Dissolution without disappearing: multicomponent gas exchange for CO2 bubbles in a microfluidic channel

Diffusiophoresis, Diffusioosmosis, and Microfluidics: Surface-Flow-Driven Phenomena in the Presence of Flow

Diffusiophoresis is the spontaneous motion of particles under a concentration gradient of solutes. Since the first recognition by Derjaguin and colleagues in 1947 in the form of capillary osmosis, the phenomenon has been broadly investigated theoretically and experimentally. Early studies were mostly theoretical and were largely interested in surface coating applications, which considered the directional transport of coating particles. In the past decade, advances in microfluidics enabled controlled demonstrations of diffusiophoresis of micro- and nanoparticles. The electrokinetic nature and the typical scales of interest of the phenomenon motivated various experimental studies using simple microfluidic configurations. In this review, I will discuss studies that report diffusiophoresis in microfluidic systems, with the focus on the fundamental aspects of the reported results. In particular, parameters and influences of diffusiophoresis and diffusioosmosis in microfluidic systems and their combinations are highlighted.

Microfluidic mass transfer of CO2 at elevated pressures: implications for carbon storage in deep saline aquifers

Carbon capture and sequestration (CCS) in a deep saline aquifer is one of the most promising technologies to mitigate anthropologically emitted carbon dioxide. Accurately quantifying the mass transport of CO₂ at pore-scales is crucial but challenging for successful CCS deployment. Here, we conduct high-pressure microfluidic experiments, mimicking reservoir conditions up to 9.5 MPa and 35 °C, to elucidate the microfluidic mass transfer process of CO₂ at three different states (i.e., gas, liquid, and supercritical phase) into water. We measure the size change of CO₂ micro-bubbles/droplets generated using a microfluidic T-junction to estimate the volumetric mass transfer coefficient (k_La), quantifying the rate change of CO₂ concentration under the driving force of concentration gradient. The results show that bubbles/droplets under high-pressure conditions reach a steady state faster than low pressure. The measured volumetric mass transfer coefficient increases with the Reynolds number (based on the liquid slug) and is nearly independent of the injection pressure for both the gas and liquid phases. In addition, k_La significantly enlarges with increasing high pressure at the supercritical state. Compared with various chemical engineering applications using millimeter-sized capillaries (with typical k_La measured ranging from ≈0.005 to 0.8 s^-1), the microfluidic results show a significant increase in the volumetric mass transfer of CO₂ into water by two to three orders of magnitude, O (10²-10³), with decreasing hydrodynamic diameter (of ≈50 μm).

CO2-Driven diffusiophoresis for maintaining a bacteria-free surface

Dissolution and dissociation of CO2 in an aqueous phase induce diffusiophoretic motion of suspended particles with a nonzero surface charge. We report CO2-driven diffusiophoresis of colloidal particles and bacterial cells in a circular Hele-Shaw geometry. Combining experiments and model calculations, we identify the characteristic length and time scales of CO2-driven diffusiophoresis in relation to system dimensions and CO2 diffusivity. The motion of colloidal particles driven by a CO2 gradient is characterized by measuring the average velocities of particles as a function of distance from the CO2 sources. In the same geometrical configurations, we demonstrate that the directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO2 source is diffusiophoresis, not chemotaxis. Such a directional response of the cells to CO2 (or an ion) concentration gradient shows that diffusiophoresis of bacteria is achieved independent of cell shape, motility and the Gram stain (cell surface structure). Long-time experiments suggest potential applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces, by reducing the population of the cells near CO2 sources.

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.

Hydrodynamic shrinkage of liquid CO2 Taylor drops in a straight microchannel

Hydrodynamic shrinkage of liquid CO2 drops in water under a Taylor flow regime is studied using a straight microchannel (length/width ~100). A general form of a mathematical model of the solvent-side mass transfer coefficient (k s) is developed first. Based on formulations of the surface area (A) and the volume (V) of a general Taylor drop in a rectangular microchannel, a specific form of k s is derived. Drop length and speed are experimentally measured at three specified positions of the straight channel, namely, immediately after drop generation (position 1), the midpoint of the channel (position 2) and the end of the channel (position 3). The reductions of drop length (L x , x  =  1, 2, 3) from position 1 to 2 and down to 3 are used to quantify the drop shrinkage. Using the specific model, k s is calculated mainly based on L x and drop flowing time (t). Results show that smaller CO2 drops produced by lower flow rate ratios ([Formula: see text]) are generally characterized by higher (nearly three times) k s and Sherwood numbers than those produced by higher [Formula: see text], which is essentially attributed to the larger effective portion of the smaller drop contributing in the mass transfer under same levels of the flowing time and the surface-to-volume ratio (~104 m-1) of all drops. Based on calculated pressure drops of the segmented flow in microchannel, the Peng-Robinson equation of state and initial pressures of drops at the T-junction in experiments, overall pressure drop (ΔP t) in the straight channel as well as the resulted drop volume change are quantified. ΔP t from position 1-3 is by average 3.175 kPa with a ~1.6% standard error, which only leads to relative drop volume changes of 0.3‰ to 0.52‰.

Shrinking microbubbles with microfluidics: mathematical modelling to control microbubble sizes

Microbubbles have applications in industry and life-sciences. In medicine, small encapsulated bubbles (<10 μm) are desirable because of their utility in drug/oxygen delivery, sonoporation, and ultrasound diagnostics. While there are various techniques for generating microbubbles, microfluidic methods are distinguished due to their precise control and ease-of-fabrication. Nevertheless, sub-10 μm diameter bubble generation using microfluidics remains challenging, and typically requires expensive equipment and cumbersome setups. Recently, our group reported a microfluidic platform that shrinks microbubbles to sub-10 μm diameters. The microfluidic platform utilizes a simple microbubble-generating flow-focusing geometry, integrated with a vacuum shrinkage system, to achieve microbubble sizes that are desirable in medicine, and pave the way to eventual clinical uptake of microfluidically generated microbubbles. A theoretical framework is now needed to relate the size of the microbubbles produced and the system's input parameters. In this manuscript, we characterize microbubbles made with various lipid concentrations flowing in solutions that have different interfacial tensions, and monitor the changes in bubble size along the microfluidic channel under various vacuum pressures. We use the physics governing the shrinkage mechanism to develop a mathematical model that predicts the resulting bubble sizes and elucidates the dominant parameters controlling bubble sizes. The model shows a good agreement with the experimental data, predicting the resulting microbubble sizes under different experimental input conditions. We anticipate that the model will find utility in enabling users of the microfluidic platform to engineer bubbles of specific sizes.

Diffusion-Driven Dissolution or Growth of a Liquid Drop Embedded in a Continuous Phase of Another Liquid via Phase-Field Ternary Mixture Model

We simulate the diffusion-driven dissolution or growth of a single-component (resp. two-component) drop embedded in a continuous phase of a binary (resp. single-component) liquid. Our theoretical approach follows a standard diffuse-interface model of partially miscible ternary liquid mixtures, which is based on a regular solution model assumption together with a Flory-Huggins and Cahn-Hilliard representation of the excess and nonlocal components of the Gibbs free energy of mixing. Based on 2D simulation results, we show that for a single-component drop embedded in a continuous phase of a binary liquid (which is highly miscible with either one component of the continuous phase but essentially immiscible with the other) the size of the drop can either shrink to zero or reach a stationary value, depending on whether the global composition of the mixture is within the one-phase region or the unstable range of the phase diagram. On the other hand, for an isolated two-component drop embedded in a continuous phase of a single-component liquid (which is essentially immiscible with either one component of the drop but miscible with the other) the size of the drop can either grow or shrink and, in particular, it will eventually go to zero if the global composition of the mixture is within the one-phase region; otherwise, for system locations in the unstable range the size of the drop tends to a constant value as the composition within the drop reaches its final equilibrium value.

Stabilization of foams by the combined effects of an insoluble gas species and gelation

Liquid foams are unstable due to aging processes such as drainage, coalescence or coarsening. Since these processes modify the foam structure, they can be a severe limitation to the elaboration of solid foams with controlled structures inherited from their liquid precursors. Such applications call for a thorough understanding of foam stabilization. Here we study how coarsening can be inhibited by the combined effects of a mixture of gas containing a species insoluble in the foaming solution and of gelation of the foaming solution. We present experiments with model ordered liquid foams and hydrogel foams. They allow us to identify the underlying physical mechanisms of stabilization and their governing parameters, namely the bubble radius R_o, the foam shear modulus G and the number η_o of insoluble trapped gas molecules per bubble. We propose a scaling model that predicts the stability diagram of an ideal monodisperse perfectly ordered foam as a function of R_o, G and η_o, in qualitative agreement with our data. We show that the domain of stable foams is governed by a characteristic elasto-capillary radius set by the ratio of surface tension to storage modulus.

Honey, I shrunk the bubbles: microfluidic vacuum shrinkage of lipid-stabilized microbubbles

We present a microfluidic technique that shrinks lipid-stabilized microbubbles from O(100) to O(1) μm in diameter - the size that is desirable in applications as ultrasound contrast agents. We achieve microbubble shrinkage by utilizing vacuum channels that are adjacent to the microfluidic flow channels to extract air from the microbubbles. We tune a single parameter, the vacuum pressure, to accurately control the final microbubble size. Finally, we demonstrate that the resulting O(1) μm diameter microbubbles have similar stability to microfluidically generated microbubbles that are not exposed to vacuum shrinkage. We anticipate that, with additional scale-up, this simple approach to shrink microbubbles generated microfluidically will be desirable in ultrasound imaging and therapeutic applications.

Stabilization of carbon dioxide (CO2) bubbles in micrometer-diameter aqueous droplets and the formation of hollow microparticles

We report an approach to stabilize carbon dioxide (CO2) gas bubbles encapsulated in micrometer-diameter aqueous drops when water in the aqueous drops is evaporated. CO2-in-water-in-oil double emulsion drops are generated using microfluidic approaches and evaporation is conducted in the presence of sodium dodecyl sulfate (SDS), poly(vinyl alcohol) (PVA) and/or graphene oxide (GO) particles dispersed in the aqueous phase of the double emulsion drops. We examine the roles of the bubble-to-drop size ratio, PVA and GO concentration in the stabilization of CO2 bubbles upon water evaporation and show that thin-shell particles with encapsulated CO2 bubbles can be obtained under optimized conditions. The developed approach offers a new strategy to study CO2 dissolution and stability on the microscale and the synthesis of novel gas-core microparticles.

Growth control of sessile microbubbles in PDMS devices

In a microfluidic environment, the presence of bubbles is often detrimental to the functionality of the device, leading to clogging or cavitation, but microbubbles can also be an indispensable asset in other applications such as microstreaming. In either case, it is crucial to understand and control the growth or shrinkage of these bodies of air, in particular in common soft-lithography devices based on polydimethylsiloxane (PDMS), which is highly permeable to gases. In this work, we study the gas transport into and out of a bubble positioned in a microfluidic device, taking into account the direct gas exchange through PDMS as well as the transport of gas through the liquid in the device. Hydrostatic pressure regulation allows for the quantitative control of growth, shrinkage, or the attainment of a stable equilibrium bubble size. We find that the vapor pressure of the liquid plays an important role for the balance of gas transport, accounting for variability in experimental conditions and suggesting additional means of bubble size control in applications.