Accurate PIV Measurement on Slip Boundary using Single-Pixel Algorithm

To accurately measure the near-wall flow by particle image velocimetry (PIV) is a big challenge, especially for the slip boundary condition. Apart from high-precision measurements, an appropriate PIV algorithm is important to resolve the near-wall velocity profile. In our study, single-pixel algorithm is employed to calculate the near-wall flow, which is demonstrated to be capable of accurately resolving the flow velocity near the slip boundary condition. Based on the synthetic particle images, the advantages of the single-pixel algorithm are manifested in comparison with the conventional window correlation algorithm. Specially, the single-pixel algorithm has higher spatial resolution and accuracy, and lower systematic error and random error for the case of slip boundary condition. Furthermore, for experimental verification, micro-PIV measurements are conducted over a liquid-gas interface and the single-pixel algorithm is successfully applied to the calculation of near-wall velocity under the slip boundary condition, especially the negative slip velocity. The current work demonstrates the advantage of the single-pixel algorithm in analyzing the complex flow under the slip boundary condition, such as drag reduction, wall skin friction evaluation and near-wall vortex structure measurement.

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.

Evolution of Water-in-Oil Droplets in T-Junction Microchannel by Micro-PIV

Water-in-oil droplets have huge importance in chemical and biotechnology applications, despite their difficulty being produced in microfluidics. Moreover, existing studies focus more on the different shape of microchannels instead of their size, which is one of the critical factors that can influence flow characteristics of the droplets. Therefore, the present work aims to study the behaviours of water-in-oil droplets at the interfacial surface in an offset T-junction microchannel, having different radiuses, using micro-PIV software. Food-grade palm olein and distilled water seeded with polystyrene microspheres particles were used as working fluids, and their captured images showing their generated droplets’ behaviours focused on the junction of the respective microfluidic channel, i.e., radiuses of 400 µm, 500 µm, 750 µm and 1000 µm, were analysed via PIVlab. The increasing in the radius of the offset T-junction microchannel leads to the increase in the cross-sectional area and the decrease in the distilled water phase’s velocity. The experimental velocity of the water droplet is in agreement with theoretical values, having a minimal difference as low as 0.004 mm/s for the case of the microchannel with a radius of 750 µm. In summary, a small increase in the channel’s size yields a significant increase in the overall flow of a liquid.

Experimental Visualization and Analysis of Multiphase Immiscible Flow in Fractured Micromodels Using Micro-Particle Image Velocimetry

The study of fluid flow through fractured porous media has drawn immense interest in the fields of soil hydrology, enhanced oil recovery (EOR) and others. In this work, a low cost fractured micromodel with regular pore geometry is fabricated and visualization experiments are performed to study the flow field produced by single-phase and two-phase immiscible flow. The fractured micromodel is fabricated using Polydimethylsiloxane (PDMS) substrate. The micro-PIV method is applied to map the flow velocity, both at the throat and near the fracture region of micromodel. In two-phase flow, imbibition flow experiments are performed to investigate the effects of fracture on the front migration caused by the trapping mechanism of residual fluid (displaced phase). The velocity distribution obtained for the two-phase flow revealed many peculiarities that are completely different from the single-phase flow pattern. These peculiarities create instabilities that yield random preferential flow paths near the pockets of stagnant fluid. Such dynamic events are quantified by mapping the velocity magnitude of flow fields. No effects of fracture are seen in the single-phase flow where uniform flow patterns are observed in the porous region. However, for the two-phase flow, more pockets of trapped fluids are found at the junction of two fractures.

The Complexity of Porous Media Flow Characterized in a Microfluidic Model Based on Confocal Laser Scanning Microscopy and Micro-PIV

In this study, the complexity of a steady-state flow through porous media is revealed using confocal laser scanning microscopy (CLSM). Micro-particle image velocimetry (micro-PIV) is applied to construct movies of colloidal particles. The calculated velocity vector fields from images are further utilized to obtain laminar flow streamlines. Fluid flow through a single straight channel is used to confirm that quantitative CLSM measurements can be conducted. Next, the coupling between the flow in a channel and the movement within an intersecting dead-end region is studied. Quantitative CLSM measurements confirm the numerically determined coupling parameter from earlier work of the authors. The fluid flow complexity is demonstrated using a porous medium consisting of a regular grid of pores in contact with a flowing fluid channel. The porous media structure was further used as the simulation domain for numerical modeling. Both the simulation, based on solving Stokes equations, and the experimental data show presence of non-trivial streamline trajectories across the pore structures. In view of the results, we argue that the hydrodynamic mixing is a combination of non-trivial streamline routing and Brownian motion by pore-scale diffusion. The results provide insight into challenges in upscaling hydrodynamic dispersion from pore scale to representative elementary volume (REV) scale. Furthermore, the successful quantitative validation of CLSM-based data from a microfluidic model fed by an electrical syringe pump provided a valuable benchmark for qualitative validation of computer simulation results. Graphic Abstract

Quantifying the Dynamics of Water-CO2 Multiphase Flow in Microfluidic Porous Media Using High-Speed Micro-PIV

Multiphase flow in porous media is central to a large range of applications in the energy and environmental sectors, such as enhanced oil recovery, groundwater remediation, and geologic CO2 storage and sequestration (CCS). Herein we present an experimental study of pore-scale flow dynamics of liquid CO2 and water in two-dimensional (2D) heterogeneous porous micromodels employing high-speed microscopic particle image velocimetry (micro-PIV). This novel technique allowed us to spatially and temporally resolve the dynamics of multiphase flow of CO2 and water under reservoir-relevant conditions for varying wettabilities and thus to evaluate the impact of wettability on the observed physics and dynamics. The preliminary results show that multiphase flow of liquid CO2 and water in hydrophilic micromodels is strongly dominated by successive pore-scale burst events, resulting in velocities of two orders of magnitude larger than the bulk velocity. When the surface wettability was altered such that imbibtion takes place, capillarity and instability are significantly suppressed, leading to more compact and axi-symmetric displacement of water by liquid CO2 with generally low flow velocities. To our knowledge, this work represents the first of its kind, and will be useful for advancing our fundamental understanding and facilitating pore-scale model development and validation.