micro-PIV Technique Research Articles

Influence of dispersed phase flow-rate pulsations on the liquid–liquid parallel flow in a T-junction microchannel

Active flow control in microfluidic devices to broaden the range of desired conditions is a significant practical concern. In this study, we experimentally investigate liquid–liquid parallel flow under the dispersed phase flow-rate pulsations in a T-junction microchannel. Sinusoidal flow rate disturbances with variations in amplitude and period were applied to dispersed phases of different viscosities. The study involves flow visualization and velocity field measurements using the micro-PIV technique for both free and excited flow conditions. The analysis of flow pattern maps, velocity fields, velocity gradients, and phase-averaged velocity profiles in a T-junction region and far downstream provided insights into the evolution of disturbances, which is different in low- and high-viscosity liquids. In the less viscous liquid, transverse waves amplify longitudinal ones due to relaxation of the liquid–liquid interface, while in the high-viscosity dispersed phase, longitudinal waves are damped significantly, with the transverse wave amplitude remaining almost constant. As a result, we revealed two distinct mechanisms of disturbance wave propagation and the loss of parallel flow stability, which are both dependent on the Ohnesorge number of the dispersed phase. Destabilization of the parallel flow led to the formation of plugs with a narrow length distribution. Single-mode, double-mode, triple-mode, and multi-mode plug formation regimes were identified. The results of the study can potentially be used to extend the range of segmented flow regimes and generate plugs of a desired length.

Pore-scale experiments reveal distinct flow field of polymer flooding with viscoelasticity loss by high salinity

It has been confirmed that high salinity significantly reduces the oil recovery from polymer flooding, which is generally considered to be due to the loss of polymer viscoelasticity. However, how the lost viscoelasticity affects the polymer flow field remains unclear. Therefore, the micro-PIV technique and rheological experiments were utilized in this study to investigate the pore-scale flow field of polyacrylamide (HPAM) under different salinities (NaCl). The results showed that as the salinity increased from 0 g/L to 10 g/L, the storage modulus and loss modulus of HPAM decreased by 84.5%− 97.9% and 87.3%− 98.3%, respectively. Regarding the pore-scale flow field, the average velocity of HPAM under low (0 g/L) and high (10 g/L) salinities was 33.4% lower and 19.5% higher than that of water flooding, respectively. Besides, high salinity resulted a higher velocity ratio between main stream and margin regions (11.8) than that of water flooding (2.0), which suggested a greater heterogeneity and a lower sweep efficiency of the flow field in pore networks. Thus, the lower sweep efficiency with high salinity is one of the dominant causes for the reduction in oil recovery. Furthermore, the breaching in main stream and the plugging in margin regions indicated that the shear resistance stability of polymer solution was diminished under high salinity. Consequently, the flow velocity in the main stream with a higher shear rate increased, whereas the margin region with a lower shear rate was plugged, which reduced the sweep efficiency. This study provides a further understanding of the porous media flow mechanisms in polymer applications.

A New Method to Develop Homogeneous and Heterogeneous Porous Micromodels Applicable to Enhanced Oil Recovery and Flow Visualization Experiments

AbstractIn this paper, a new method to fabricate micromodels having homogeneous and heterogeneous porous structures is reported to gain fundamental insight into the flow through porous media. The technique of microparticle image velocimetry (PIV) is used to map the pore-scale velocity field inside the micromodels. A thin perforated metal sheet composed of uniformly distributed circular holes is used as the master pattern, and the replica of the negative of this perforated sheet is transferred to a polydimethylsiloxane (PDMS) substrate using a method similar to the soft lithography. This method allows an efficient fabrication of micromodels having different porosity by adjusting and selecting the perforated sheets of different hole sizes. The prepared micromodels were tested for its applicability and reliability by carrying out the measurements of pore-scale velocity distribution using the micro-PIV technique. The experiments with micromodels with high porosity but different grain arrangements showed qualitative as well as quantitative differences in the velocity field. The pressure drop across the two ends of micromodel is also measured. The variation of pressure difference with the flowrate is found to be nonlinear with a significant effect on the patterns of micropillars. However, at low porosity, the variation of pressure difference with the flowrate is found linear and there is almost no influence of the micropillar patterns. The flow visualization measurements are also conducted with the dual porosity micromodels, and the flow patterns were examined by analyzing the velocity vector maps.

Coalescence of a Ferrofluid Drop at Its Bulk Surface with or without a Magnetic Field.

The coalescence of a ferrofluid drop at its bulk surface, with or without a magnetic field, was investigated experimentally by a high-speed camera. Shape deformations of both the pendant ferrofluid drop and the bulk surface in the axial direction were observed during the approaching process even in the absence of a magnetic field. The angle of the upper pendant peak at the first contact decreases with the magnetic flux density, while the lower ferrofluid peak displays an opposite trend. The coalescing width of the ferrofluid drop follows a power-law relationship. The exponent of 0.64 under medium and high magnetic fields as well as the case without magnetic field confirms the inertial regime of drop coalescence. Under the low magnetic field, the significant exponent increasing from 0.59 to 3.02 at about 4 ms is in coincidence with the sudden change to a smooth coalescing section according to the visualized images. A high-speed microparticle image velocimetry (micro-PIV) technique was employed with a transparent model fluid to reveal the flow fields during the drop coalescence instead of opaque ferrofluids.

Continuous-flow synthesis of amphiphilic rhodamine B-polymethylsilsesquioxane fluorescent microspheres for micro-PIV analysis

• Rhodamine B- polymethylsilsesquioxane fluorescent microspheres were prepared. • Synthetic process was performed in a one-step continuous-flow microreaction system. • The hydrophilic and hydrophobic RhB-PMSQ microspheres were synthesized. • Amphiphilic RhB-PMSQ particles were used as tracer particles for micro-PIV analysis. Tracer particle is a key element in the micro-PIV flow field measurements. The oil-dispersible tracer particles are mainly obtained by post modification, resulting in relatively poor uniformity and dispersibility. In this work, we developed a molecular-level amphiphilic tracer particle, i.e., rhodamine B-polymethylsilsesquioxane (RhB-PMSQ) fluorescent microspheres. The RhB-PMSQ microspheres were synthesized in a continuous-flow microreaction system with the aqueous solution of NH 4 OH and RhB as the continuous phase and methyltrimethoxysilane (MTMS) as the dispersed phase. The microreaction system consisted of a T -junction micromixer, a capillary reactor for MTMS hydrolysis, and a millimeter-scale tubular reactor for condensation reaction. By controlling the condensation degree of surface Si-OH groups, the hydrophilic and hydrophobic RhB-PMSQ microspheres could be synthesized with the same reaction system, and they could be uniformly dispersed in the aqueous phase and oil phase, respectively. The density, pore structure, and fluorescence property of the RhB-PMSQ microspheres were investigated in detail. The RhB-PMSQ microspheres were employed as the tracer particles of water and liquid paraffin, and the two-phase velocity profiles were measured via micro-PIV technique in a step T -junction microchannel. The micro-PIV results indicated that amphiphilic RhB-PMSQ microspheres could serve as the tracer particles for the flow field measurement both of aqueous phase and oil phase.

Magnetophoretic Control of Diamagnetic Particles Inside an Evaporating Droplet.

The present study reports the magnetophoresis of diamagnetic particles in an evaporating ferrofluid droplet. Both solid and ring magnet arrangements are used to investigate the effect of magnetic field distribution. The distance of the magnet from the droplet is varied to study the effect of magnetic field strength. The magnetic field distribution is computed using COMSOL multiphysics software. Magnetometer measurements have been carried out to validate the simulation results. The motion of particles and the drying pattern of evaporating ferrofluid droplets are visualized using the confocal microscopy technique. Both bright-field and fluorescence imagings have been carried out to observe the differential deposition of the fluorescent particle (microparticle) and magnetic nanoparticles in the absence and presence of a magnetic field. The velocity of diamagnetic particles as a function of magnetic field distribution and strength has been studied using the micro-PIV technique. In the absence of the magnetic field, a ring-shaped deposition pattern is observed. The mixture of microparticles (diamagnetic) and nanoparticles (magnetic) is deposited between the outer and inner edges of the ring. The diamagnetic particles occupy the inner and outer edges of the ring. Magnetic particles travel toward the higher magnetic field zone and diamagnetic particles move toward the smaller magnetic field zone when a magnetic field is applied by a solid magnet placed over the droplet. This can be attributed to the negative magnetic force originating from the difference between the susceptibility of magnetic and nonmagnetic particles. The negative magnetic force on the microparticle increases as the magnetic field intensity increases, causing the microparticle to convect faster toward the contact line. The deposition behavior can be reversed or suppressed using a ring magnet in place of a solid magnet. In this case, the negative magnetic force is stronger at the contact line region of the droplet and decreases as it approaches the center region of the droplet. The deposition behavior of diamagnetic particle depends on the balance between the Marangoni force and the magnetophoretic force. Overall, the present study demonstrates the capability of the controlled deposition of diamagnetic polystyrene particles by suitable arrangement of the solid and ring magnet.

Vapor mediated interaction of two condensing droplets

The present work investigates the internal flow structure of two condensing droplets of aqueous solution during interaction. Velocity of fluid inside the droplets was investigated by confocal micro-PIV technique. Simulation study has also been carried out to supplement the understanding of internal hydrodynamics inside droplets. Condensation on the droplets was carried out inside a closed chamber by generating a difference in vapor pressure between the droplet interface and a reservoir fluid surrounding the droplet in the room temperature without any cooling. Condensation on the droplet leads to spatial variation of solute concentration inside the droplet causing buoyancy driven Rayleigh convection. Fluid flow pattern inside a single condensing droplet is symmetric in nature whereas the fluid flow pattern for two condensing droplets is asymmetric in nature. The neighboring droplet influences the internal convection through vapor mediated interaction between the two condensing droplets. The condensing droplet senses the neighboring droplet from a distance without any physical contact. The asymmetric nature of the flow pattern is attributed to the modification in the distribution of condensation flux of the droplet in the presence of the neighboring droplet. The flow pattern observed for interacting droplets during condensation is opposite to that of evaporating droplets. The effect of the neighboring droplet on the internal convection of the condensing droplet reduces with increase in separation distance between the droplets.

Surface tension driven flow of blood in a rectangular microfluidic channel: Effect of erythrocyte aggregation

Microfluidic platforms have increasingly been explored for in vitro blood diagnostics and for studying complex microvascular processes. The perfusion of blood in such devices is typically achieved through pressure-driven setups. Surface tension driven blood flow provides an alternative flow delivery option, and various studies in the literature have examined the behavior of blood flow in such fluidic devices. In such flows, the influence of red blood cell (RBC) aggregation, the phenomenon majorly responsible for the non-Newtonian nature of blood, requires particular attention. In the present work, we examine differences in the surface tension driven flow of aggregating and non-aggregating RBC and Newtonian suspensions, in a rectangular microchannel. The velocity fields were obtained using micro-PIV techniques. The analytical solution for blood velocity in the channel is developed utilizing the power law model for blood viscosity. The results showed that RBC aggregation has an impact at the late stages of the flow, observed mainly in the bluntness of the velocity profiles. At the initial stages of the flow, the shearing conditions are found moderately elevated, preventing intense RBC aggregate formation. As the flow decelerates in the channel, RBC aggregation increases, affecting the flow characteristics.

Micro-PTV technique application to velocity field measurements in immiscible liquid-liquid plug flow in microchannels

The present paper presents the results of micro-PTV technique application to velocity fields measurements in plug flow of immiscible liquids in T-shaped straight and curved microchannels. The data processing algorithms are described; the procedure for velocity circulation assessment was proposed and applied to measure velocity circulation in aqueous plugs at different flow rates. The comparison with micro-PIV technique application results is performed.

Assessment of Measurement Accuracy of a Micro-PIV Technique for Quantitative Visualization of Al2O3 and MWCNT Nanofluid Flows

Nanofluids, which are liquids containing nanoparticles, are used to modify heat transfer performance in various systems. To explain the mechanism of heat transfer modification with nanofluids, many theories have been suggested based on numerical simulations without experimental validation because there is no suitable experimental method for measuring the velocity fields of nanofluid flows. In this study, the measurement accuracy of micro-particle image velocimetry (μ-PIV) is systemically quantified with Al2O3 and multi-walled carbon nanotube (MWCNT) nanofluids. Image quality, cross-correlation signal-to-noise ratio, displacement difference, and spurious vector ratio are investigated with static images obtained at various focal plane positions along the beam pathway. Applicable depth is enough to investigate micro-scale flows when the concentrations of Al2O3 and MWCNT nanofluids are lower than 0.01% and 0.005%, respectively. The velocity fields of Hagen–Poiseuille flow are measured and compared with theoretical velocity profiles. The measured velocity profiles present good agreement with the theoretical profiles throughout. This study provides the criteria for μ-PIV application and demonstrates that μ-PIV is a promising technique for measuring the velocity field information of nanofluids.

Rotating microchannel flow velocity measurements using the stationary micro-PIV technique with application to lab-on-a-CD devices

The measurement of microfluidic flows is an essential instrument to understand the governing physical mechanisms at small scales. This fact has motivated the adaptation of well-established “macroscale” experimental technics to deal with the specificities of microfluidic flows; a prominent example is the micro particle image velocimetry (micro-PIV) technique. In a different manner, the progress experienced by experimental techniques to measure flows in rotating frames has been more limited, with most studies concerned with macroscale turbomachinery applications. It turns out that the scale reduction in this field establishes a new and important flow class, known as centrigually-driven microfluidics, with application to lab-on-a-CD devices. However, the experimental characterization of rotating microflows has been, so far, limited to bulk flow measurements and/or visualization practices. For that reason, in this work, we propose extending the stationary micro-PIV technique to undertake quantitative, whole-field, velocity measurements inside rotating microchannel flow platforms. For this task, actual lab-on-a-CD prototypes are used. This work develops in two parts. First, we describe the most relevant changes in the micro-PIV equipment viewing the introduction of the test section rotation, namely: (i) hardware changes related to the micro-PIV/CD synchronization and (ii) software changes aiming at the preservation of the velocity measurement accuracy, through the removal of the circumferential velocity component. While this last step follows a well-known methodology, called image de-rotation, we propose tackling it in a new and automated fashion by means of the image registration method, whose implementation and advantages are explained in detail here. The second part of this work evaluates the capabilities of the modified micro-PIV technique by critically assessing the results of preliminary tests undertaken in dynamical regimes where rotation is dominant. Here, we present for the first time velocity profile measurements of centrifugally-driven microchannel flows, which display marked structural differences from classical stationary pressure-driven flows. The quality of these experimental profiles is further examined through comparisons with computational fluid dynamics simulations, based on the lattice Boltzmann method. Overall, this study indicates the effectiveness of the proposed micro-PIV system, which is able to accurately capture the most relevant physical features of rotating microfluidic flows over regions sufficiently far away from the walls. On the other hand, inside the boundary layers, the present micro-PIV measurements remain difficult to execute; the reasons for this limitation are discussed and clearly identified in the present preliminary studies, which pave the way for future studies in the field.

Micro-PIV characterization of the flow in a milli-labyrinth-channel used in drip irrigation

The baffle-fitted labyrinth-channel is largely used in drip irrigation systems. The existing baffles, which play an important role for generating head losses and ensure the flow regulation on the irrigation network, produce vorticity regions where the velocity is low or equal to zero. These vortices are likely to favor the deposition of particles or biochemical development causing dripper clogging which drastically reduces its performance. Flow topology in the dripper labyrinth-channel must be described to analyze dripper clogging sensibility and the effect on irrigation efficiency. Also, a question remains about the regime of this low Reynolds number flow. In the present study, the flow is characterized experimentally by micro-particle image velocimetry (micro-PIV) technique on ten-pattern repeating baffles used in drip irrigation dripper. Square cross-section is of about 1.4 mm $$^2$$ . Studied inlet Reynolds number varies from 345 to 690, which is equivalent to 1.4–2.8 l h $$^{-1}$$ . The mean velocity distribution and turbulence quantities within the labyrinth-channel flow are presented and discussed in this paper. The results underline that flow regime is turbulent and non-isotropic.

Onset of transition in the flow of polymer solutions through microtubes

Experiments are performed to characterize the onset of laminar–turbulent transition in the flow of high-molecular-weight polymer solutions in rigid microtubes of diameters in the range $390~\unicode[STIX]{x03BC}\text{m}{-}470~\unicode[STIX]{x03BC}\text{m}$ using the micro-PIV technique. By considering flow in tubes of such small diameters, the present study probes higher values of elasticity numbers ($E\equiv \unicode[STIX]{x1D706}\unicode[STIX]{x1D708}/R^{2}$) compared to existing studies, where $\unicode[STIX]{x1D706}$ is the longest relaxation time of the polymer solution, $R$ is the tube radius and $\unicode[STIX]{x1D708}$ is the kinematic viscosity of the polymer solution. For the Newtonian case, our experiments indicate that the natural transition (without the aid of any forcing mechanism) occurs at Reynolds number ($Re$) $2000\pm 100$. As the concentration of polymer is increased, initially there is a delay in the onset of the transition and the transition Reynolds number increases to $2500$. Further increase in concentration of the polymer results in a decrease in the Reynolds number for transition. At sufficiently high concentrations, the added polymer tends to destabilize the flow and the transition is observed to happen at $Re$ as low as $800$. It is also observed that the addition of polymers, regardless of their concentration, reduces the magnitude of the velocity fluctuations after transition. Dye-stream visualization is further used to corroborate the onset of transition in the flow of polymer solutions. The present work thus shows that addition of polymer, at sufficiently high concentrations, destabilizes the flow when compared to that of a Newtonian fluid, thereby providing additional evidence for ‘early transition’ or ‘elasto-inertial turbulence’ in the flow of polymer solutions. The data for the transition Reynolds number $Re_{t}$ from our experiments (for tubes of different diameters, and for two different polymers at varying concentrations) collapse well according to the scaling relation $Re_{t}\propto 1/\sqrt{E(1-\unicode[STIX]{x1D6FD})}$, where $\unicode[STIX]{x1D6FD}$ is the ratio of solvent viscosity to the viscosity of the polymer solution.

Surface roughness influences on the behaviour of flow inside microchannels

This work discusses influence of the surface roughness on the behavior of liquids flowing inside microchannels. By measuring the flow profile using the micro-PIV technique, the flow of water inside two rectangular microchannels of different wall roughness and in a circular smooth microchannel was studied. Comparisons were made among the experimental results, showing that a metrological approach concerning surface characteristics of microdevices is required to ensure reliability of the measurements for flow analyses in microfluidic processes.

Enhancement of measurement accuracy of X-ray PIV in comparison with the micro-PIV technique.

The X-ray PIV (particle image velocimetry) technique has been used as a non-invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two-dimensional velocity field data from the three-dimensional volumetric information contained in X-ray images is technically unclear. In this study, a new two-dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X-ray PIV and micro-PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro-PIV experiments were conducted for single-plane and summation images, which provide similar positional information of particles as X-ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (VS) and centre-plane images (VC). The velocity differences between VS and VC along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two-dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two-dimensional velocity field information at arbitrary positions could be successfully extracted from X-ray images by using the correction coefficient and several velocity parameters derived from VS.

Convection inside condensing and evaporating droplets of aqueous solution.

We experimentally study the fluid convection inside a condensing droplet of aqueous NaCl solution and compare it with that of an evaporating droplet. The droplets are sandwiched between two horizontal hydrophobic surfaces and surrounded by a reservoir with solution of different concentration. Condensation and evaporation of the droplets occur due to the vapor pressure difference between the droplet and the reservoir solution. The micro-PIV technique has been used to study the velocity field inside the droplets. Buoyancy driven Rayleigh convection is observed inside both the condensing and evaporating droplets. In the condensing droplet, water condenses on the liquid-air interface creating a low density region near the interface. There is upward movement of fluid along the condensing interface towards the top region of the droplet which recirculates back from the center region of the droplet in the downward direction. In contrast, the fluid moves in the downward direction along the interface in the case of an evaporating droplet with an upward plume like flow at the center region of the droplet. Both evaporating and condensing droplets show a recirculating loop inside the droplets of opposite direction.

Experimental study of liquid-liquid plug flow in a T-shaped microchannel

Plug flow regime in immiscible liquid-liquid flow in a T-shaped microchannel is experimentally studied in the present work. For three different immiscible liquids plug length is measured using high-speed visualization of the flow. Micro-PIV technique is applied for instantaneous velocity field measurements in plugs. Velocity circulation in plugs is calculated for different bulk flow rates.

Evaporation-induced natural convection of a liquid slug of binary mixture inside a microchannel: effect of confinement

Both experimental and simulation studies have been carried out on internal convection of an evaporating liquid slug of aqueous NaCl solution inside a microcapillary. Effect of confinement due to the extended channel length beyond the interface of the liquid slug has been investigated by placing the liquid slug inside the microcapillary of different lengths. Micro-PIV technique has been used for measurement of velocity field inside the liquid slug. Simulation studies have been carried out using COMSOL Multiphysics software for reporting the evaporative flux distribution on the meniscus and the concentration field distribution inside the liquid slug. The combined experimental and simulation studies successfully explain the underlying flow physics. Evaporation from the liquid–air interface of the slug induces buoyancy-driven Rayleigh convection. Evaporative flux of the interface depends on the extended length of the microcapillary beyond the liquid slug. The presence of extended channel region beyond the meniscus suppresses the evaporation from the meniscus due to the absence of evaporation flux normal to the channel wall. Evaporation occurs primarily from only one meniscus when the slug is located at one end of a long channel. Evaporation occurs from both the menisci when both the menisci are directly exposed to the atmosphere. Evaporation from only one meniscus of a slug leads to one recirculation bubble inside the liquid slug, whereas evaporation from both the menisci leads to two recirculation bubbles inside the liquid slug. Liquid slug with asymmetric extended channel length beyond the liquid slug interface leads to asymmetric evaporative flux, concentration field distribution and recirculation bubble size. The extended channel length beyond an evaporating liquid slug can influence/control the performance of a digital microfluidic system/device.

Two-fluid mixing inside a sessile micro droplet using magnetic beads actuation

The present work reports the mixing between two fluids within a sessile droplet using actuation of magnetic microspheres due to the cyclic magnetic field produced by four electromagnets. Gaussmeter measurements and COMSOL simulation have been used for characterization of the magnetic field. Mixing experiments are conducted for different strength of actuation current and actuation frequencies to the electromagnets. Mixing effectiveness between the fluids is evaluated using the “μ-LIF technique”. Micro-PIV technique is used to measure the flow field inside the droplet in the presence of magnetic field in order to explain the mixing mechanism. A recirculating flow in synchronization with the applied periodic magnetic field is observed inside the droplet that enhances the viscous interaction between the magnetic microspheres and fluids leading to efficient mixing of fluids. The strength of the actuation current and actuation frequency are observed to be the control parameters for effective mixing inside a droplet. A critical actuation frequency is observed above which mixing between the two fluids is degraded with increase in frequency to the electromagnets.

Influence of an adjacent droplet on fluid convection inside an evaporating droplet of binary mixture

Abstract We study the fluid convection inside two adjacent evaporating droplets of binary mixture. Micro-PIV technique has been used to measure the velocity field inside the droplet. The experimental observation on internal convection inside droplet has been explained using the simulation results of evaporative flux distribution and concentration field inside the droplet. The interaction between the two adjacent evaporating droplets has been studied by varying the separation distance between the two droplets. Evaporation from the droplet induces buoyancy driven Rayleigh convection inside the droplet. Fluid convection inside a single droplet shows a symmetrical flow pattern with two recirculating bubbles in X–Z and Y–Z wall normal planes. This behavior is attributed to the symmetric evaporative flux distribution on the droplet surface and the resulting symmetric concentration field inside the droplet. Presence of an adjacent evaporating droplet leads to asymmetric evaporative flux distribution on the droplet surface due to the influence of the neighboring droplet on the free stream mass fraction. This asymmetric evaporative flux distribution on the droplet surface results in asymmetric concentration field inside the droplet. As a result, the fluid convection in two droplets configuration shows asymmetric flow pattern with one recirculation bubble in X–Z wall normal plane and two recirculation bubbles in Y–Z wall normal plane.