Bubbles and foams in microfluidics

High-Speed Generation of Microbubbles with Constant Cumulative Production in a Glass Capillary Microfluidic Bubble Generator

This work reports a simple bubble generator for the high-speed generation of microbubbles with constant cumulative production. To achieve this, a gas-liquid co-flowing microfluidic device with a tiny capillary orifice as small as 5 μm is fabricated to produce monodisperse microbubbles. The diameter of the microbubbles can be adjusted precisely by tuning the input gas pressure and flow rate of the continuous liquid phase. The co-flowing structure ensures the uniformity of the generated microbubbles, and the surfactant in the liquid phase prevents coalescence of the collected microbubbles. The diameter coefficient of variation (CV) of the generated microbubbles can reach a minimum of 1.3%. Additionally, the relationship between microbubble diameter and the gas channel orifice is studied using the low Capillary number (Ca) and Weber number (We) of the liquid phase. Moreover, by maintaining a consistent gas input pressure, the CV of the cumulative microbubble volume can reach 3.6% regardless of the flow rate of the liquid phase. This method not only facilitates the generation of microbubbles with morphologic stability under variable flow conditions, but also ensures that the cumulative microbubble production over a certain period of time remains constant, which is important for the volume-dominated application of chromatographic analysis and the component analysis of natural gas.

Gas transport mechanisms through gas-permeable membranes in microfluidics: A perspective

Gas-permeable membranes (GPMs) and membrane-like micro-/nanostructures offer precise control over the transport of liquids, gases, and small molecules on microchips, which has led to the possibility of diverse applications, such as gas sensors, solution concentrators, and mixture separators. With the escalating demand for GPMs in microfluidics, this Perspective article aims to comprehensively categorize the transport mechanisms of gases through GPMs based on the penetrant type and the transport direction. We also provide a comprehensive review of recent advancements in GPM-integrated microfluidic devices, provide an overview of the fundamental mechanisms underlying gas transport through GPMs, and present future perspectives on the integration of GPMs in microfluidics. Furthermore, we address the current challenges associated with GPMs and GPM-integrated microfluidic devices, taking into consideration the intrinsic material properties and capabilities of GPMs. By tackling these challenges head-on, we believe that our perspectives can catalyze innovative advancements and help meet the evolving demands of microfluidic applications.

Prebiotic Foam Environments to Oligomerize and Accumulate RNA

When water interacts with porous rocks, its wetting and surface tension properties create air bubbles in large number. To probe their relevance as a setting for the emergence of life, we microfluidically created foams that were stabilized with lipids. A persistent non-equilibrium setting was provided by a thermal gradient. The foam's large surface area triggers capillary flows and wet-dry reactions that accumulate, aggregate and oligomerize RNA, offering a compelling habitat for RNA-based early life as it offers both wet and dry conditions in direct neighborhood. Lipids were screened to stabilize the foams. The prebiotically more probable myristic acid stabilized foams over many hours. The capillary flow created by the evaporation at the water-air interface provided an attractive force for molecule localization and selection for molecule size. For example, self-binding oligonucleotide sequences accumulated and formed micrometer-sized aggregates which were shuttled between gas bubbles. The wet-dry cycles at the foam bubble interfaces triggered a non-enzymatic RNA oligomerization from 2',3'-cyclic CMP and GMP which despite the small dry reaction volume was superior to the corresponding dry reaction. The found characteristics make heated foams an interesting, localized setting for early molecular evolution.

Nonlinear Phenomena in Microfluidics

This review focuses on experimental work on nonlinear phenomena in microfluidics, which for the most part are phenomena for which the velocity of a fluid flowing through a microfluidic channel does not scale proportionately with the pressure drop. Examples include oscillations, flow-switching behaviors, and bifurcations. These phenomena are qualitatively distinct from laminar, diffusion-limited flows that are often associated with microfluidics. We explore the nonlinear behaviors of bubbles or droplets when they travel alone or in trains through a microfluidic network or when they assemble into either one- or two-dimensional crystals. We consider the nonlinearities that can be induced by the geometry of channels, such as their curvature or the bas-relief patterning of their base. By casting posts, barriers, or membranes─situated inside channels─from stimuli-responsive or flexible materials, the shape, size, or configuration of these elements can be altered by flowing fluids, which may enable autonomous flow control. We also highlight some of the nonlinearities that arise from operating devices at intermediate Reynolds numbers or from using non-Newtonian fluids or liquid metals. We include a brief discussion of relevant practical applications, including flow gating, mixing, and particle separations.

Effect of gravity on the orientation and detachment of cubic particles adsorbed at soap film or liquid interfaces

We investigate the interaction that occurs between a light solid cube falling under gravity and a horizontal soap film that is pinned to a circular ring. We observe in both experiments and quasi-static simulations that the final orientation of a cube that becomes entrapped by a soap film is strongly dependent on the Bond number. A cube is rotated by a soap film into one of three main orientations in a process that is driven by energy minimisation. The likelihood of observing each of these final orientations is shown to depend on the Bond number, and the most energetically favourable orientation depends on the terminal height reached by the cube. We also find a critical value for the Bond number, above which a cube is no longer supported by a soap film and detachment occurs, to be less than one.

Bubbles in microfluidics: an all-purpose tool for micromanipulation

In recent decades, the integration of microfluidic devices and multiple actuation technologies at the microscale has greatly contributed to the progress of related fields. In particular, microbubbles are playing an increasingly important role in microfluidics because of their unique characteristics that lead to specific responses to different energy sources and gas-liquid interactions. Many effective and functional bubble-based micromanipulation strategies have been developed and improved, enabling various non-invasive, selective, and precise operations at the microscale. This review begins with a brief introduction of the morphological characteristics and formation of microbubbles. The theoretical foundations and working mechanisms of typical micromanipulations based on acoustic, thermodynamic, and chemical microbubbles in fluids are described. We critically review the extensive applications and the frontline advances of bubbles in microfluidics, including microflow patterns, position and orientation control, biomedical applications, and development of bubble-based microrobots. We lastly present an outlook to provide directions for the design and application of microbubble-based micromanipulation tools and attract the attention of relevant researchers to the enormous potential of microbubbles in microfluidics.

Generation and Dynamics of Janus Droplets in Shear-Thinning Fluid Flow in a Double Y-Type Microchannel

Droplets composed of two different materials, or Janus droplets, have diverse applications, including microfluidic digital laboratory systems, DNA chips, and self-assembly systems. A three-dimensional computational study of Janus droplet formation in a double Y-type microfluidic device filled with a shear-thinning fluid is performed by using the multiphaseInterDyMFoam solver of the OpenFOAM, based on a finite-volume method. The bi-phase volume-of-fluid method is adopted to track the interface with an adaptive dynamic mesh refinement for moving interfaces. The formation of Janus droplets in the shear-thinning fluid is characterized in five different states of tubbing, jetting, intermediate, dripping and unstable dripping in a multiphase microsystem under various flow conditions. The formation mechanism of Janus droplets is understood by analyzing the influencing factors, including the flow rates of the continuous phase and of the dispersed phase, surface tension, and non-Newtonian rheological parameters. Studies have found that the formation of the Janus droplets and their sizes are related to the flow rate at the inlet under low capillary numbers. The rheological parameters of shear-thinning fluid have a significant impact on the size of Janus droplets and their formation mechanism. As the apparent viscosity increases, the frequency of Janus droplet formation increases, while the droplet volume decreases. Compared with Newtonian fluid, the Janus droplet is more readily generated in shear-thinning fluid due to the interlay of diminishing viscous force, surface tension, and pressure drop.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Programmable Porous Polymers via Direct Bubble Writing with Surfactant-Free Inks

Fabrication of macroporous polymers with functionally graded architecture or chemistry bears transformative potential in acoustic damping, energy storage materials, flexible electronics, and filtration but is hardly reachable with current processes. Here, we introduce thiol-ene chemistries in direct bubble writing, a recent technique for additive manufacturing of foams with locally controlled cell size, density, and macroscopic shape. Surfactant-free and solvent-free graded three-dimensional (3D) foams without drying-induced shrinkage were fabricated by direct bubble writing at an unparalleled ink viscosity of 410 cP (40 times higher than previous formulations). Functionalities including shape memory, high glass transition temperatures (>25 °C), and chemical gradients were demonstrated. These results extend direct bubble writing from aqueous inks to nonaqueous formulations at high liquid flow rates (3 mL min-1). Altogether, direct bubble writing with thiol-ene inks promises rapid one-step fabrication of functional materials with locally controlled gradients in the chemical, mechanical, and architectural domains.

Micrometric Monodisperse Solid Foams as Complete Photonic Bandgap Materials

Solid foams with micrometric pores are used in different fields (filtering, 3D cell culture, etc.), but today, controlling their foam geometry at the pore level, their internal structure, and the monodispersity, along with their mechanical properties, is still a challenge. Existing attempts to create such foams suffer either from slow speed or size limitations (above 80 μm). In this work, by using a temperature-regulated microfluidic process, 3D solid foams with highly monodisperse open pores (PDI lower than 5%), with sizes ranging from 5 to 400 μm and stiffnesses spanning 2 orders of magnitude, are created for the first time. These features open the way for exciting applications, in cell culture, filtering, optics, etc. Here, the focus is set on photonics. Numerically, these foams are shown to open a 3D complete photonic bandgap, with a critical index of 2.80, thus compatible with the use of rutile TiO₂. In the field of photonics, such structures represent the first physically realizable self-assembled FCC (face-centered cubic) structure that possesses this functionality.

Stable liquid foams from a new polyfluorinated surfactant

Liquid foams exhibiting long-term stability are a key-challenge in material design. Based on this perspective, new pyridinium polyfluorinated surfactants were synthesized from simple building blocks enabling unusually stable liquid foams. While the batch-generated foams were used for qualitative foaming evaluation, microfluidics allowed a quantitative insight into the aging effects of monodisperse foams.

Microfluidic Study of the Electrocoalescence of Aqueous Droplets in Crude Oil

In electrocoalescence, an electric field is applied to a dispersion of conducting water droplets in a poorly conducting oil to force the droplets to merge in the direction of the field. Electrocoalescence is used in petroleum refining to separate water from crude oil and in droplet-based microfluidics to combine droplets of water in oil and to break emulsions. Using a microfluidic design to generate a two-dimensional (2D) emulsion, we demonstrate that electrocoalescence in an opaque crude oil can be visualized with optical microscopy and studied on an individual droplet basis in a chamber whose height is small enough to make the dispersions two dimensional and transparent. From reconstructions of images of the 2D electrocoalescence, the electrostatic forces driving the droplet merging are calculated in a numerically exact manner and used to predict observed coalescence events. Hence, the direct simulation of the electrocoalescence-driven breakdown of 2D emulsions in microfluidic devices can be envisioned.

Microfluidics Mediated Production of Foams for Biomedical Applications

Within the last decade, there has been increasing interest in liquid and solid foams for several industrial uses. In the biomedical field, liquid foams can be used as delivery systems for dermatological treatments, for example, whereas solid foams are frequently used as scaffolds for tissue engineering and drug screening. Most of the foam functionalities are largely correlated to their mechanical properties and their structure, especially bubble/pore size, shape, and interconnectivity. However, the majority of conventional foaming fabrication techniques lack pore size control which can induce important inhomogeneities in the foams and subsequently decrease their performance. In this perspective, new advanced technologies have been introduced, such as microfluidics, which offers a highly controlled production, allowing for design customization of both liquid foams and solid foams obtained through liquid-templating. This short review explores both the fabrication and the characterization of foams, with a focus on solid polymer foams, and sheds the light on how microfluidics can overcome some existing limitations, playing a crucial role in their production for biomedical applications, especially as scaffolds in tissue engineering.

Elastic behavior of confined soap froth

The elastic response of ordered 3D soap froth, in which N layers of cells are confined between two rigid walls, is analyzed. Surface Evolver simulations are used to compute the equilibrium structure, which consists of a layer of Fejes-Toth cells at each wall and N - 2 core layers of Kelvin cells. The reference state corresponds to the plate spacing ho that achieves isotropic stress; and the foam confinement is varied by changing h. The foam is sheared in two orthogonal directions to determine elastic behavior up to the elastic limit or yield strain, which corresponds to the onset of topological transitions. The shear moduli in both directions decrease as N increases and slowly converge to the values of bulk Kelvin foam; the dependence on N is well described by a three-layer model consisting of Fejes-Toth cells and a core of Kelvin cells that deform like bulk foam. The influence of foam confinement on the elastic limit is studied. The topological transitions are compared to those in bulk Kelvin foam.

Modeling the Excess Velocity of Low-Viscous Taylor Droplets in Square Microchannels

Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to determine the excess velocity of Taylor droplets in square microchannels. This velocity difference between the droplet and the total superficial velocity of the flow has a direct influence on the droplet residence time and is linked to the pressure drop. Since the droplet does not occupy the entire channel cross-section, it enables the continuous phase to bypass the droplet through the corners. A consideration of the continuity equation generally relates the excess velocity to the mean flow velocity. We base the quantification of the bypass flow on a correlation for the droplet cap deformation from its static shape. The cap deformation reveals the forces of the flowing liquids exerted onto the interface and allows estimating the local driving pressure gradient for the bypass flow. The characterizing parameters are identified as the bypass length, the wall film thickness, the viscosity ratio between both phases and the C a number. The proposed model is adapted with a stochastic, metaheuristic optimization approach based on genetic algorithms. In addition, our model was successfully verified with high-speed camera measurements and published empirical data.

T-Shaped Microfluidic Junction Processing of Porous Alginate-Based Films and Their Characteristics

In this work, highly monodisperse porous alginate films from bubble bursting were formed on a glass substrate at ambient temperature, by a T-shaped microfluidic junction device method using polyethylene glycol (PEG) stearate and phospholipid as precursors in some cases. Various polymer solution concentrations and feeding liquid flow rates were applied for the generation of monodisperse microbubbles, followed by the conversion of the bubbles to porous film structures on glass substrates. In order to compare the physical properties of polymeric solutions, the effects of alginate, PEG stearate (surfactant), and phospholipid concentrations on the flowability of the liquid in a T-shaped microfluidic junction device were studied. To tailor microbubble diameter and size distribution, a method for controlling the thinning process of the bubbles' shell was also explored. In order to control pore size, shape, and surface as well as internal structure morphologies in the scalable forming of alginate polymeric films, the effect of the feeding liquid's flow rate and concentrations of PEG-stearate and phospholipid was also studied. Digital microscopy images revealed that the as-formed alginate films at the flow rate of 100 µL·min-1 and the N2 gas pressure of 0.8 bar have highly monodisperse microbubbles with a polydispersity index (PDI) of approximately 6.5%. SEM captures also revealed that the as-formed alginate films with high PDI value have similar monodisperse porous surface and internal structure morphologies, with the exception that the as-formed alginate films with the help of phospholipids were mainly formed under our experimental environment. From the Fourier-transform infrared spectroscopy (FTIR), Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements, we concluded that no chemical composition changes, thermal influence, and crystal structural modifications were observed due to the T-shaped microfluidic junction device technique. The method used in this work could expand and enhance the use of alginate porous films in a wide range of bioengineering applications, especially in tissue engineering and drug delivery, such as studying release behaviors to different internal and surface morphologies.

Nanoparticle-Coated Microbubbles for Combined Ultrasound Imaging and Drug Delivery

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, to be echogenic, and to release the cargo of drug-containing nanoparticles with an ultrasound trigger. This Article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability, and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way toward more efficient nanoparticle-mediated drug delivery. This Article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

Controlled open-cell two-dimensional liquid foam generation for micro- and nanoscale patterning of materials

Liquid foam consists of liquid film networks. The films can be thinned to the nanoscale via evaporation and have potential in bottom-up material structuring applications. However, their use has been limited due to their dynamic fluidity, complex topological changes, and physical characteristics of the closed system. Here, we present a simple and versatile microfluidic approach for controlling two-dimensional liquid foam, designing not only evaporative microholes for directed drainage to generate desired film networks without topological changes for the first time, but also microposts to pin the generated films at set positions. Patterning materials in liquid is achievable using the thin films as nanoscale molds, which has additional potential through repeatable patterning on a substrate and combination with a lithographic technique. By enabling direct-writable multi-integrated patterning of various heterogeneous materials in two-dimensional or three-dimensional networked nanostructures, this technique provides novel means of nanofabrication superior to both lithographic and bottom-up state-of-the-art techniques.

Nip the bubble in the bud: a guide to avoid gas nucleation in microfluidics

Gas bubbles are almost a routine occurrence encountered by researchers working in the field of microfluidics. The spontaneous and unexpected nature of gas bubbles represents a major challenge for experimentalists and a stumbling block for the translation of microfluidic concepts to commercial products. This is a startling example of successful scientific results in the field overshadowing the practical hurdles of day-to-day usage. We however believe such hurdles can be overcome with a sound understanding of the underlying conditions that lead to bubble formation. In this tutorial, we focus on the two main conditions that result in bubble nucleation: surface nuclei and gas supersaturation in liquids. Key theoretical concepts such as Henry's law, Laplace pressure, the role of surface properties, nanobubbles and surfactants are presented along with a view of practical implementations that serve as preventive and curative measures. These considerations include not only microfluidic chip design and bubble traps but also often-overlooked conditions that regulate bubble formation, such as gas saturation under pressure or temperature gradients. Scenarios involving electrolysis, laser and acoustic cavitation or T-junction/co-flow geometries are also explored to provide the reader with a broader understanding on the topic. Interestingly, despite their often-disruptive nature, gas bubbles have also been cleverly utilized for certain practical applications, which we briefly review. We hope this tutorial will provide a reference guide in helping to deal with a familiar foe, the "bubble".

Self-assembly of droplets in three-dimensional microchannels

Self-assembly of droplets guided by microfluidic channels have potential applications ranging from high throughput assays to materials synthesis, but such demonstrations have been limited primarily to two-dimensional (2D) assembly of droplets in planar microfluidic devices. We demonstrated the use of three-dimensional (3D) microchannels to self-assemble droplets into ordered 2D and 3D arrays by designing microchannels with axial gradients in height and controlling the volume fraction of the droplets in the channel. In contrast to previous demonstrations, ordered 2D arrays of droplets were assembled at low volume fractions of the dispersed phase. Interestingly, we found that the self-assembly of droplets in microchannels was highly path dependent. The assembly of droplets was governed by transitions in the cross-sectional shapes of the microchannel, not the final geometry of the chamber for the assembly of droplet, which is a hitherto rarely explored phenomenon. The assembled droplets were used as templates for the fabrication of millimeter scale, anisotropic hydrogel fibers with ordered pore sizes (∼250 μm). These demonstrations suggested that 3D microchannels would be a viable platform for the manipulation of droplets, and applicable for the continuous synthesis of complex materials with 3D morphologies.

Ultrasound transmission through monodisperse 2D microfoams

While the acoustic properties of solid foams have been abundantly characterized, sound propagation in liquid foams remains poorly understood. Recent studies have investigated the transmission of ultrasound through three-dimensional polydisperse liquid foams (Pierre et al., 2013, 2014, 2017). However, further progress requires to characterize the acoustic response of better-controlled foam structures. In this work, we study experimentally the transmission of ultrasounds through a single layer of monodisperse bubbles generated by microfluidics techniques. In such a material, we show that the sound velocity is only sensitive to the gas phase. Nevertheless, the structure of the liquid network has to be taken into account through a transfer parameter analogous to the one in a layer of porous material. Finally, we observe that the attenuation cannot be explained by thermal dissipation alone, but is compatible with viscous dissipation in the gas pores of the monolayer.

Patterning Bubbles by the Stick-Slip Motion of the Advancing Triple Phase Line on Nanostructures

The stick-slip motion of the triple phase contact line (TCL) has wide applications in inkjet printing, surface coatings, functional material assembly, and device fabrication. Here, for the first time, we report that on an alumina substrate with nanostructures, the stick-slip motion of the advancing TCL during spreading of an emulsion droplet can serve as an effective nanopatterning process. Air enclosed in the substrate nanostructures can be exchanged with liquid during the "stick" phase, resulting in the formation of bubbles arranged in a ring pattern. The process takes place in two stages: rings of air form first and then, as the volume of air increases, they separate into air bubbles as a result of the Plateau Rayleigh instability. During the first stage, the rings form due to the stick-slip of the advancing TCL and are ascribed to hydrogen-bonding interactions. Ultimate bubble size is dependent on the substrate pore dimensions. The process was simulated using finite-element analysis to elucidate the mechanism associated with subsequent bubble formation. The simulations corroborate well with the experimental results. This stick-slip motion of the advancing TCL provides new insights into the phenomena associated with droplet spreading and wetting, and the ability to control the formation of patterned bubbles will be promising in applications ranging from microfluidics to printing of functional materials and devices based on bubble templates and applications requiring submerged hydrophobic surface.

Controlling the distance of highly confined droplets in a capillary by interfacial tension for merging on-demand

Droplet microfluidics is a powerful technology that finds many applications in chemistry and biomedicine. Among different configurations, droplets confined in a capillary (or plugs) present a number of advantages: they allow positional identification and simplify the integration of complex multi-steps protocols. However, these protocols rely on the control of droplet speed, which is affected by a complex and still debated interplay of various physico-chemical parameters like droplet length, viscosity ratio between droplets and carrier fluid, flow rate and interfacial tension. We present here a systematic investigation of the droplet speed as a function of their length and interfacial tension, and propose a novel, simple and robust methodology to control the relative distance between consecutive droplets flowing in microfluidic channels through the addition of surfactants either into the dispersed and/or into the continuous phases. As a proof of concept application, we present the possibility to accurately trigger in space and time the merging of two confined droplets flowing in a uniform cross-section circular capillary. This approach is further validated by monitoring a conventional enzymatic reaction used to quantify the concentration of H2O2 in a biological sample, showing its potentialities in both continuous and stopped assay methods.

Simulating the interaction between a descending super-quadric solid object and a soap film

We investigate the interaction that occurs between a light solid object and a horizontal soap film of a ‘bamboo’ foam contained in a cylindrical tube. We vary the shape of the descending object from a sphere to a cube by changing a single shape parameter. We investigate in detail how the soap film deforms and determine the forces that the film exerts on the object, depending on the radius of the cylindrical tube, and the shape, orientation and position of the object. We show that a cubic particle in a particular orientation experiences the largest drag force, and that this orientation is also the most likely outcome of dropping a cube from an arbitrary orientation through a bamboo foam.

Electroosmosis near surfactant laden liquid-air interfaces

Generation of an electroosmostic (EO) flow near a liquid-gas interface covered with ionic surfactants is experimentally investigated. A combination of microscopic flow measurements with a molecular characterization of the interface by second harmonic generation (SHG) shows that under an electrical forcing, although a liquid flow is generated below the free surface, the surfactants remain immobile. The zeta potential was then determined and compared to the surfactant surface coverage. This combination of experimental techniques opens the route to simultaneously probe the liquid flow near a soapy interface and the corresponding surfactant repartition affecting the hydrodynamic boundary condition.

Capture of colloidal particles by a moving microfluidic bubble

Foams can be stabilized for long periods by the adsorption of solid particles on the liquid-gas interfaces. Although such long-term observations are common, mechanistic descriptions of the particle adsorption process are scarce, especially in confined flows, in part due to the difficulty of observing the particles in the complex gas-liquid dispersion of a foam. Here, we characterise the adsorption of micron-scale particles onto the interface of a bubble flowing in a colloidal aqueous suspension within a microfluidic channel. Three parameters are systematically varied: the particle size, their concentration, and the mean velocity of the colloidal suspension. The bubble coverage is found to increase linearly with position in the channel for all conditions but with a slope that depends on all three parameters. The optimal coverage is found for 1 μm particles at low flow rates and high concentrations. In this regime the particles pass the bubbles through the gutters between the interface and the channel corners, where the complex 3D flow leads them onto the interface. The largest particles cannot enter into the gutters and therefore provide very poor coverage. In contrast, particle aggregates can sediment onto the microchannel floor ahead of the bubble and get swept up by the advancing interface, thus improving the coverage for both large and medium particle sizes. These observations provide new insight on the influence of boundaries for particle adsorption at an air-liquid interface.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca^4/3 depending on the distance from the nose of the bubble, where Ca = μU_b/γ is the flow capillary number based on the bubble velocity U_b, liquid viscosity μ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance L_w between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, L_w always scales proportional to Ca^α, where 1.7 < α < 2 for the range of 10^-5 < Ca < 10^-2. Moreover, L_w increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of L_w provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

Electrokinetic transport in liquid foams

Investigating electrokinetic transport in a liquid foam is at the confluence of two well developed research areas. On one hand, the study of electrokinetic flows (i.e. surface-driven flows generated close to a charged interface) is fairly well understood in regards the solid/liquid interface. On the other hand, the flow of liquid in a 3D deformable network, i.e a foam, under a volume force such as gravity has been thoroughly studied over the past decade. The overlapping zone of these two frameworks is of great interest for both communities as it gives rise to challenging new questions such as: what is the importance of the nature of the charged interface, created by mobile and soluble surfactants in the case of foam, on electrokinetic transport? How does a foam behave when submitted to a surface-driven flow? Can we compensate a volume-driven flow, i.e. gravity, by a surface-driven flow, i.e. electroosmosis? In this review, we will explore these questions on three different scales: a surfactant laden interface, a foam film and a macroscopic foam.

Curvature driven motion of a bubble in a toroidal Hele-Shaw cell

We investigate the equilibrium properties of a single area-minimizing bubble trapped between two narrowly separated parallel curved plates. We begin with the case of a bubble trapped between concentric spherical plates. We develop a model which shows that the surface energy of the bubble is lower when confined between spherical plates than between flat plates. We confirm our findings by comparing against Surface Evolver simulations. We then derive a simple model for a bubble between arbitrarily curved parallel plates. The energy is found to be higher when the local Gaussian curvature of the plates is negative and lower when the curvature is positive. To check the validity of the model, we consider a bubble trapped between concentric tori. In the toroidal case, we find that the sensitivity of the bubble's energy to the local curvature acts as a geometric potential capable of driving bubbles from regions with negative to positive curvature.

Electro-osmosis at surfactant-laden liquid-gas interfaces: beyond standard models

Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and nanofluidic systems. While EO has been studied extensively at liquid-solid interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and bubbles, remains to be explored. Here we perform molecular dynamics (MD) simulations of EO in a film of aqueous electrolyte covered with fluid layers of ionic surfactants and surrounded by gas. Following the experimental procedure, we compute the zeta potential from the EO velocity, defined as the velocity difference between the middle of the liquid film and the surrounding gas. We show that the zeta potential can be smaller or larger than existing predictions depending on the surfactant coverage. We explain the failure of previous descriptions by the fact that surfactants and bound ions move as rigid bodies and do not transmit the electric driving force to the liquid locally. Considering the reciprocal streaming current effect, we then develop an extended model, which can be used to predict the experimental zeta potential of surfactant-laden liquid-gas interfaces.

Microscale distribution and dynamic surface tension of pulmonary surfactant normalize the recruitment of asymmetric bifurcating airways

We investigate the influence of bifurcation geometry, asymmetry of daughter airways, surfactant distribution, and physicochemical properties on the uniformity of airway recruitment of asymmetric bifurcating airways. To do so, we developed microfluidic idealized in vitro models of bifurcating airways, through which we can independently evaluate the impact of carina location and daughter airway width and length. We explore the uniformity of recruitment and its relationship to the dynamic surface tension of the lining fluid and relate this behavior to the hydraulic (PHyd) and capillary (PCap) pressure drops. These studies demonstrate the extraordinary importance of PCap in stabilizing reopening, even in highly asymmetric systems. The dynamic surface tension of pulmonary surfactant is integral to this stability because it modulates PCap in a velocity-dependent manner. Furthermore, the surfactant distribution at the propagating interface can have a very large influence on recruitment stability by focusing surfactant preferentially to specific daughter airways. This implies that modification of the surfactant distribution through novel modes of ventilation could be useful in inducing uniformly recruited lungs, aiding in gas exchange, and reducing ventilator-induced lung injury.

NEW & NOTEWORTHY The dynamic surface tension of pulmonary surfactant is integral to the uniformity of asymmetric bifurcation airway recruitments because it modulates capillary pressure drop in a velocity-dependent manner. Also, the surfactant distribution at the propagating interface can have a very large influence on recruitment stability by focusing surfactant preferentially to specific daughter airways. This implies that modification of the surfactant distribution through novel modes of ventilation could be useful in inducing uniformly recruited lungs, reducing ventilator-induced lung injury.

Using flow technologies to direct the synthesis and assembly of materials in solution

In the pursuit of materials with structure-related function, directing the assembly of materials is paramount. The resultant structure can be controlled by ordering of reactants, spatial confinement and control over the reaction/crystallisation times and stoichiometries. These conditions can be administered through the use of flow technologies as evidenced by the growing widespread application of microfluidics for the production of nanomaterials; the function of which is often dictated or circumscribed by size. In this review a range of flow technologies is explored for use in the control of self-assembled systems: including techniques for reagent ordering, mixing control and high-throughput optimisation. The examples given encompass organic, inorganic and biological systems and focus on control of shape, function, composition and size.Graphical abstract.

Porous Polymeric Films from Microbubbles Generated Using a T-Junction Microfluidic Device

In this work, a simple microfluidic junction with a T geometry and coarse (200 μm diameter) capillaries was used to generate monodisperse microbubbles with an alginate polymer shell. Subsequently, these bubbles were used to prepare porous alginate films with good control over the pore structure. The lack of pore size, shape, and surface control in scalable forming of polymeric films is a major application-limiting drawback at present. Controlling the thinning process of the shell of the bubbles to tune the surface of the resulting structures was also explored. Films were prepared with nanopatterned surfaces by controlling the thinning of the bubble shell, with the aid of surfactants, to induce efficient bursting (fragmentation) of bubbles to generate nanodroplets, which become embedded within the film surface. This novel feature greatly expands and enhances the use of hydrophilic polymers in a wide range of biomedical applications, particularly in drug delivery and tissue engineering, such as studying cellular responses to different morphological surfaces.

Yield-stress fluids foams: flow patterns and controlled production in T-junction and flow-focusing devices

We study the formation of yield-stress fluid foams in millifluidic flow-focusing and T-junction devices. First, we provide a phase diagram for the unsteady operating regimes of bubble production when the gas pressure and the yield-stress fluid flow rate are imposed. Three regimes are identified: a co-flow of gas and yield-stress fluid, a transient production of bubble and a flow of yield-stress fluid only. Taking wall slip into account, we provide a model for the pressure at the onset of bubble formation. Then, we detail and compare two simple methods to ensure steady bubble production: regulation of the gas pressure or flow-rate. These techniques, which are easy to implement, thus open pathways for controlled production of dry yield-stress fluid foams as shown at the end of this article.

Dynamics of bubble breakup at a T junction

The gas-liquid interfacial dynamics of bubble breakup in a T junction was investigated. Four regimes were observed for a bubble passing through the T junction. It was identified by the stop flow that a critical width of the bubble neck existed: if the minimum width of the bubble neck was less than the critical value, the breakup was irreversible and fast; while if the minimum width of the bubble neck was larger than the critical value, the breakup was reversible and slow. The fast breakup was driven by the surface tension and liquid inertia and is independent of the operating conditions. The minimum width of the bubble neck could be scaled with the remaining time as a power law with an exponent of 0.22 in the beginning and of 0.5 approaching the final fast pinch-off. The slow breakup was driven by the continuous phase and the gas-liquid interface was in the equilibrium stage. Before the appearance of the tunnel, the width of the depression region could be scaled with the time as a power law with an exponent of 0.75; while after that, the width of the depression was a logarithmic function with the time.

Microfluidic Foaming: A Powerful Tool for Tailoring the Morphological and Permeability Properties of Sponge-like Biopolymeric Scaffolds

Ordered porous polymeric materials can be engineered to present highly ordered pore arrays and uniform and tunable pore size. These features prompted a number of applications in tissue engineering, generation of meta materials, and separation and purification of biomolecules and cells. Designing new and efficient vistas for the generation of ordered porous materials is an active area of research. Here we investigate the potential of microfluidic foaming within a flow-focusing (FF) geometry in producing 3D regular sponge-like polymeric matrices with tailored morphological and permeability properties. The challenge in using microfluidic systems for the generation of polymeric foams is in the high viscosity of the continuous phase. We demonstrate that as the viscosity of the aqueous solution increases, the accessible range of foam bubble fraction (Φb) and bubble diameter (Db) inside the microfluidic chip tend to narrow progressively. This effect limits the accessible range of geometric properties of the resulting materials. We further show that this problem can be rationally tackled by appropriate choice of the concentration of the polymer. We demonstrate that via such optimization, the microfluidic assisted synthesis of porous materials becomes a facile and versatile tool for generation of porous materials with a wide range of pore size and pore volume. Moreover, we demonstrate that the size of interconnects among pores-for a given value of the gas fraction-can be tailored through the variation of surfactant concentration. This, in turn, affects the permeability of the materials, a factor of key importance in flow-through applications and in tissue engineering.