Droplet formation in microfluidic T-junction generators operating in the transitional regime. III. Dynamic surfactant effects

A review of electrochemical sensing in droplet systems: Droplet and digital microfluidics

BACKGROUND

Microfluidic technologies based on droplets provide discrete volumes within which chemical and/or biological processes can take place. Two major platforms in this space include droplet microfluidics (emulsions within channels) and digital microfluidics (discrete droplet manipulation by electric fields). The integration of electrochemical sensing with both microfluidic platforms offers advantages in miniaturization and portability, as sensors can be integrated directly within the microfluidic devices and instrumentation is relatively compact.

RESULTS

This review provides background on droplet and digital microfluidic technologies and electrochemical sensing before moving to methods and applications. A discussion of the various strategies to integrate sensing electrodes with both droplet and digital microfluidics and the merits of each method are included. A review of the many different applications of these integrated systems is provided.

SIGNIFICANCE AND NOVELTY

To date, there are no reviews that solely focus on the integration of electrochemical sensing with droplet and digital microfluidics. There are many advantages to combining electrochemical sensing with these platforms, especially for applications where portability or small form factors are paramount. While early reports on integrating electrochemical sensing with droplet and digital microfluidics are more than a decade old, the field is still relatively nascent, offering opportunity for many applications.

Studying surfactant mass transport through dynamic interfacial tension measurements: A review of the models, experiments, and the contribution of microfluidics

Surfactant mass transport towards an interface plays a critical role during formation of emulsions, foams and in industrial processes where two immiscible phases coexist. The understanding of these mechanisms as experimentally observed by dynamic interfacial tension measurements, is crucial. In this review, theoretical models describing both equilibrated systems and surfactant kinetics are covered. Experimental results from the literature are analysed based on the nature of surfactants and the tensiometry methods used. The innovative microfluidic techniques that have become available to study both diffusion and adsorption mechanisms during surfactant mass transport are discussed and compared with classical methods. This review focuses on surfactant transport during formation of droplets or bubbles; stabilisation of dispersed systems is not discussed here.

Effective colloidal emulsion droplet regulation in flow-focusing glass capillary microfluidic device via collection tube variation

Colloidal emulsion droplets, created using glass capillary microfluidic devices, have been found in a myriad of applications, serving as subtle microcarriers, delicate templates, etc. To meet the objective requirements under varying circumstances, it is crucial to efficiently control the morphology and dimensions of the droplets on demand. The glass capillary collection tube is a crucial component of the flow-focusing microfluidic system due to its close association with the geometrical confinement of the multiphasic flow. However, there are currently no guidelines for the design of the morphology and dimensions of the glass capillary collection tube, which shall result in a delay in assessing serviceability until after the microfluidic device is prepared, thereby causing a loss of time and effort. Herein, an experimental study was conducted to investigate the effect of the geometrical characteristics of glass capillary collection tubes on the production of colloidal emulsion droplets. After characterizing the generated colloidal emulsion droplets, it was found that the geometrical variations of the glass capillary collection tube resulted in numerical disparities of droplets due to different degrees of flow-focusing effects. The stronger flow-focusing effect produced smaller droplets at a higher frequency, and the dimensional variation of colloidal emulsion droplets was more responsive to varying flow rates. Furthermore, the transformation from colloidal single-core double-emulsion droplets to multi-core double-emulsion droplets also changed with the flow rate due to the glass capillary collection tube morphology-determined varying flow-focusing effect. These experimental findings can offer qualitative guidance for the design of glass capillary microfluidic devices in the preliminary stage, thus facilitating the smooth production of desired colloidal emulsion droplets.

Volatile Aroma Surfactants: The Evaluation of the Adsorption-Evaporation Behavior under Dynamic and Equilibrium Conditions

Multicomponent heterogeneous systems containing volatile amphiphiles are relevant to the fields ranging from drug delivery to atmospheric science. Research presented here discloses the individual interfacial activity and adsorption-evaporation behavior of amphiphilic aroma molecules at the liquid-vapor interface. The surface tension of solutions of nonmicellar volatile surfactants linalool and benzyl acetate, fragrances as such, was compared with that of the conventional surfactant sodium dodecyl sulfate (SDS) under equilibrium as well as under no instantaneous equilibrium, including a fast-adsorbing regime. In open systems, the increase in the surface tension on a time scale of ∼10 min is evaluated using a phenomenological model. The derived characteristic mass transfer constant is shown to be specific to both the desorption mechanism and the chemistry of the volatile amphiphile. Fast-adsorbing behavior disclosed here, as well as the synergetic effect in the mixtures with conventional micellar surfactants, justifies the advantages of volatile amphiphiles as cosurfactants in dynamic interfacial processes. The demonstrated approach to derive specific material parameters of fragrance molecules can be used for an application-targeted selection of volatile cosurfactants, e.g., in emulsification and foaming, inkjet printing, microfluidics, spraying, and coating technologies.

Droplet formation of biological non-Newtonian fluid in T-junction generators. II. Model for final droplet volume prediction

This work represents the second part of a two-part series on the dynamics of droplet formation in a T-junction generator under the squeezing regime when using solutions of red blood cells as the dispersed phase. Solutions containing red blood cells are non-Newtonian; however, these solutions do not behave in the same way as other non-Newtonian fluids currently described in the literature. Hence, available models do not capture nor predict important features useful for the design of T-junction microfluidic systems, including droplet volume. The formation of a red blood cell-containing droplet consists of three stages: a lag stage, a filling stage, and a necking stage, with the lag stage only observed in narrow dispersed phase channel setups. Unlike other shear-thinning fluids, thread elongation into the main channel at the end of the necking stage is not observed for red blood cell solutions. In this work, a model that predicts the final droplet volume of a red blood cell containing droplets in T-junction generators is presented. The model combines a detailed analysis of the geometrical shape of the droplet during the formation process, with force and Laplace pressure balances to obtain the penetration depth (b_{fill}^{*}) and the critical neck thickness (2r_{pinch}^{*}) of the droplet. The performance of the model was validated by comparing the operational parameters (droplet volume, the spacing between the droplet, and the generation frequency) with the experimental data across a range of the dimensionless parameters (flow rate ratios, continuous phase viscosities, and channel geometries).

Droplet formation of biological non-Newtonian fluid in T-junction generators. I. Experimental investigation

The extension of microfluidics to many bioassay applications requires the ability to work with non-Newtonian fluids. One case in point is the use of microfluidics with blood having different hematocrit levels. This work is the first part of a two-part study and presents the formation dynamics of blood droplets in a T-junction generator under the squeezing regime. In this regime, droplet formation with Newtonian fluids depends on T-junction geometry; however, we found that in the presence of the non-Newtonian fluid such as red blood cells, the formation depends on not only to the channel geometry, but also the flow rate ratio of fluids, and the viscosity of the phases. In addition, we analyzed the impact of the red blood cell concentration on the formation cycle. In this study, we presented the experimental data of the blood droplet evolution through the analysis of videos that are captured by a high-speed camera. During this analysis, we tracked several parameters such as droplet volume, spacing between droplets, droplet generation frequency, flow conditions, and geometrical designs of the T junction. Our analysis revealed that, unlike other non-Newtonian fluids, where the fourth stage exists (stretching stage), the formation cycle consists of only three stages: lag, filling, and necking stages. Because of the detailed analysis of each stage, a mathematical model can be generated to predict the final volume of the blood droplet and can be utilized as a guide in the operation of the microfluidic device for biochemical assay applications; this is the focus of the second part of this study [Phys. Rev. E 105, 025106 (2022)10.1103/PhysRevE.105.025106].

Comparison of surfactant mass transfer with drop formation times from dynamic interfacial tension measurements in microchannels

Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.

Determination of Dynamic Interfacial Tension during the Generation of Tiny Droplets in the Liquid-Liquid Jetting Flow Regime

Liquid-liquid dispersion coupled with droplet formation and mass transfer of surfactants is one of the most typical phenomena in many chemical processes. As in every aspect of this process, the interfacial tension variation caused by the unsaturated adsorption of surfactants on the droplet surface plays an important role. This article focuses on microdroplet formation and the dynamic interfacial behavior of surfactants in the jetting regime. In a capillary embedded step T-junction device, controllable preparation of monodisperse droplets is achieved, and a correlation for predicting droplet sizes is established. A method for measuring the dynamic interfacial tension is provided. Mass transfer coefficients are then calculated for Tween 20 during the droplet formation process by a semiempirical correlation. The results indicate that dynamic interfacial tensions are lower than those obtained when the surfactant is adsorbed to equilibrium. Based on the tip-streaming phenomenon, mass transfer coefficients for Tween 20 can reach up to ∼10^-3 m/s, higher than those obtained in coaxial microfluidic devices. All the preliminary results shed light on the nature of droplet formation and will be of significance for application in industrial apparatuses.

All-Fiber Measurement of Surface Tension Using a Two-Hole Fiber

An all-fiber approach is presented to measure surface tension. The experimental realization relies on the use of a specialty fiber, a so-called two-hole fiber (THF), which serves a two-fold purpose: providing a capillary channel to produce bubbles while having the means to measure the power reflected at the end facet of the fiber core. We demonstrate that provided a controlled injection of gas into the hollow channels of the THF, surface tension measurements are possible by simply tracking the Fresnel reflection at the distal end of the THF. Our results show that the characteristic times involved in the bubble formation process, from where the surface tension of the liquids under test is retrieved, can be measured from the train of pulses generated by the continuous formation and detachment of bubbles.

Acoustic-Controlled Bubble Generation and Fabrication of 3D Polymer Porous Materials

Porous materials have a variety of applications such as catalysis, gas separation, sensing, tissue engineering, sewage treatment, and so on. However, there are still challenges in the synthesis of porous materials with light weight, high porosity, and superhydrophobicity. Herein, we demonstrate one acoustic-controlled microbubble generation method, which is used to synthesize 3D polymer porous materials. The acoustic-controlled microbubble generation based on focused surface acoustic wave (FSAW) is suitable for not only the generation of gas-in-oil microbubbles but also the gas-in-water microbubbles. The size of microbubbles can be real-time controlled by adjusting the frequency or the driving voltage of the FSAW. The as-prepared poly(vinyl alcohol) (PVA) foams composed of microbubbles can be used as a template to fabricate the PVA-based porous gel materials through freezing-thawing cyclic processing, and the various sized bubbles result in different porosity of the PVA-based porous gel materials. Moreover, excellent properties like oleophilicity and superhydrophobicity of the PVA-based porous gel materials can be obtained through a further hydrophobic modification treatment. The oil/water separation experiments have been done to demonstrate the good absorption and reliability of the modified porous gel materials, which are capable of multiple uses.

Aroma Molecules as Dynamic Volatile Surfactants: Functionality beyond the Scent

Understanding of nonequilibrium processes at dynamic interfaces is indispensable for advancing design and fabrication of solid-state and soft materials. The research presented here unveils specific interfacial behavior of aroma molecules and justifies their usage as multifunctional volatile surfactants. As nonconventional volatile amphiphiles, we study commercially available poorly water-soluble compounds from the classes of synthetic and essential flavor oils. Their disclosed distinctive feature is a high dynamic interfacial activity, so that they decrease the surface tension of aqueous solutions on a time scale of milliseconds. Another potentially useful property of such amphiphiles is their volatility, so that they notably evaporate from interfaces on a time scale of seconds. This behavior allows for control of wetting and spreading processes. A revealed synergetic interfacial behavior of mixtures of conventional and volatile surfactants is attributed to a decrease of the activation barrier as a result of high statistical availability of new sites at the surface upon evaporation of the volatile component. Our results offer promising advantages in manufacturing technologies which involve newly creating interfaces, such as spraying, coating technologies, ink-jet printing, microfluidics, laundry, and stabilization of emulsions in cosmetic and food industry, as well as in geosciences for controlling aerosol formation.

Accounting for corner flow unifies the understanding of droplet formation in microfluidic channels

While shear emulsification is a well understood industrial process, geometrical confinement in microfluidic systems introduces fascinating complexity, so far prohibiting complete understanding of droplet formation. The size of confined droplets is controlled by the ratio between shear and capillary forces when both are of the same order, in a regime known as jetting, while being surprisingly insensitive to this ratio when shear is orders of magnitude smaller than capillary forces, in a regime known as squeezing. Here, we reveal that further reduction of—already negligibly small—shear unexpectedly re-introduces the dependence of droplet size on shear/capillary-force ratio. For the first time we formally account for the flow around forming droplets, to predict and discover experimentally an additional regime—leaking. Our model predicts droplet size and characterizes the transitions from leaking into squeezing and from squeezing into jetting, unifying the description for confined droplet generation, and offering a practical guide for applications.

T-junctions are a tool for droplet generation; they are well-described by models that distinguish for squeezing and jetting regimes for different capillary numbers. By considering the usually neglected corner flow, the authors identify an additional leaking regime for very low capillary numbers.

A new way to measure viscosity in droplet-based microfluidics for high throughput analysis

In this work, we propose a new way to measure the viscosity of samples in a microfluidic device. By analysing the shape of droplets after an expansion, we can measure the viscosity of the phase inside the droplet knowing the surface tension between the two liquids, the flow rate, the geometry of the channel and the viscosity of the continuous phase. This work paves the way for future high throughput studies in the framework of digital microfluidics.

Numerical Study of Surfactant Dynamics during Emulsification in a T-Junction Microchannel

Microchannel emulsification requires large amounts of surfactant to prevent coalescence and improve emulsions lifetime. However, most numerical studies have considered surfactant-free mixtures as models for droplet formation in microchannels, without taking into account the distribution of surfactant on the droplet surface. In this paper, we investigate the effects of nonuniform surfactant coverage on the microfluidic flow pattern using an extended lattice-Boltzmann model. This numerical study, supported by micro-particle image velocimetry experiments, reveals the likelihood of uneven distribution of surfactant during the droplet formation and the appearance of a stagnant cap. The Marangoni effect affects the droplet breakup by increasing the shear rate. According to our results, surfactant-free and surfactant-rich droplet formation processes are qualitatively different, such that both the capillary number and the Damköhler number should be considered when modeling the droplet generation in microfluidic devices. The limitations of traditional volume and pressure estimation methods for determining the dynamic interfacial tension are also discussed on the basis of the simulation results.

Ultralow Interfacial Tension Measurement through Jetting/Dripping Transition

In this paper, we present a dynamic microfluidic tensiometer able to perform measurements over more than four decades and which is suitable for high throughput experimentations. This tensiometer is able to withstand hard conditions such as high pressure, high temperature, high salinity, and crude oil. It is made of two coaxial capillaries in which two immiscible fluids are injected. Depending on the flow rate of each phase, either droplets or jetting will be obtained. The transition between these two regimes relies on the Rayleigh-Plateau instability. This transition can be theoretically computed thanks to a linear analysis based on the convective and absolute instabilities theory. From this model, the interfacial tension between the two phases can be calculated.

Investigation of dynamic surface tension in gas–liquid absorption using a microflow interfacial tensiometer

Dynamic surface tension in gas–liquid absorption is studied using a microflow device.

Passive and active droplet generation with microfluidics: a review

Precise and effective control of droplet generation is critical for applications of droplet microfluidics ranging from materials synthesis to lab-on-a-chip systems. Methods for droplet generation can be either passive or active, where the former generates droplets without external actuation, and the latter makes use of additional energy input in promoting interfacial instabilities for droplet generation. A unified physical understanding of both passive and active droplet generation is beneficial for effectively developing new techniques meeting various demands arising from applications. Our review of passive approaches focuses on the characteristics and mechanisms of breakup modes of droplet generation occurring in microfluidic cross-flow, co-flow, flow-focusing, and step emulsification configurations. The review of active approaches covers the state-of-the-art techniques employing either external forces from electrical, magnetic and centrifugal fields or methods of modifying intrinsic properties of flows or fluids such as velocity, viscosity, interfacial tension, channel wettability, and fluid density, with a focus on their implementations and actuation mechanisms. Also included in this review is the contrast among different approaches of either passive or active nature.

On-chip droplet production regimes using surface acoustic waves

Aqueous droplets suspended in an immiscible carrier fluid are a key tool in microfluidic chemical analysis platforms. The approaches for producing droplets in microfluidic devices can be divided into three general categories: batch emulsification, continuous production and tailored on-demand production. The major distinctions between each category are the rate of production and the degree of control over the droplet formation process in terms of the size and quantity. On-demand methods are highly desirable when, for example, small numbers or even single droplets of one sample type are required at a time. Here, we present a method for the on-demand production of femtolitre droplets, utilising a pressure source generated by high frequency surface acoustic waves (SAW). An increase in the continuous phase flow rate is enabled by a quasi-3D feature at the droplet production nozzle. A wide range of accessible flow rates permits the identification of different physical regimes in which droplets of different dimensions are produced. In the system investigated droplets measuring as little as 200 fl have been produced, ∼1/60th of the minimum volume previously reported. The experimental findings are supported by a numerical model which demonstrates the link between the number of droplets formed and the pulse length used.

Mass-Transfer-Controlled Dynamic Interfacial Tension in Microfluidic Emulsification Processes

Varied interfacial tension caused by the unsaturated adsorption of surfactants on dripping droplet surfaces is experimentally studied. The mass transfer and adsorption of surfactants, as well as the generation of fresh interfaces, are considered the main factors dominating the surfactant adsorption ratio on droplet surfaces. The diffusion and convective mass transfer of the surfactants are first distinguished by comparing the adsorption depth and the mass flux boundary layer thickness. A characterized mass transfer time is then calculated by introducing an effective diffusion coefficient. A time ratio is furthermore defined by dividing the droplet generation time by the characteristic mass transfer time, t/tm, in order to compare the rates of surfactant mass transfer and droplet generation. Different control mechanisms for different surfactants are analyzed based on the range of t/t(m), and a criterion time ratio using a simplified characteristic mass transfer time, t(m)*, is finally proposed for predicting the appearance of dynamic interfacial tension.

Effects of viscoelasticity on droplet dynamics and break-up in microfluidic T-Junctions: a lattice Boltzmann study

The effects of viscoelasticity on the dynamics and break-up of fluid threads in microfluidic T-junctions are investigated using numerical simulations of dilute polymer solutions at changing the Capillary number (Ca), i.e. at changing the balance between the viscous forces and the surface tension at the interface, up to Ca ≈ 3×10(-2). A Navier-Stokes (NS) description of the solvent based on the lattice Boltzmann models (LBM) is here coupled to constitutive equations for finite extensible non-linear elastic dumbbells with the closure proposed by Peterlin (FENE-P model). We present the results of three-dimensional simulations in a range of Ca which is broad enough to characterize all the three characteristic mechanisms of break-up in the confined T-junction, i.e. squeezing, dripping and jetting regimes. The various model parameters of the FENE-P constitutive equations, including the polymer relaxation time τP and the finite extensibility parameter L2, are changed to provide quantitative details on how the dynamics and break-up properties are affected by viscoelasticity. We will analyze cases with Droplet Viscoelasticity (DV), where viscoelastic properties are confined in the dispersed (d) phase, as well as cases with Matrix Viscoelasticity (MV), where viscoelastic properties are confined in the continuous (c) phase. Moderate flow-rate ratios Q ≈ O(1) of the two phases are considered in the present study. Overall, we find that the effects are more pronounced in the case with MV, as the flow driving the break-up process upstream of the emerging thread can be sensibly perturbed by the polymer stresses.

Understanding Miltefosine-Membrane Interactions Using Molecular Dynamics Simulations

Coarse-grained molecular dynamics simulations are used to calculate the free energies of transfer of miltefosine, an alkylphosphocholine anticancer agent, from water to lipid bilayers to study its mechanism of interaction with biological membranes. We consider bilayers containing lipids with different degrees of unsaturation: dipalmitoylphosphatidylcholine (DPPC, saturated, containing 0%, 10%, and 30% cholesterol), dioleoylphosphatidylcholine (DOPC, diunsaturated), palmitoyloleoylphosphatidylcholine (POPC, monounsaturated), diarachidonoylphosphatidylcholine (DAPC, polyunsaturated), and dilinoleylphosphatidylcholine (DUPC, polyunsaturated). These free energies, calculated using umbrella sampling, were used to compute the partition coefficients (K) of miltefosine between water and the lipid bilayers. The K values for the bilayers relative to that of pure DPPC were found to be 5.3 (DOPC), 7.0 (POPC), 1.0 (DAPC), 2.2 (DUPC), 14.9 (10% cholesterol), and 76.2 (30% cholesterol). Additionally, we calculated the free energy of formation of miltefosine-cholesterol complexes by pulling the surfactant laterally in the DPPC + 30% cholesterol system. The free energy profile that we obtained provides further evidence that miltefosine tends to associate with cholesterol and has a propensity to partition into lipid rafts. We also quantified the kinetics of the transport of miltefosine through the various bilayers by computing permeance values. The highest permeance was observed in DUPC bilayers (2.28 × 10(-2) m/s) and the lowest permeance in the DPPC bilayer with 30% cholesterol (1.10 × 10(-7) m/s). Our simulation results show that miltefosine does indeed interact with lipid rafts, has a higher permeability in polyunsaturated, loosely organized bilayers, and has higher flip-flop rates in specific regions of cellular membranes.

Droplet confinement and leakage: Causes, underlying effects, and amelioration strategies

The applicability of droplet-based microfluidic systems to many research fields stems from the fact that droplets are generally considered individual and self-contained reaction vessels. This study demonstrates that, more often than not, the integrity of droplets is not complete, and depends on a range of factors including surfactant type and concentration, the micro-channel surface, droplet storage conditions, and the flow rates used to form and process droplets. Herein, a model microfluidic device is used for droplet generation and storage to allow the comparative study of forty-four different oil/surfactant conditions. Assessment of droplet stability under these conditions suggests a diversity of different droplet failure modes. These failure modes have been classified into families depending on the underlying effect, with both numerical and qualitative models being used to describe the causative effect and to provide practical solutions for droplet failure amelioration in microfluidic systems.