Microscopic Particle Image Research Articles

Microscopic Particle Image Velocimetry Analysis of Multiphase Flow in a Porous Media Micromodel.

Pore-scale oil displacement behavior was investigated in a porous media micromodel using microscopic particle image velocimetry (μPIV). Porous media micromodels consisting of an ordered square array of cylindrical pillars with 50 and 70% porosities were fabricated with photolithography. The oil displacement was performed with the injection of water at flow rates of 37.5, 75, and 150 μL/h. These flow rates correspond to Reynolds number of 1.1 × 10-2, 2.2 × 10-2, and 4.4 × 10-2, respectively in the 50% porous channel, and 1.84 × 10-3, 3.69 × 10-3, and 7.38 × 10-3, respectively in the 70% porous channel. The capillary numbers for these flow rates are 2.18 × 10-5, 4.36 × 10-5, and 8.72 × 10-5, respectively in the 50% porous channel, and 1.56 × 10-5, 3.12 × 10-5, and 6.23 × 10-5, respectively in the 70% porous channel. The micromodel is initially saturated with oil, with the invading water phase following the path of least resistance as it displaces the oil. The μPIV data were used to construct probability density functions (PDFs) which show an initial, nonzero, peak in transverse velocity as the water enters the micromodel. The PDFs broaden with time, indicating that the water is spreading, before retracting to a peak velocity of 0 mm/s, indicating that the water displacement has achieved equilibrium. We developed a model based on conservation of mass to describe the efficiency of the displacement process. All flow conditions demonstrate peak displacement efficiency when the amount of oil phase displacement is ∼9 pore volumes in 50% porous channel and ∼4 pore volumes in 70% porous channel.

Acoustic driven circulation around cylindrical obstructions in microchannels

We introduce an approach to generate direction-controlled circulation around cylindrical obstructions in channels using a piezoelectric transducer embedded porous-channel device fabricated by photolithography. To transmit acoustic signals into the channel, a single piezoelectric transducer was attached, operating at voltage levels of 5, 10, 15, and 20 V. Microscopic particle image velocimetry was employed to analyze the flow patterns in the channels. The analysis revealed two opposing circulation tendencies around the pillars located at two opposite sides of the channel in the longitudinal direction. The strength of circulation was found to be minimal in the middle of the channel and increased gradually toward the two ends of the channels. Furthermore, we observed that the circulation strength was maximum near the axial centerline and minimum at the boundaries along the width of the channels. Comparing the voltage levels, the higher voltage signals produced a higher strength of circulation than the lower voltage signals in all cases. Additionally, we found that the strength of circulation increased almost linearly and then decayed exponentially in the radial direction from the surfaces of the pillars. The observed velocity fields around individual cylinders matched well with the Görtler vortex model. The reported circulation phenomenon around pillars can be applied in non-contact fluid stirring and mixing in bio-chemical systems and lab-on-a-chip systems and may also provide additional degrees of freedom in object tweezing, trapping, and levitation.

Smart Manufacturing Implementation of a Continuous Downstream Precipitation and Filtration Process for Antibody Purification

Abstract Recently, continuous bioprocessing has gained momentum in biomanufacturing and can alleviate many of the hurdles faced in batch or semi-batch operations. Moreover, the parallel development of smart manufacturing (SM) allows the rapid small-scale prototyping and large-scale implementation of continuous bioprocesses. With this background, this paper presents the laboratory-scale implementation of a continuous precipitation-filtration process that can ultimately be used for therapeutic protein capture purification. The experimental setup includes four static mixers, four peristaltic pumps, one hollow fiber dewatering filtration module, and multiple pressure sensors and weigh scales. The system also includes an in-line advanced microscopic particle imaging probe that provides real-time images and derived metrics of the precipitate particle morphologies and a fiber optic 880 nm optical absorbance probe. A polyclonal human serum antibody mixture (hIgG) (10 g/L) was used as a stand-in for a monoclonal antibody therapeutic along with 7 % w/v polyethylene glycol (PEG, volume exclusion agent) and 10 mM zinc chloride (cross-linking agent) as the precipitants to demonstrate the principles of operation and control of a precipitation-based process using SM technology. An integrated input/output (I/O) system was used to acquire pressure, flow rate, and weigh scale data and also to communicate with the pumps to change flow rates in real-time. Edge computers communicate with the I/O system and the imaging probe and host the software layer. The software layer enables real-time data acquisition, data-driven and first-principles model predictions, closed-loop control of precipitate particle morphology using pump flow rate of PEG, and cloud communications with the Clean Energy Smart Manufacturing Innovation Institute Smart Manufacturing Innovation Platform. The paper presents the initial results obtained with this integrated system, demonstrating the potential of SM strategies to enhance the production of life-saving biopharmaceutical products.

Surfactant sorption on a single air bubble in an ultrasonic standing acoustic wave field

Ultrasound application presents a promising non-intrusive way to enhance and facilitate mass transfer in aqueous systems. This enhanced mass transfer can influence the sorption processes in multiphase flows. Previous studies investigating the impact of ultrasound on sorption, have reported an increase in either desorption due to the rise in liquid temperature or adsorption due to the additional convective mass transfer resulting from acoustic streaming. In this study, low intensity ultrasound with a frequency of 36kHz was deployed to evaluate the sorption process of Triton X-100 on the surface of a single bubble, placed along the standing acoustic wave using profile analysis tensiometry. Furthermore, microscopic particle image velocimetry was used to understand the role acoustic streaming might play during different stages of the sorption process. Contrary to expectations, the results showed no considerable change in surface tension and sorption dynamics after sonicating both fresh and surfactant-loaded bubbles. The results of this study suggest that the observations from previous studies may be attributed to the additional energy input of the acoustic wave into the system rather than the presence of an external acoustic field.

Highly efficient passive Tesla valves for microfluidic applications

A multistage optimization method is developed yielding Tesla valves that are efficient even at low flow rates, characteristic, e.g., for almost all microfluidic systems, where passive valves have intrinsic advantages over active ones. We report on optimized structures that show a diodicity of up to 1.8 already at flow rates of 20 μl s−1 corresponding to a Reynolds number of 36. Centerpiece of the design is a topological optimization based on the finite element method. It is set-up to yield easy-to-fabricate valve structures with a small footprint that can be directly used in microfluidic systems. Our numerical two-dimensional optimization takes into account the finite height of the channel approximately by means of a so-called shallow-channel approximation. Based on the three-dimensionally extruded optimized designs, various test structures were fabricated using standard, widely available microsystem manufacturing techniques. The manufacturing process is described in detail since it can be used for the production of similar cost-effective microfluidic systems. For the experimentally fabricated chips, the efficiency of the different valve designs, i.e., the diodicity defined as the ratio of the measured pressure drops in backward and forward flow directions, respectively, is measured and compared to theoretical predictions obtained from full 3D calculations of the Tesla valves. Good agreement is found. In addition to the direct measurement of the diodicities, the flow profiles in the fabricated test structures are determined using a two-dimensional microscopic particle image velocimetry (μPIV) method. Again, a reasonable good agreement of the measured flow profiles with simulated predictions is observed.

Defocusing μPTV With Increased Measurement Depth Using Shadowgraphy

In this study we can show that shadowgraphy is a feasible alternative to fluorescence for microscopic particle imaging. Furthermore, using the particle shadow image geometry (diameter for defocusing methods or axis lengths for astigmatism methods), the particle location along the optical axis can be determined, enabling a three-dimensional flow measurement with a single camera. However, the rate of change of the particle shadow image geometry as a function of the distance to the focal plane is one order of magnitude smaller as compared to fluorescence/scattering imaging approaches, such that the measurement depths is effectively increased. This increase is due to the fact that the signal to noise ratio of the relatively small particle shadow images is large, even if the particle is located far away from the focal plane, as the intensity is distributed over a small sensor area. In addition, due to their smaller dimensions, more particle shadow images can be captured on the camera sensor, yielding a larger information density for microscopic particle imaging. A measurement of a micro-channel (channel height: 650 µm) flow showed a good agreement with the ideal parabolic flow profile, proving the viability of this microscopic particle shadow imaging approach as a suitable addition to the existing fluorescence-based methods.

Interfacial Behavior of Particle-Laden Bubbles under Asymmetric Shear Flow.

The behavior of moving bubbles has mostly been studied in an axisymmetric flow field. To extend the knowledge to practical conditions, we investigate the interfacial and hydrodynamic properties of bubbles under asymmetric shear forces. Experiments are performed with a buoyant bubble at the tip of a capillary placed in a defined shear flow in the presence of surfactants, nanoparticles, and glass beads. The response of the interface to the surrounding asymmetric flow is measured under successive reduction of the surface area. Profile analysis tensiometry is utilized to investigate the dynamic surface tension and the surface rheology of the surfactant- and nanoparticle-laden interfaces. Microscopic particle image and tracking velocimetry are used to study the bulk flow and the interfacial mobility of the buoyant bubble. According to our results, the rotational component of the shear flow provokes an interfacial flow, which redistributes the adsorbed surfactants and particles at the interface. In the presence of NPSCs, a contiguous network of particles forms at the interface through densification of surface structures. We show that this interconnected nanoparticle network eventually stops the interfacial flow and decreases the mobility of the glass beads at the interface. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field. Our findings can serve as a basis to formulate boundary conditions for refined modeling and to predict the hydrodynamics of bubbles and droplets.

Pore-Scale Dynamics of Liquid CO2–Water Displacement in 2D Axisymmetric Porous Micromodels Under Strong Drainage and Weak Imbibition Conditions: High-Speed μPIV Measurements

Resolving pore-scale transient flow dynamics is crucial to understanding the physics underlying multiphase flow in porous media and informing large-scale predictive models. Surface properties of the porous matrix play an important role in controlling such physics, yet interfacial mechanisms remain poorly understood, in part due to a lack of direct observations. This study reports on an experimental investigation of the pore-scale flow dynamics of liquid CO2 and water in two-dimensional (2D) circular porous micromodels with different surface characteristics employing high-speed microscopic particle image velocimetry (μPIV). The design of the micromodel minimized side boundary effects due to the limited size of the domain. The high-speed μPIV technique resolved the spatial and temporal dynamics of multiphase flow of CO2 and water under reservoir-relevant conditions, for both drainage and imbibition scenarios. When CO2 displaced water in a hydrophilic micromodel (i.e., drainage), unstable capillary fingering occurred and the pore flow was dominated by successive pore-scale burst events (i.e., Haines jumps). When the same experiment was repeated in a nearly neutral wetting micromodel (i.e., weak imbibition), flow instability and fluctuations were virtually eliminated, leading to a more compact displacement pattern. Energy balance analysis indicates that the conversion efficiency between surface energy and external work is less than 30%, and that kinetic energy is a disproportionately smaller contributor to the energy budget. This is true even during a Haines jump event, which induces velocities typically two orders of magnitude higher than the bulk velocity. These novel measurements further enabled direct observations of the meniscus displacement, revealing a significant alteration of the pore filling mechanisms during drainage and imbibition. While the former typically featured burst events, which often occur only at one of the several throats connecting a pore, the latter is typically dominated by a cooperative filling mechanism involving simultaneous invasion of a pore from multiple throats. This cooperative filling mechanism leads to merging of two interfaces and releases surface energy, causing instantaneous high-speed events that are similar, yet fundamentally different from, burst events. Finally, pore-scale velocity fields were statistically analyzed to provide a quantitative measure of the role of capillary effects in these pore flows.

Micro-PIV Measurements of Multiphase Flow in Deforming Porous Media Subject to Mineral Dissolution

Mineral dissolution is studied in novel calcite-based porous micromodels under single- and multiphase conditions, with a focus on the interactions of mineral dissolution with pore flow. Microscopic particle image velocimetry (PIV) was utilized to simultaneously characterize the local velocity field and the instantaneous shapes of the dissolving grains. The preliminary results provide a unique view of the coupled dynamics between pore flow and mineral dissolution.

Hydrodynamics of Euglena Gracilis during locomotion

The hydrodynamics and locomotion mechanism of Euglena Gracilis (E. Gracilis) is investigated using microscopic shadow imaging and micro particle image velocimetry (MicroPIV). Three distinct locomotion modes were observed: translation, spin, and left/right turn. Since the flagellum was not possible to image, the strokes were identified by evaluating the flow field around the protist. The flow field information is obtained using a phase-separated PIV evaluation, which uses a histogram-thresholding based dynamic masking approach. The temporal resolution of the experiment was sufficient to identify the sequence of two modes, translation and spin, and the stroke-pulling frequency. The flow field result during a stroke is compared with existing Stokeslet dipole theories and flow disturbance decay with distance is investigated. The results indicate that the organism has a complex locomotion technique that allows the change of direction, axial orientation and propulsion.

Advection kinetics induced self-assembly of colloidal nanoflakes into microscale floral structures

The article explores the governing role of the internal soluto-thermal hydrodynamics and advective transport within sessile colloidal droplets on the self-assembly of nanostructures to form floral patterns. Water–acetone mixture and Bi2O3 nanoflakes based complex fluids are used as the experimental liquids. Micro-liter sessile droplets are allowed to vaporize and the dry-out patterns are examined using scanning electron microscopy. The presence of distributed self-assembled rose-like structures is observed and is postulated to be formed by the hydrodynamic interactions within the drying droplet. The population density, structure and shape of the floral structures are noted to be dependent on the binary fluid composition and nanomaterial concentration. Detailed microscopic particle image velocimetry and infrared thermography analysis is undertaken to qualitatively and quantitatively describe the solutal Marangoni advection within the evaporating droplets. It has been shown that the kinetics, regime and spatial distribution of the internal flows are dominantly responsible factors towards the advection influenced clustering, aggregation and self-assembly of the nanoflakes. In addition, the size of the nanostructures and the viscous character of the complex fluid are also noted to play dominant roles. The resulting interplay of hydrodynamic behavior, adhesion and cohesion forces during the droplet dry-out phase, and thermodynamic energy minimization leads to formation of such floral structures. The findings may have strong implications towards modulating micro-hydrodynamics induced self-assembly in complex fluids.

Psi-PIV: a novel framework to study unsteady microfluidic flow

In microscopic particle image velocimetry (micro-PIV), correlation averaging over multiple frames is often required, leading to a loss in temporal resolution, therefore limiting the measurement accuracy for unsteady flows. Here, we present a new PIV method suitable to study steady and unsteady laminar flows between parallel plates (i.e., Hele-Shaw flow), which is a common flow configuration in microfluidic applications. Our method reduces the effective seeding density and yields similar if not higher signal-to-noise ratio (SNR) compared to conventional micro-PIV. We call this algorithm Psi-PIV. Psi-PIV requires a much smaller number of frames to reach the same SNR compared to the widely used correlation averaging method. This leads to a significant improvement of the temporal resolution. The Psi-PIV algorithm is used in an experimental investigation of steady and unsteady flows in a Hele-Shaw cell. Our experiment shows that Psi-PIV reduces the number of required frames by 8 times and 30 times compared to the frames required by conventional PIV for steady and unsteady laminar flow, respectively. In this study, PIV and Psi-PIV use a single-pass cross-correlation to present the underlying difference between the two approaches.Graphic abstract

Experimental and Numerical Investigations on the Flow Characteristics within Hydrodynamic Entrance Regions in Microchannels.

Flow characteristics within entrance regions in microchannels are important due to their effect on heat and mass transfer. However, relevant research is limited and some conclusions are controversial. In order to reveal flow characteristics within entrance regions and to provide empiric correlation estimating hydrodynamic entrance length, experimental and numerical investigations were conducted in microchannels with square cross-sections. The inlet configuration was elaborately designed in a more common pattern for microdevices to diminish errors caused by separation flow near the inlet and fabrication faults so that conclusions which were more applicable to microchannels could be drawn. Three different microchannels with hydraulic diameters of 100 μm, 150 μm, and 200 μm were investigated with Reynolds (Re) number ranging from 0.5 to 50. For the experiment, deionized water was chosen as the working fluid and microscopic particle image velocimetry (micro-PIV) was adopted to record and analyze velocity profiles. For numerical simulation, the test-sections were modeled and incompressible laminar Navier–Stokes equations were solved with commercial software. Strong agreement was achieved between the experimental data and the simulated data. According to the results of both the experiments and the simulations, new correlations were proposed to estimate entrance length. Re numbers ranging from 12.5 to 15 was considered as the transition region where the relationship between entrance length and Re number converted. For the microchannels and the Reynolds number range investigated compared with correlations for conventional channels, noticeable deviation was observed for lower Re numbers (Re < 12.5) and strong agreement was found for higher Re numbers (Re > 15).

Hydrodynamic investigation of a novel shear-generating device for the measurement of anchorage-dependent cell adhesion intensity.

Proliferation of anchorage-dependent cells occurs after adhesion to a suitable surface. Thus, quantitative information about the force of cells adhesion to microcarriers at early culture phases is vital for scaling up such system. In this work, a newly designed shear-generating device was proposed, based on a previously proposed contraction flow device designed for suspended cells. A design equation for the new device was also proposed to correlate the generated energy dissipation rate (EDR) with the cross-sectional area and flow rate. Microscopic-particle image velocimetry was measured to validate the simulation results, and good agreement was achieved. The newly designed device was then used to measure the adhesion force of MDCK and PK cells, and the results showed that the critical EDR was 3000W/m3 for MDCK and 5000W/m3 for PK cells. This quantitative information is of great value for better understanding shearing effects during the scaling up of anchorage-dependent cell cultures.

Inner and outer layer turbulence over a superhydrophobic surface with low roughness level at low Reynolds number

The inner and outer layers of a turbulent channel flow over a superhydrophobic surface (SHS) are characterized using simultaneous long-range microscopic particle tracking velocimetry (micro-PTV) and particle image velocimetry, respectively. The channel flow is operated at a low Reynolds number of ReH = 4400 (based on full channel height and 0.174 m/s bulk velocity), equivalent to Reτ = 140 (based on half channel height and friction velocity). The SHS is produced by spray coating, and the root-mean-square of wall roughness normalized by wall-unit is k+rms = 0.11. The micro-PTV shows 0.023 m/s slip velocity over the SHS (about 13% of the bulk velocity), which corresponds to a slip-length of ∼200 μm. A drag reduction of ∼19% based on the slope of the linear viscous sublayer and 22% based on an analytical expression of Rastegari and Akhavan [J. Fluid Mech. 773, R4 (2015)] realized. The reduced Reτ over the SHS based on the corresponding friction velocity is ∼125, which is in the lower limit of a turbulence regime. The results show the increase of streamwise Reynolds stresses 〈u2〉 for the SHS in the linear viscous sublayer due to the slip boundary condition. The 〈u2〉 peak does not change in magnitude while it is displaced closer to the wall in physical distance. The wall-normal Reynolds stress 〈v2〉 over the SHS and smooth surface is observed to overlap near the wall at y+ &amp;lt; 10, while 〈v2〉 for the SHS is smaller further away from the wall in physical dimensions. At y+ = 30, 〈v2〉 is 30% smaller for the SHS. A small increase of Reynolds shear stress for the SHS is observed at y+ &amp;lt; 10, while about 30% reduction is observed at y+ = 30. The observed variation of Reynolds stresses is associated with the relatively small roughness of the surface. If Reynolds stresses are normalized based on the corresponding friction velocity, the non-dimensional stresses show a large increase of 〈u2〉 and a small increase of 〈uv〉 over the SHS at y+ &amp;lt; 20. Farther away from the wall at y+ &amp;gt; 20, the scaling of Reynolds stresses based on the corresponding uτ results in their overlap for the smooth and SHSs. The drag reduction is mainly associated with the reduction of viscous wall-shear stress, while the variation in Reynolds shear stress at the wall is negligible. The quadrant analysis of turbulent fluctuations shows attenuation of stronger sweep motions at y+ &amp;lt; 15, while ejections are attenuated in the buffer layer at y+ = 20 until 30.

Three-dimensional microscopic light field particle image velocimetry

A microscopic particle image velocimetry ( $$\mu \text {PIV}$$ ) technique is developed based on light field microscopy and is applied to flow through a microchannel containing a backward-facing step. The only hardware difference from a conventional $$\mu$$ PIV setup is the placement of a microlens array at the intermediate image plane of the microscope. The method combines this optical hardware alteration with post-capture computation to enable 3D reconstruction of particle fields. From these particle fields, we measure three-component velocity fields, but find that accurate velocity measurements are limited to the two in-plane components at discrete depths through the volume (i.e., 2C-3D). Results are compared with a computational fluid dynamics simulation.

Internal flow in droplets within a concentrated emulsion flowing in a microchannel

Droplet microfluidics has enabled a wide variety of high-throughput biotechnical applications through the use of monodisperse micro-droplets as bioreactors. Previous fluid dynamics studies of droplet microfluidics have focused on single droplets or emulsions at low volume fractions. The study of concentrated emulsions at high volume fractions is important for further increasing the throughput of droplet microfluidics, but the fluid dynamics of such emulsions in confined microchannels is not well understood. This paper describes the use of microscopic particle image velocimetry to quantify the flow inside individual droplets within a concentrated emulsion having volume fraction φ ∼ 85% flowing as a monolayer in a straight microfluidic channel. The effects of confinement (namely, the number of rows of droplets across the width of the channel) and viscosity ratio on the internal flow patterns inside the drops at a fixed capillary number of 10−3 and a Reynolds number of 10−2 to 10−1 are studied. The results show that rotational structures inside the droplets always exist and are independent of viscosity ratio for the conditions tested. The structures depend on droplet mobility, the ratio of the velocity of the droplet to the velocity of the continuous phase. These values, in turn, depend on the confinement of the emulsion and the location of the droplets in the channel. Although this work presents two-dimensional measurements at the mid-height of the microchannel only, the results reveal flow patterns that are never described before in single drops or dilute emulsions.

Characteristics of Liquid Flow in Microchannels at very Low Reynolds Numbers

AbstractThe flow characteristics of deionized water and kerosene through smooth rectangular microchannels with varying hydraulic diameters are studied experimentally. Through the special design of the experimental setup, a wide range of measured average liquid velocities in microchannels is achieved, corresponding to broad‐ranging Reynolds numbers. The experimental data indicate that the pressure drop exhibits a linear relationship with the average velocity of the fluid in microchannels whose scales are larger than 2 μm. Moreover, the experimental Darcy friction factor is consistent with the classical laminar flow theory of a Newtonian fluid. Microscopic particle image velocimetry was applied to measure the velocity distribution inside the macrochannels. The experimental results agree well with the classical hydrodynamic theory.

Characterization of zebrafish larvae suction feeding flow using μPIV and optical coherence tomography

The hydrodynamics of suction feeding is critical for the survival of fish larvae; failure to capture food during the onset of autonomous feeding can rapidly lead to starvation and mortality. Fluid mechanics experiments that investigate the suction feeding of suspended particles are limited to adult fishes, which operate at large Reynolds numbers. This manuscript presents the first literature results in which the external velocity fields generated during suction feeding of early zebrafish larvae (2500–20,000 μm total length) are reported using time-resolved microscopic particle image velocimetry. For the larval stages studied, the maximum peak suction velocity of the inflow bolus is measured at a finite distance from the mouth tip and ranges from 1 to 8 mm/s. The average pressure gradient and the velocity profile proximal to the buccal (mouth) cavity are calculated, and two distinct trends are identified. External recirculation regions and reverse flow feeding cycles are also observed and quantified. One of the unresolved questions in fish suction feeding is the shape and dynamics of the buccal cavity during suction feeding; optical coherence tomography imaging is found to be useful for reconstructing the mouth kinematics. The projected area of the mouth cavity during the feeding cycle varies up to 160 and 22 % for the transverse and mid-sagittal planes, respectively. These findings can inspire novel hydrodynamically efficient biomedical and microfluidic devices.

On the effect of velocity gradients on the depth of correlation in μPIV

The present work revisits the effect of velocity gradients on the depth of the measurement volume (depth of correlation) in microscopic particle image velocimetry (μPIV). General relations between the μPIV weighting functions and the local correlation function are derived from the original definition of the weighting functions. These relations are used to investigate under which circumstances the weighting functions are related to the curvature of the local correlation function. Furthermore, this work proposes a modified definition of the depth of correlation that leads to more realistic results than previous definitions for the case when flow gradients are taken into account. Dimensionless parameters suitable to describe the effect of velocity gradients on μPIV cross correlation are derived and visual interpretations of these parameters are proposed. We then investigate the effect of the dimensionless parameters on the weighting functions and the depth of correlation for different flow fields with spatially constant flow gradients and with spatially varying gradients. Finally this work demonstrates that the results and dimensionless parameters are not strictly bound to a certain model for particle image intensity distributions but are also meaningful when other models for particle images are used.