Initial microfluidic dissolution regime of CO2 bubbles in viscous oils

CO2 Microbubbles in Silicone Oil (Part II: Henry's Constant and Anomalous Diffusion)

This work demonstrates the utility of microfluidic devices for characterizing diffusion mechanisms. We determined Henry's constant and characterized the diffusion process of gaseous CO2 in silicone oil. Using microfluidic techniques, we analyzed the evolution of the CO2 bubble size in a solvent flowing through a microchannel system. The reduction in bubble size due to the mass transfer of gaseous CO2 into the solvent fluid primarily affects their length. A microfluidic device was used to produce bubbles, consisting of a pressure-driven injection system for the gas and a flow-driven system for the liquid. Additionally, an optical device was coupled for tracking and studying the bubbles in the microchannels, enabling us to study their spatial and temporal evolution using image analysis. From this study, we found two diffusion regimes. The first is a superdiffusive process for short times. In this regime, due to the high concentration gradient values at the gas-liquid interface, we observed a higher rate of carbon dioxide transfer to the silicone oil. At longer times, we see that the gas transfer rate significantly decreases compared to the previous regime, leading to a subdiffusive process. In this latter regime, it was found that if we increase the gas pressure, the system approaches a normal diffusive process that coincides with previously conducted studies by other researchers. It is suggested that the subdiffusion could be due to the high degree of confinement of the bubbles within the microchannel, similar to what occurs in porous media, the high viscosity of the fluid, and the low gas pressure used in the tests. The microfluidic device proved to be a very efficient method for determining the diffusion process and Henry's constant in this case. Its easy fabrication and low cost make this type of device appropriate for substance characterization.

CO2 Microbubbles in Silicone Oil (Part I: Study of the Frequency of Formation)

The objective of this work is the study of gas microbubble dissolution in a carrier liquid. To achieve this, we will analyze, using microfluidic techniques, the formation and evolution of carbon dioxide (CO2) gas microbubbles in silicone oil, monitoring the size, position, and distance between the formed bubbles as they advance through a microchannel. This work consists of two parts (Part I and Part II): in Part I, we analyze the mechanisms determining the variation in the spatial frequency of bubbles as they move through the microchannel, while Part II examines the evolution of their size and demonstrates the utility of the device for obtaining diffusion coefficients and Henry's constant for the gases used. The microchip containing the microchannels has a serpentine shape, allowing extensive bubble trajectories to be captured in a single image. Regarding the study of bubble spatial frequency (Part I), it was found to exhibit three regimes during its displacement through the microchannel. Initially, the frequency increases because the bubbles block the microchannel, preventing liquid transfer through the residual film formed between the channel wall and the gas. Then, there is a transition stage during which the liquid manages to penetrate the residual film due to a local increase in pressure gradient. The pressure gradient overcomes the reduction in film thickness due to the decrease in carrier phase flow rate, which is affected by gas dissolution in the liquid. Therefore, the frequency begins to stabilize until the bubbles lose contact with the microchannel wall and assume a spherical shape, being transported at the carrier liquid velocity. These studies contribute to the understanding of transport phenomena and interfaces in CO2 foams in oils, which are of great interest in industries such as oil recovery and refrigerants.

Universal Correlation for Droplet Fragmentation in a Microfluidic T-Junction

Despite extensive research on droplet dynamics at microfluidic T-junction, for different droplet lengths and Capillary numbers, there remains limited understanding of their dynamics at different viscosity ratio. In this study, we adopt a modeling framework in a three-dimensional (3D) configuration to numerically investigate the droplet dynamics as it passes through a symmetric T-junction with varying Capillary numbers, droplet lengths, and viscosity ratios. We present a 3D regime map for the first time to demarcate the droplet breakup and no breakup regimes. Herein, we propose a simple surface equation accounting for the critical Capillary number for breakup, in terms of viscosity ratio and dimensionless droplet length. The proposed universal relationship aligns well with experimental and computational findings from the existing literature. Furthermore, we reveal a new droplet breakup characteristic at high viscosity ratio and high Capillary number where the droplet spreads almost twice its initial value before splitting. Overall, this research provides comprehensive understanding of droplet dynamics at the T-junction and has significant implications for several related applications, including the large-scale synthesis of microdroplets using microchannel networks.

Microfluidic approaches for accessing thermophysical properties of fluid systems

Thermophysical properties of fluid systems under high pressure and high temperature conditions are highly desirable as they are used in many industrial processes both from a chemical engineering point of view and to push forward the development of modeling approaches.

Catalytic activity of nickel nanoparticles stabilized by adsorbing polymers for enhanced carbon sequestration

This work shows the potential of nickel (Ni) nanoparticles (NPs) stabilized by polymers for accelerating carbon dioxide (CO₂) dissolution into saline aquifers. The catalytic characteristics of Ni NPs were investigated by monitoring changes in diameter of CO₂ microbubbles. An increase in ionic strength considerably reduces an electrostatic repulsive force in pristine Ni NPs, thereby decreasing their catalytic potential. This study shows how cationic dextran (DEX), nonionic poly(vinyl pyrrolidone) (PVP), and anionic carboxy methylcellulose (CMC) polymers, the dispersive behaviors of Ni NPs can be used to overcome the negative impact of salinity on CO₂ dissolution. The cationic polymer, DEX was less adsorbed onto NPs surfaces, thereby limiting the Ni NPs’ catalytic activity. This behavior is due to a competition for Ni NPs’ surface sites between the cation and DEX under high salinity. On the other hand, the non/anionic polymers, PVP and CMC could be relatively easily adsorbed onto anchoring sites of Ni NPs by the monovalent cation, Na⁺. Considerable dispersion of Ni NPs by an optimal concentration of the anionic polymers improved their catalytic capabilities even under unfavorable conditions for CO₂ dissolution. This study has implications for enhancing geologic sequestration into deep saline aquifers for the purposes of mitigating atmospheric CO₂ levels.

The hydrodynamics of segmented two-phase flow in a circular tube with rapidly dissolving drops

This article discusses boundary integral simulations of dissolving drops flowing through a cylindrical tube for large aspect ratio drops. The dynamics of drop dissolution is determined by three dimensionless parameters: λ, the viscosity of the drop fluid relative to the suspending fluid; Ca, the capillary number defining the ratio of the hydrodynamic force to the interfacial tension force; and k, a dissolution constant based on the velocity of dissolution. For a single dissolving drop, the velocity in the upstream region is greater than the downstream region, and for sufficiently large k, the downstream velocity can be completely reversed, particularly at low Ca. The upstream end of the drop travels faster and experiences greater deformation than the downstream end. The film thickness, δ, between the drop and the tube wall is governed by a delicate balance between dissolution and changes in the outer fluid velocity resulting from a fixed pressure drop across the tube and mass continuity. Therefore, δ, and consequently, the drop average velocity, can increase, decrease or be relatively invariant in time. For two drops flowing in succession, while low Ca drops maintain a nearly constant separation distance during dissolution, at sufficiently large Ca, for all values of k, dissolution increases the separation distance between drops. Under these conditions, the liquid segments between two adjacent drops can no longer be considered as constant volume stirred tanks. These results will guide the choices of geometry and operating parameters that will facilitate the characterization of fast gas-liquid reactions via two-phase segmented flows.

Fingering instability and mixing of a blob in porous media

The curvature of the unstable part of the miscible interface between a circular blob and the ambient fluid in two-dimensional homogeneous porous media depends on the viscosity of the fluids. The influence of the interface curvature on the fingering instability and mixing of a miscible blob within a rectilinear displacement is investigated numerically. The fluid velocity in porous media is governed by Darcy's law, coupled with a convection-diffusion equation that determines the evolution of the solute concentration controlling the viscosity of the fluids. Numerical simulations are performed using a Fourier pseudospectral method to determine the dynamics of a miscible blob (circular or square). It is shown that for a less viscous circular blob, there exist three different instability regions without any finite R-window for viscous fingering, unlike the case of a more viscous circular blob. Critical blob radius for the onset of instability is smaller for a less viscous blob as compared to its more viscous counterpart. Fingering enhances spreading and mixing of miscible fluids. Hence a less viscous blob mixes with the ambient fluid quicker than the more viscous one. Furthermore, we show that mixing increases with the viscosity contrast for a less viscous blob, while for a more viscous one mixing depends nonmonotonically on the viscosity contrast. For a more viscous blob mixing depends nonmonotonically on the dispersion anisotropy, while it decreases monotonically with the anisotropic dispersion coefficient for a less viscous blob. We also show that the dynamics of a more viscous square blob is qualitatively similar to that of a circular one, except the existence of the lump-shaped instability region in the R-Pe plane. We have shown that the Rayleigh-Taylor instability in a circular blob (heavier or lighter than the ambient fluid) is independent of the interface curvature.

Calculating the volume of elongated bubbles and droplets in microchannels from a top view image

We present a theoretical model to calculate the volume of bubbles and droplets in segmented microflows from given dimensions of the microchannel and measured lengths of bubbles and droplets.

Microfluidic studies of carbon dioxide

Carbon dioxide (CO2) sequestration, storage and recycling will greatly benefit from comprehensive studies of physical and chemical gas-liquid processes involving CO2. Over the past five years, microfluidics emerged as a valuable tool in CO2-related research, due to superior mass and heat transfer, reduced axial dispersion, well-defined gas-liquid interfacial areas and the ability to vary reagent concentrations in a high-throughput manner. This Minireview highlights recent progress in microfluidic studies of CO2-related processes, including dissolution of CO2 in physical solvents, CO2 reactions, the utilization of CO2 in materials science, and the use of supercritical CO2 as a "green" solvent.