micro Channels for Passive Fluid Dynamics in Nano Lab on Chip Domain

A fluid dynamics in a micro channel for analytical chemistry and different aspects of this type of flow for specific application has remained a long-acting problem in the last two decades considering its numerus applications in various fields, thus, Surface tension and wall adhesive forces are often used to transport fluid through micro channels in Micro Electromechanical system devices or to measure the transport and position of small amounts of fluid using micropipettes. Here we took the advantages of wall adhesion and surface tension at the air/fluid interface, fluid rises through the channel and study also calculate the velocity, pressure and shape and position of the fluid surface, the model consist of a capillary channel of radius 50μm and a chamber. The study demonstrated that the fluid freely flown into the chamber 2mm/s without using any external mechanism.

Changes in Day 1 Post-Operative Intraocular Pressure Following Sutureless 23-Gauge and Conventional 20-Gauge Pars Plana Vitrectomy

Background/Aims: The benefits associated with transconjunctival sutureless vitrectomy techniques are continuing to be defined. The purpose of this study was to compare the incidence of extreme changes in day 1 intraocular pressure (IOP) following 23-gauge sutureless vitrectomy compared with conventional 20-gauge vitrectomy.Methods: Fifty consecutive 23-gauge and 50 consecutive 20-gauge cases were included; eyes with a history of previous vitreoretinal surgery were excluded. 23-gauge surgery was completed with passive fluid air exchange where no long acting tamponade was indicated. The surgery remained sutureless unless a leak was visible at the end of the procedure. Data were collected by retrospective case note review. Statistical analysis was carried out using Fisher’s exact and chi-square tests.Results: Twenty-two percent (11/50) of 23-gauge vitrectomies required suturing of one or more ports. On the first post-operative day hypotony (IOP<5mmHg) occurred in 1/50 eyes in the 20- and 23-gauge groups. Raised pressure (IOP>21mmHg) occurred in 30% (15/50) of eyes in the 20-gauge group and 8% (4/50) of eyes in the 23-gauge group; IOP>30mmHg only occurred in the 20-gauge group (3/50).Conclusion: Fluid air exchange following 23-gauge vitrectomy is associated with very low risk of day 1 hypotony. This predominantly sutureless technique appears to reduce the incidence and magnitude of early post-operative IOP elevation compared with conventional 20-gauge vitrectomy.

Chloride transport-driven alveolar fluid secretion is a major contributor to cardiogenic lung edema

Alveolar fluid clearance driven by active epithelial Na(+) and secondary Cl(-) absorption counteracts edema formation in the intact lung. Recently, we showed that impairment of alveolar fluid clearance because of inhibition of epithelial Na(+) channels (ENaCs) promotes cardiogenic lung edema. Concomitantly, we observed a reversal of alveolar fluid clearance, suggesting that reversed transepithelial ion transport may promote lung edema by driving active alveolar fluid secretion. We, therefore, hypothesized that alveolar ion and fluid secretion may constitute a pathomechanism in lung edema and aimed to identify underlying molecular pathways. In isolated perfused lungs, alveolar fluid clearance and secretion were determined by a double-indicator dilution technique. Transepithelial Cl(-) secretion and alveolar Cl(-) influx were quantified by radionuclide tracing and alveolar Cl(-) imaging, respectively. Elevated hydrostatic pressure induced ouabain-sensitive alveolar fluid secretion that coincided with transepithelial Cl(-) secretion and alveolar Cl(-) influx. Inhibition of either cystic fibrosis transmembrane conductance regulator (CFTR) or Na(+)-K(+)-Cl(-) cotransporters (NKCC) blocked alveolar fluid secretion, and lungs of CFTR(-/-) mice were protected from hydrostatic edema. Inhibition of ENaC by amiloride reproduced alveolar fluid and Cl(-) secretion that were again CFTR-, NKCC-, and Na(+)-K(+)-ATPase-dependent. Our findings show a reversal of transepithelial Cl(-) and fluid flux from absorptive to secretory mode at hydrostatic stress. Alveolar Cl(-) and fluid secretion are triggered by ENaC inhibition and mediated by NKCC and CFTR. Our results characterize an innovative mechanism of cardiogenic edema formation and identify NKCC1 as a unique therapeutic target in cardiogenic lung edema.

Effect of the direction of the electric field on the interfacial instability between a passive fluid and a viscoelastic polymer

In this paper, we study the linear stability of the interface between an Upper Convective Maxwell fluid and a hydrodynamically passive fluid subject to an electric field applied either parallel or normal to the flat interface between the two fluids. The fluids are leaky-dielectric and we apply surface-coupled model. We solve the model equations analytically and study the dispersion and neutral curves for various parameters representing the applied potential, the fluid’s elasticity, the physical and the electrical properties of the fluids, and the heights of the fluids in the presence of both normal and parallel electric fields. It is found that the critical wavenumber is independent of the Weissenberg number. However, increasing the Weissenberg number increases the maximum growth rate for both the normal and the parallel fields. The critical wavenumber increases with the dimensionless applied voltage for the normal field. Lastly for the normal field, for some values of the dimensionless parameters, the growth rate reached very large values representing some type of singularity as has been observed in the literature. However, for the same values of the parameters no singularity is observed for the parallel field.

Spectral analysis of mixing in chaotic flows via the mapping matrix formalism: Inclusion of molecular diffusion and quantitative eigenvalue estimate in the purely convective limit

This paper extends the mapping matrix formalism to include the effects of molecular diffusion in the analysis of mixing processes in chaotic flows. The approach followed is Lagrangian, by considering the stochastic formulation of advection-diffusion processes via the Langevin equation for passive fluid particle motion. In addition, the inclusion of diffusional effects in the mapping matrix formalism permits to frame the spectral properties of mapping matrices in the purely convective limit in a quantitative way. Specifically, the effects of coarse graining can be quantified by means of an effective Péclet number that scales as the second power of the linear lattice size. This simple result is sufficient to predict the scaling exponents characterizing the behavior of the eigenvalue spectrum of the advection-diffusion operator in chaotic flows as a function of the Péclet number, exclusively from purely kinematic data, by varying the grid resolution. Simple but representative model systems and realistic physically realizable flows are considered under a wealth of different kinematic conditions–from the presence of large quasi-periodic islands intertwined by chaotic regions, to almost globally chaotic conditions, to flows possessing “sticky islands”–providing a fairly comprehensive characterization of the different numerical phenomenologies that may occur in the quantitative analysis of mapping matrices, and ultimately of chaotic mixing processes.