A novel method to fabricate micromodels with varying depth (2.5-D) was developed, which allows more realistic investigation on flow in natural 3-D porous media.
We show that smaller gas bubbles grow at the expense of larger bubbles and all bubbles approach the same surface curvature after long times in porous media. This anticoarsening effect is contrary to typical Ostwald ripening and leads to uniformly sized bubbles in a homogeneous medium. Evolution dynamics of bubble populations were measured, and mathematical models were developed that fit the experimental data well. Ostwald ripening is shown to be the driving mechanism in this anticoarsening phenomenon; however, the relationship between surface curvature and bubble size determined by the pore-throat geometric confinement reverses the ripening direction.
The dynamic behavior of microemulsion-forming water-oil-amphiphiles mixtures is investigated in a 2.5D micromodel. The equilibrium phase behavior of such mixtures is well-understood in terms of macroscopic phase transitions. However, what is less understood and where experimental data are lacking is the coupling between the phase change and the bulk flow. Herein, we study the flow of an aqueous surfactant solution-oil mixture in porous media and analyze the dependence of phase formation and spatial phase configurations on the bulk flow rate. We find that a microemulsion forms instantaneously as a boundary layer at the initial surface of contact between the surfactant solution and oil. The boundary layer is temporally continuous because of the imposed convection. In addition to the imposed flow, we observe spontaneous pulsed Marangoni flows that drag the microemulsion and surfactant solution into the oil stream, forming large (macro)emulsion droplets. The formation of the microemulsion phase at the interface distinguishes the situation from that of the more common Marangoni flow with only two phases present. Additionally, an emulsion forms via liquid-liquid nucleation or the Ouzo effect (i.e., spontaneous emulsification) at low flow rates and via mechanical mixing at high flow rates. With regard to multiphase flow, contrary to the common belief that the microemulsion is the wetting liquid, we observe that the minor oil phase wets the solid surface. We show that a layered flow pattern is formed because of the out-of-equilibrium phase behavior at high volumetric flow rates (order of 2 m/day) where advection is much faster than the diffusive interfacial mass transfer and transverse mixing, which promote equilibrium behavior. At lower flow rates (order of 30 cm/day), however, the dynamic and equilibrium phase behaviors are well-correlated. These results clearly show that the phase change influences the macroscale flow behavior.
Summary Injecting oil-in-water (O/W) emulsions stabilized with nanoparticles (NPs) or surfactants is a promising option for enhanced oil recovery (EOR) in harsh-condition reservoirs. Stability and rheology of the flowing emulsion in porous media are key factors for the effectiveness of the EOR method. The objective of this study is to use microfluidics to (1) quantitatively evaluate the synergistic effect of surfactants and NPs on emulsion dynamic stability and how NPs affect the emulsion properties, and to (2) investigate how emulsion properties affect the sweep performance in emulsion flooding. A microfluidic device with well-defined channel geometry of a high-permeability pathway and multiple parallel low-permeability pathways was created to represent a fracture/matrix dual-permeability system. Measurement of droplet coalescence frequency during flow is used to quantify the dynamic stability of emulsions. An NP aqueous suspension (2 wt%) shows excellent ability to stabilize the macro-emulsion when mixed with a trace amount of surfactant (0.05 wt%), revealing a synergistic effect between NPs and surfactant. For a stable emulsion, when a pore throat is present in the high-permeability pathway, it was observed that flowing emulsion droplets compress each other and then block the high-permeability pathway at a throat structure, which forces the wetting phase into low-permeability pathways. Droplet size shows little correlation with this blocking effect. Water content was observed to be much higher in the low-permeability pathways than in the high-permeability pathways, indicating different emulsion texture and viscosity in channels of different sizes. Consequently, the assumption of bulk emulsion viscosity in the porous medium is not applicable in the description and modeling of the emulsion-flooding process. Flow of emulsions stabilized by an NP/surfactant mixture shows droplet packing in high-permeability regions that is denser than those stabilized by surfactant only, at high-permeability regions, which is attributed to the enhanced interaction between droplets caused by NPs in the thin liquid film between neighboring oil/water (O/W) interfaces. This effect is shown to enhance the performance of emulsion-blockage effect for sweep-efficiency improvement, showing the advantage of NPs as an emulsion stabilizer during an emulsion-based EOR process.
In this work, we developed a novel and simple microfluidic method for the fabrication of self-assembled monodispersed photonic crystal microbeads with core–shell structures using solvent extraction. Monodispersed aqueous droplets encapsulating colloidal photonic crystal particles were produced in a T-junction microfluidic device, and the controlled transport of water from the aqueous droplets to the oil phase created spherical colloidal crystal microbeads with controlled shell–core structures by extraction-derived self-organization of the colloidal nanoparticles. While the solidification of colloidal particles from emulsion droplets in an oven took tens of hours, the present extraction-derived method reduces the time required for solidification to several minutes. Compared with recent microwave-assisted consolidation methods which showed a particle material dependency, our new method exhibited no such limitation. The results showed that the packing quality of colloidal crystals, which can be precisely controlled by adjusting the extraction rate and surfactant, was high enough to show photonic band-gap characteristics. The reflectance of our photonic microbeads responded precisely to any change in physical properties including the size of colloidal particles and refractive index. A mechanism of the extraction-derived self-assembly of colloidal particles was developed and then supposed by theoretical derivations and experimental results. Finally, the universality of the method was demonstrated by fabricating SiO2 photonic crystal microbeads.
The flow of multiple fluid phases in porous media often results in trapped droplets of the nonwetting phase. Recent experimental and theoretical studies have suggested that nanoparticle aqueous dispersions may be effective at mobilizing trapped droplets of nonwetting fluid (oil) in porous media. Hypotheses to explain the observation include the nanoparticles' modification of solid wettability, droplet stabilization, and changes in interfacial tension and interface rheology. However, because it is difficult to observe droplet behavior on the pore scale, how those factors contribute to oil droplet mobilization has not been fully understood. In this work, we investigated the nanoparticles' role in nanoparticle-based improved recovery of the nonwetting phase through the direct observation of the mobilization of trapped oil droplets in microfluidic structures that mimic pore-throat geometries. A microfluidic platform was constructed for this study, on which different displacing liquids including aqueous surfactant solutions and nanoparticle suspensions were tested. We found that the nanoparticle concentration is positively related to the oil mobilization efficiency. An approximate mathematical model for calculating the maximum size of an oil droplet trapped in a pore-throat geometry for a fixed flow rate matches the experiment result for displacing liquid with no nanoparticles. The model still holds when the nanoparticle suspension is a displacing liquid. We concluded that nanoparticles mobilize oil in these geometries in a mechanism similar to that for surfactants, which is an increase in capillary number rather than an effect of other fluidic or interfacial properties such as the dynamics adsorption of nanoparticle or dilational rheology of a nanoparticle-adsorbed interface.
In this study, we developed a new method for the direct measurement of differential pressures in a co-flow junction microfluidic device using a Capillary Laplace Gauge (CLG). The CLG - used inside the microchannel device--was designed using a tapered glass-capillary set up in co-flow junction architecture with a three-phase liquid-liquid-gas system with two flowing liquid phases and an entrained gas phase. By taking advantage of the Laplace equation, basic geometric relations and an integrated image analysis program, the movement of the entrained gas phase with the flow of the liquid-phases is tracked and monitored, allowing the gauge to function as an ultra-sensitive, integrated, differential pressure sensor measuring fluctuations in the liquid-dispersed phase channel pressure as small as tens of Pascals caused by droplet formation. The gauge was used to monitor the drop formation and breakup process in a co-flow junction microfluidic device under different flow conditions across a large range (1 × 10(-3) to 2.0 × 10(-1)) of capillary numbers. In addition to being able to monitor short and long term dispersed phase pressure fluctuation trends for both single drop and large droplet populations, the gauge was also used to clearly identify a transition between the dripping and jetting flow regimes. Overall, the combination of a unique, integrated image analysis program with this new type of sensor serves as a powerful tool with great potential for a variety of different research and industrial applications requiring sensitive microchannel pressure measurements.
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