Microscopic Particle Image Velocimetry Research Articles

Quantifying the flow dynamics of supercritical CO2–water displacement in a 2D porous micromodel using fluorescent microscopy and microscopic PIV

The multi-phase flow of liquid/supercritical CO2 and water (non-wetting and wetting phases, respectively) in a two-dimensional silicon micromodel was investigated at reservoir conditions (80 bar, 24 °C and 40 °C). The fluorescent microscopy and microscopic particle image velocimetry (micro-PIV) techniques were combined to quantify the flow dynamics associated with displacement of water by CO2 (drainage) in the porous matrix. To this end, water was seeded with fluorescent tracer particles, CO2 was tagged with a fluorescent dye and each phase was imaged independently using spectral separation in conjunction with microscopic imaging. This approach allowed simultaneous measurement of the spatially-resolved instantaneous velocity field in the water and quantification of the spatial configuration of the two fluid phases. The results, acquired with sufficient time resolution to follow the dynamic progression of both phases, provide a comprehensive picture of the flow physics during the migration of the CO2 front, the temporal evolution of individual menisci, and the growth of fingers within the porous microstructure. During that growth process, velocity jumps 20–25 times larger in magnitude than the bulk velocity were measured in the water phase and these bursts of water flow occurred both in-line with and against the bulk flow direction. These unsteady velocity events support the notion of pressure bursts and Haines jumps during pore drainage events as previously reported in the literature [1–3]. After passage of the CO2 front, shear-induced flow was detected in the trapped water ganglia in the form of circulation zones near the CO2–water interfaces as well as in the thin water films wetting the surfaces of the silicon micromodel. To our knowledge, the results presented herein represent the first quantitative spatially and temporally resolved velocity-field measurements at high pressure for water displacement by liquid/supercritical CO2 injection in a porous micromodel.

A methodology for velocity field measurement in multiphase high‐pressure flow of CO2 and water in micromodels

AbstractThis paper presents a novel methodology for capturing instantaneous, temporally and spatially resolved velocity fields in an immiscible multiphase flow of liquid/supercritical CO2 and water through a porous micromodel. Of interest is quantifying pore‐scale flow processes relevant to geological CO2 sequestration and enhanced oil recovery, and in particular, at thermodynamic conditions relevant to geological reservoirs. A previously developed two‐color microscopic particle image velocimetry approach is combined with a high‐pressure apparatus, facilitating flow quantification of water interacting with supercritical CO2. This technique simultaneously resolves (in space and time) the aqueous phase velocity field as well as the dynamics of the menisci. The method and the experimental apparatus are detailed, and the results are presented to demonstrate its unique capabilities for studying pore‐scale dynamics of CO2‐water interactions. Simultaneous identification of the boundary between the two fluid phases and quantification of the instantaneous velocity field in the aqueous phase provides a step change in capability for investigating multiphase flow physics at the pore scale at reservoir‐relevant conditions.

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Reθ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.

A microscopic particle image velocimetry method for studying the dynamics of immiscible liquid–liquid interactions in a porous micromodel

The development of an experimental protocol to investigate the flow field produced by the interaction of two immiscible liquids flowing through a porous network is reported. The experimental protocol allows simultaneous quantification of the velocity distribution in a multi-liquid system based on the microscopic particle image velocimetry technique. The experimental challenges associated with this unique application are discussed, including two-liquid imaging and interface tracking, and solutions that couple refractive index matching and fluorescent signal separation are described. The technique was applied to both single- and two-liquid flows in a two-dimensional pore network comprising a staggered array of circular pillars wherein the flow was driven by a steady pressure gradient. Both drainage and imbibition were considered herein with a focus on fluid–fluid front migration and effects owing to the passage of the interface. The velocity distribution obtained for these two-liquid-phase flow scenarios revealed several peculiarities when compared to the reference case of single-liquid-phase flow. In particular, the instabilities associated with the interfacial processes propagate downstream and perturb the flow field, resulting in dramatic differences from the regular and periodic flow paths typical of steady-state, single-phase flow. Additionally, the passage of the interface does not restore previous flow patterns, but instead yields complex preferential flow paths that mutually interact with residual trapped pockets of fluid. Such dynamical events must be quantified in order to properly model the pore-scale physics central to fully understanding the wealth of practical applications represented by this model flow system.

Altered Wall Shear Stresses in Embryonic Chicken Outflow Tract due to Homocysteine Exposure

The embryonic heart is already beating before cardiac morphogenesis is complete. Therefore, the effect of blood flow on cardiogenesis has been subject of many studies. Shear stress, acting on the endocardial cells, has been shown to alter the expression of shear-responsive genes implicated in heart development. The congenital heart defects (CHDs) induced in the chicken embryo after altering blood flow and intracardiac shear stress are comparable to the type of defects observed after homocysteine (Hcy) exposure. This may suggest that altered blood flow and shear stress have a role in the etiology of Hcy-induced CHDs. The aim of this study is to quantitate wall shear stress (WSS) in the outflow tract (OFT) of Hcy-exposed chicken embryos. WSS was derived from the velocity field obtained with microscopic particle image velocimetry in the OFT of the embryonic-day-3 Hcy- and sham-treated chicken embryos. Hcy treatment consisted of L-Hcy-thiolactone 30 ?M solution injected into the neural tube of the embryos. The results suggest that Hcy has an inhibiting effect on WSS in the early embryonic chicken heart. These alterations in shear stress may cause altered gene expression and behaviour of endothelial cells, eventually contributing to the development of CHDs

A Real-Time Microscopic PIV System Using Frame Straddling High-Frame-Rate Vision

In this paper, we propose a novel concept of realtime microscopic particle image velocimetry (PIV) for apparent high-speed microchannel flows in lab-on-achip (LOC). We introduce a frame-straddling dualcamera high-speed vision system that synchronizes two different camera inputs for the same camera view with a submicrosecond time delay. In order to improve upper and lower limits of measurable velocity in microchannel flow observation, we designed an improved gradient-based optical flow algorithm that adaptively selects a pair of images in the optimal frame-straddling time between the two camera inputs based on the amplitude of the estimated optical flow. This avoids large image displacement between frames that often generates serious errors in optical flow estimation. Our method is implemented using software on a frame-straddling dual-camera high-speed vision platform that captures real-time video and processes 512 × 512 pixel images at 2000 fps for the two camera heads and controls the frame-straddling time delay between them from 0 to 0.25 ms with 9.9 ns step. Our microscopic PIV system with frame-straddling dualcamera high-speed vision simultaneously estimates the velocity distribution of high-speed microchannel flow at 1 × 108pixel/s or more. Results of experiments using real microscopic flows on microchannels thousands of µm wide on LOCs verify the performance of the real-time microscopic PIV system we developed.

Measurements of turbulence in a microscale multi-inlet vortex nanoprecipitation reactor

The microscale multi-inlet vortex reactor (MIVR) is designed for use in Flash NanoPrecipitation (FNP), a promising technique for producing nanoparticles within small particle size distribution. Fluid mixing is crucial in the FNP process, and due to mixing’s strong dependence upon fluid kinematics, investigating velocity and turbulence within the reactor is crucial to optimizing reactor design. To this end, microscopic particle image velocimetry has been used to investigate flow within the MIVR. Three Reynolds numbers are studied, namely, Rej = 53, 93 and 240. At Rej = 53, the flow is laminar and steady. Due to the strong viscous effects at this Reynolds number, distinct flow patterns are observed at different distances from the reactor top and bottom walls. The viscous effects also retard the tangential motions within the reactor, resulting in a weaker vortex than appears at the higher Reynolds numbers. As the Reynolds number is increased to 93, the flow becomes more homogeneous over the depth of the reactor due to weaker viscous effects, yet the flow is still steady. The diminishing effects of viscosity also result in a stronger vortex. At the highest Reynolds number investigated, the flow is turbulent. Turbulent statistics including tangential and radial velocity fluctuations and Reynolds shear stresses are analyzed for this case in addition to the mean velocity field. The tangential motions of the flow are strongest at Rej = 240. Both the tangential and radial velocity fluctuations increase as the flow spirals toward the center of the reactor. The magnitudes of the tangential and radial velocity fluctuations are similar, suggesting that the turbulence is locally isotropic.

Time-resolved OCT-μPIV: a new microscopic PIV technique for noninvasive depth-resolved pulsatile flow profile acquisition

In vivo acquisition of endothelial wall shear stress requires instantaneous depth-resolved whole-field pulsatile flow profile measurements in microcirculation. High-accuracy, quantitative and non-invasive velocimetry techniques are essential for emerging real-time mechano-genomic investigations. To address these research needs, a novel biological flow quantification technique, OCT-μPIV, was developed utilizing high-speed optical coherence tomography (OCT) integrated with microscopic Particle Image Velocimetry (μPIV). This technique offers the unique advantage of simultaneously acquiring blood flow profiles and vessel anatomy along arbitrarily oriented sagittal planes. The process is instantaneous and enables real-time 3D flow reconstruction without the need for computationally intensive image processing compared to state-of-the-art velocimetry techniques. To evaluate the line-scanning direction and speed, four sets of parametric synthetic OCT-μPIV data were generated using an in-house code. Based on this investigation, an in vitro experiment was designed at the fastest scan speed while preserving the region of interest providing the depth-resolved velocity profiles spanning across the width of a micro-fabricated channel. High-agreement with the analytical flow profiles was achieved for different flow rates and seed particle types and sizes. Finally, by employing blood cells as non-invasive seeding particles, in vivo embryonic vascular velocity profiles in multiple vessels were measured in the early chick embryo. The pulsatile flow frequency and peak velocity measurements were also acquired with OCT-μPIV, which agreed well with previous reported values. These results demonstrate the potential utility of this technique to conduct practical microfluidic and non-invasive in vivo studies for embryonic blood flows.

Turbulence measurements in a rectangular mesoscale confined impinging jets reactor

Mesoscale chemical reactors capable of operating in the turbulent flow regime, such as confined impinging jets reactors (CIJR), offer many advantages for rapid chemical processing at the microscale. One application where these reactors are used is flash nanoprecipitation, a method for producing functional nanoparticles. Because these reactors often operate in a flow regime just beyond transition to turbulence, modeling flows in these reactors can be problematic. Moreover, validation of computational fluid dynamics models requires detailed and accurate experimental data, the availability of which has been very limited for turbulent microscale flows. In this work, microscopic particle image velocimetry (microPIV) was performed in a mesoscale CIJR at inlet jet Reynolds numbers of 200, 1,000, and 1,500. Pointwise and spacial turbulence statistics were calculated from the microPIV data. The flow was observed to be laminar and steady in the entire reactor at a Reynolds number of 200. However, at jets Reynolds numbers of 1,000 and 1,500, instabilities as a result of the jets impinging along the centerline of the reactor lead to a highly turbulent impingement region. The peak magnitude of the normalized Reynolds normal and shear stresses within this region were approximately the same for the Reynolds numbers of 1,000 and 1,500. The Reynolds shear stress was found to exhibit a butterfly shape, consistent with a flow field dominated by an oblique rocking of the impingement zone about the center of the reactor. Finally, the spatial auto- and cross-correlations velocity fluctuations were calculated and analyzed to obtain an understanding of size of the coherent structures.

High‐Throughput Printing via Microvascular Multinozzle Arrays

Microvascular multinozzle arrays are designed and fabricated for high-throughput printing of functional materials. Ink-flow uniformity within these multigeneration, bifurcating microchannel arrays is characterized by computer modeling and microscopic particle image velocimetry (micro-PIV) measurements. Both single and dual multinozzle printheads are produced to enable rapid printing of multilayered periodic structures over large areas (≈1 m(2)).

On the effect of particle image intensity and image preprocessing on the depth of correlation in micro-PIV

The depth of correlation (DOC) is an experimental parameter, introduced to quantify the thickness of the measurement volume and thus the depth resolution in microscopic particle image velocimetry (μPIV). The theory developed to estimate the value of the DOC relies on some approximations that are not always verified in actual experiments, such as a single thin-lens optical system. In many practical μPIV experiments, a deviation of the actual DOC from its nominal value can be expected, due for instance to additional components present in the optical path of the microscope or to the use of image preprocessing before the PIV evaluation. In the presented paper, the effect of real particle image intensity distribution and image preprocessing on the thickness of the measurement volume is investigated. This is performed studying the defocusing of tracer particles and the DOC-related bias error present in μPIV measurements in a Poiseuille flow. The analysis shows that the DOC predicted using the conventional formulas can be significantly smaller than its actual value. To overcome this problem, the use of an effective NA determined experimentally from the curvature of the image autocorrelations is proposed. The accuracy of this approach to properly predict the actual size of DOC is discussed and validated on the experimental data. The effectiveness of image preprocessing to reduce the DOC-related bias error is tested and discussed as well.

An experimental study of pulsed micro-flows pertinent to continuous subcutaneous insulin infusion therapy

An experimental study was conducted to investigate the unsteady micro-flow driven by an insulin pump commonly used in continuous subcutaneous insulin infusion (CSII) therapy. A microscopic particle image velocimetry (PIV) system was used to characterize the transient behavior of the micro-flow upon the pulsed excitation of the insulin pump in order to elucidate the underlying physics for a better understanding of the microphysical process associated with the insulin delivery in CSII therapy. The effects of air bubbles entrained inside the micro-sized CSII tubing system on the insulin delivery process were also assessed based on the micro-PIV measurements. While most solutions to insulin occlusion-related problems are currently based on clinical trials, the findings derived from the present study can be used to provide a better guidance for the troubleshooting of insulin occlusion in CSII therapy.

Depth of correlation reduction due to out-of-plane shear in microscopic particle image velocimetry

The effects of out-of-plane shear on the depth of correlation in microscopic particle image velocimetry (microPIV) were analyzed by deriving an analytical model of microPIV interrogation for flowfields containing velocity gradients in the out-of-plane direction. The model is derived using a Taylor series approximation and is therefore most accurate in the limit of small shear, but it does provide valuable insights. The model shows that out-of-plane velocity gradients reduce the depth of correlation compared to flowfields without gradients, but this decrease in depth of correlation is smaller than the increase in depth of correlation for a flowfield containing only in-plane velocity gradients. By combining the analysis for flows with in-plane gradients and with the analysis for flows with out-of-plane gradients, an equation for the depth of correlation for a flowfield containing both in-plane and out-of-plane velocity gradients was derived. This equation suggests that unless the out-of-plane gradients are significantly larger than the in-plane gradients, the effect on the depth of correlation due to the out-of-plane gradients is negligible, and the depth of correlation can be very closely approximated by calculating the depth of correlation using only the in-plane velocity gradients.

Characterization of Active Cooling and Flow Distribution in Microvascular Polymers

Two- and three-dimensional microvascular networks embedded within a polymer fin were fabricated via direct write assembly to demonstrate cooling potential of vascular polymer structures. Thin fin cooling experiments were carried out utilizing water and polyalphaolefin (PAO) oil-based coolant as the working fluids. The surface temperature of the fin was monitored using an infrared camera and flow distribution within the network was evaluated by microscopic particle image velocimetry. The effective heat transfer coefficient was increased 53-fold at low Reynolds number for water cooling in both 2D and 3D geometries. However, 3D architectures offer more uniform flow distribution and the ability to efficiently adapt to blockages and reroute flow within the network. Microvascular materials are excellent candidates for compact, efficient cooling platforms for a variety of applications and 3D architectures offer unique performance enhancements.

Interaction of Synthetic Jet with Boundary Layer Using Microscopic Particle Image Velocimetry

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

Natural advection from a microcantilever heat source

Fluid motion induced by a microcantilever heat source immersed in water is assessed by microscopic particle image velocimetry. Large fluid velocity is observed near the heated cantilever and its magnitude increases with cantilever temperature. Neither Brownian nor thermophoretic motion of the tracer particles can account for these large velocities. Rather, this fluid motion is consistent with buoyancy-driven advection. The observation of natural advection due to a microscopic heat source is important for electronics cooling, performance of thermal sensors, and the thermal processing of small biological samples.

Turbulent precipitation in micromixers: CFD simulation and flow field validation

The object of this work is the investigation of the process of nanoparticles turbulent precipitation in a micromixer, the Confined Impinging Jets Mixer. This study is motivated by the increasing importance that nanoparticulate systems have for applications in several fields and by the consequent necessity of developing an economical and reliable process for the production of nanoparticles with the desired qualities, in terms of size, morphology and composition. The precipitation process is among the most promising processes for this purpose, and micromixers, such as the CIJM, are employed because they provide high mixing rates and efficiencies, which are needed because the process is highly mixing sensitive. Here a precipitation model based on classical precipitation theory and Computational Fluid Dynamics was developed and tested on barium sulphate precipitation, which is often employed as a mixing sensitive test reaction alternative to parallel competitive reactions. The use of barium sulphate precipitation also allows to assess the capability of the CIJM of producing very fine particulate systems. The flow field is modelled with a RANS approach, and validated through comparison with experimental data, obtained with the microscopic Particle Image Velocimetry. Model predictions on barium sulphate mean particles size were compared with experimental data and good agreement was found.

The impact of surface roughness on flow through a rectangular microchannel from the laminar to turbulent regimes

Modifications of fluid flow within microscale flow passages by internal surface roughness is investigated in the laminar, transitional, and turbulent regimes using pressure-drop measurements and instantaneous velocity fields acquired by microscopic particle-image velocimetry (micro-PIV). The microchannel under study is rectangular in cross-section with an aspect ratio of 1:2 (depth: width) and a hydraulic diameter of \(D_{\rm h} =600\,\upmu \hbox{m}.\) Measurements are first performed under smooth-wall conditions to establish the baseline flow characteristics within the microchannel followed by measurements for two different rough-wall cases [with RMS roughness heights of \(7.51\,\upmu \hbox{m}\) (0.0125Dh) and \(15.1\,\upmu \hbox{m}\) (0.025Dh)]. The roughness patterns under consideration are unique in that they are reminiscent of surface irregularities one might encounter in practical microchannels due to imperfect fabrication methods. The pressure-drop results reveal the onset of transition above \(Re_{\rm cr}=1{,}800\) for the smooth-wall case, consistent with the onset of transition at the macroscale, along with deviation from laminar behavior at progressively lower Re with increasing roughness. Mean velocity profiles computed from the micro-PIV ensembles at various Re for each surface condition confirm these trends, meaning \(Re_{\rm cr}\) is a strong function of roughness. The ensembles of velocity fields at each Re and surface condition in the transitional regime are subdivided into fields embodying laminar behavior and fields containing disordered motions. This decomposition reveals a clear hastening of the flow toward a turbulent state due both to the roughness dependence of Recr and an enhancement in the growth rate of the non-laminar fraction of the flow when the flow is in the early stages of transition. Nevertheless, the range of Re relative to Recr over which the flow transitions from a laminar to a turbulent state is found to be essentially the same for all three surface conditions. From a structural viewpoint, instantaneous velocity fields embodying disordered behavior in the transitional regime are found to contain large-scale motions consistent with hairpin-vortex packets irrespective of surface condition. These observations are in accordance with the characteristics of transitional and turbulent flows at the macroscale and therefore indicate that the overall structural paradigm of the flow is relatively insensitive to roughness. From a quantitative viewpoint, however, the intensity of both the velocity fluctuations and structural activity appear to increase substantially with increasing roughness, particularly in the latter stages of transition. These differences are further supported by the trends of single-point statistics of the non-laminar ensembles and quadrant analysis in which an intensification of the velocity fluctuations by surface roughness is noted in the region close to the wall, particularly for the wall-normal fluctuations.

Particle image identification and correlation analysis in microscopic holographic particle image velocimetry

This paper discusses the different analysis methods used in holographic particle image velocimetry to measure particle displacement and compares their relative performance. A digital holographic microscope is described and is used to record the light scattered by particles deposited on cover slides that are displaced between exposures. In this way, particle position and displacement are controlled and a numerical data set is generated. Data extraction using nearest neighbor analysis and correlation of either the reconstructed complex amplitude or intensity fields is then investigated.

Cavitation microstreaming and stress fields created by microbubbles

Cavitation microstreaming plays a role in the therapeutic action of microbubbles driven by ultrasound, such as the sonoporative and sonothrombolytic phenomena. Microscopic particle–image velocimetry experiments are presented. Results show that many different microstreaming patterns are possible around a microbubble when it is on a surface, albeit for microbubbles much larger than used in clinical practice. Each pattern is associated with a particular oscillation mode of the bubble, and changing between patterns is achieved by changing the sound frequency. Each microstreaming pattern also generates different shear stress and stretch/compression distributions in the vicinity of a bubble on a wall. Analysis of the micro-PIV results also shows that ultrasound-driven microstreaming flows around bubbles are feasible mechanisms for mixing therapeutic agents into the surrounding blood, as well as assisting sonoporative delivery of molecules across cell membranes. Patterns show significant variations around the bubble, suggesting sonoporation may be either enhanced or inhibited in different zones across a cellular surface. Thus, alternating the patterns may result in improved sonoporation and sonothrombolysis. The clear and reproducible delineation of microstreaming patterns based on driving frequency makes frequency-based pattern alternation a feasible alternative to the clinically less desirable practice of increasing sound pressure for equivalent sonoporative or sonothrombolytic effect. Surface divergence is proposed as a measure relevant to sonoporation.