Microfluidic ultralow interfacial tensiometry with magnetic particles

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications

Effective manipulation of liquids on open surfaces without external energy input is indispensable for the advancement of point-of-care diagnostic devices. Open-surface microfluidics has the potential to benefit health care, especially in the developing world. This review highlights the prospects for harnessing capillary forces on surface-microfluidic platforms, chiefly by inducing smooth gradients or sharp steps of wettability on substrates, to elicit passive liquid transport and higher-order fluidic manipulations without off-the-chip energy sources. A broad spectrum of the recent progress in the emerging field of passive surface microfluidics is highlighted, and its promise for developing facile, low-cost, easy-to-operate microfluidic devices is discussed in light of recent applications, not only in the domain of biomedical microfluidics but also in the general areas of energy and water conservation.

Emulsion characterization via microfluidic devices: A review on interfacial tension and stability to coalescence

Emulsions have gained significant importance in many industries including foods, pharmaceuticals, cosmetics, health care formulations, paints, polymer blends and oils. During emulsion generation, collisions can occur between newly-generated droplets, which may lead to coalescence between the droplets. The extent of coalescence is driven by the properties of the dispersed and continuous phases (e.g. density, viscosity, ion strength and pH), and system conditions (e.g. temperature, pressure or any external applied forces). In addition, the diffusion and adsorption behaviors of emulsifiers which govern the dynamic interfacial tension of the forming droplets, the surface potential, and the duration and frequency of the droplet collisions, contribute to the overall rate of coalescence. An understanding of these complex behaviors, particularly those of interfacial tension and droplet coalescence during emulsion generation, is critical for the design of an emulsion with desirable properties, and for the optimization of the processing conditions. However, in many cases, the time scales over which these phenomena occur are extremely short, typically a fraction of a second, which makes their accurate determination by conventional analytical methods extremely challenging. In the past few years, with advances in microfluidic technology, many attempts have demonstrated that microfluidic systems, characterized by micrometer-size channels, can be successfully employed to precisely characterize these properties of emulsions. In this review, current applications of microfluidic devices to determine the equilibrium and dynamic interfacial tension during droplet formation, and to investigate the coalescence stability of dispersed droplets applicable to the processing and storage of emulsions, are discussed.

Continuous, autonomous subsurface cargo shuttling by nature-inspired meniscus-climbing systems

Water-walking insects can harness capillary forces by changing their body posture to climb or descend the meniscus between the surface of water and a solid object. Controlling surface tension in this manner is necessary for predation, escape and survival. Inspired by this behaviour, we demonstrate autonomous, aqueous-based synthetic systems that overcome the meniscus barrier and shuttle cargo subsurface to and from a landing site and a targeted drop-off site. We change the sign of the contact angle of a coacervate sac containing an aqueous phase or of a hydrogel droplet hanging from the surface by controlling the normal force acting on the sac or droplet. The cyclic buoyancy-induced cargo shuttling occurs continuously, as long as the supply of reactants diffusing to the sac or droplet from the surrounding aqueous phase is not exhausted. These findings may lead to potential applications in autonomously driven reaction or delivery systems and micro-/milli-robotics.

Dielectrophoretic medium exchange around droplets for on-chip fabrication of layer-by-layer microcapsules

Continuous medium exchange within a microchannel represents a highly sought-after technique in functionalizing micro-objects with coating layers, enabling a myriad of applications ranging from biomedical engineering to materials science. Herein, we introduce a unique medium exchange approach, namely, tilted-angle dielectrophoresis, to accomplish layer-by-layer (LbL) coating on droplets in a wide microchannel. Pairs of adjacent tilted parallel electrodes arranged in a zigzag fashion are exploited to consecutively and repeatedly guide particles/droplets travelling through three parallel laminar streams comprising two reagents and a washing buffer. The performance of medium exchange is demonstrated using PS microparticles and oil droplets. We show that multi-cycle medium exchange, droplet transfer accompanied with purification, and multi-mode medium exchange around different micro-objects are achieved by conveniently regulating the applied voltage and the inlet flow rate, indicating a flexible, versatile and label-free alternative for characterizing and handling colloidal particles. Furthermore, LbL coating on droplets utilizing the presented strategy is implemented in the parallel coating-chemical and washing streams to obtain multiple layers of microcapsules. The linearly increasing fluorescence intensity of the coated droplets with each subsequent fluorescent coating demonstrates the capability of the tilted-angle dielectrophoretic medium exchanger for on-chip generation of LbL microcapsules on demand. The presented medium exchange strategy, together with its unique features of simple geometric configuration, facile control and multifunctionality, can provide a refined alternative for further expanding the utility scope in functional particles and cells.

Plasmonic nanobowtiefluidic device for sensitive detection of glioma extracellular vesicles by Raman spectrometry

Cancer cells shed into biofluids extracellular vesicles (EVs) - nanoscale membrane particles carrying diagnostic information. EVs shed by heterogeneous populations of tumor cells offer a unique opportunity to access biologically important aspects of disease complexity. Glioblastoma (GBM) exemplifies cancers that are incurable, because their temporal dynamics and molecular complexity evade standard diagnostic methods and confound therapeutic efforts. Liquid biopsy based on EVs offers unprecedented real-time access to complex tumour signatures, but it is not used clinically due to inefficient testing methods. We report on a nanostructured microfluidic-device that employs SERS for unambiguous identification of EVs from different GBM cell populations. The device features fabless plasmonic nanobowties for label-free and non-immunological SERS detection of EVs. This nanobowtiefluidic device combines the advanced characteristics of plasmonic nanobowties with a high throughput sample-delivery system for concentration of the analytes in the vicinity of the detection site. We showed theoretically and experimentally that the fluidic device assists the monolayer distribution of the EVs, which dramatically increase the probability of EV's existence in the laser illumination area. In addition, the optimized fabless nanobowtie structures with an average electric field enhancement factor of 9 × 10⁵ achieve distinguishable and high intensity SERS signals. Using the nanobowtiefluidic and micro-Raman equipment, we were able to distinguish a library of peaks expressed in GBM EV subpopulations from two distinct glioblastoma cell lines (U373, U87) and compare them to those of non-cancerous glial EVs (NHA) and artificial homogenous vesicles (e.g. DOPC/Chol). This cost-effective and easy-to-fabricate SERS platform and a portable sample-delivery system for discerning the sub-population of GBM EVs and non-cancerous glial EVs may have broader applications to different types of cancer cells and their molecular/oncogenic signature.

Phase-Dependent Surfactant Transport on the Microscale: Interfacial Tension and Droplet Coalescence

Liquid-liquid emulsion systems are usually stabilized by additives, known as surfactants, which can be observed in various environments and applications such as oily bilgewater, water-entrained diesel fuel, oil production, food processing, cosmetics, and pharmaceuticals. One important factor that stabilizes emulsions is the lowered interfacial tension (IFT) between the fluid phases due to surfactants, inhibiting the coalescence. Many studies have investigated the surfactant transport behavior that leads to corresponding time-dependent lowering of the IFT. For example, the rate of IFT decay depends on the phase in which the surfactant is added (dispersed vs continuous) due in part to differences in the near-surface depletion depth. Other key factors, such as the viscosity ratio between the dispersed and continuous phases and Marangoni stress, will also have an impact on surfactant transport and therefore the coalescence and emulsion stability. In this feature article, the measurement techniques for dynamic IFT are first reviewed due to their importance in characterizing surfactant transport, with a specific focus on macroscale versus microscale techniques. Next, equilibrium isotherm models as well as dynamic diffusion and kinetic equations are discussed to characterize the surfactant and the time scale of the surfactant transport. Furthermore, recent studies are highlighted showing the different IFT decay rates and its long-time equilibrium value depending on the phase into which the surfactant is added, particularly on the microscale. Finally, recent experiments using a hydrodynamic Stokes trap to investigate the impact of interfacial surfactant transport, or "mobility", and the phase containing the surfactant on film drainage and droplet coalescence will be presented.

Static and dynamic behavior of magnetic particles at fluid interfaces

This perspective work reviews the current status of research on magnetic particles at fluid interfaces. The article gives both a unified overview of recent experimental advances and theoretical studies centered on very different phenomena that share a common characteristic: they involve adsorbed magnetic particles that range in size from a few nanometers to several millimeters. Because of their capability of being remotely piloted through controllable external fields, magnetic particles have proven essential as building blocks in the design of new techniques, smart materials and micromachines, with new tunable properties and prospective applications in engineering and biotechnology. Once adsorbed at a fluid-fluid interfase, in a process that can be facilitated via the application of magnetic field gradients, these particles often result sorely confined to two dimensions (2D). In this configuration, inter-particle forces directed along the perpendicular to the interface are typically very small compared to the surface forces. Hence, the confinement and symmetry breaking introduced by the presence of the surface play an important role on the response of the system to the application of an external field. In monolayers of particles where the magnetic is predominant interaction, the states reached are strongly determined by the mode and orientation of the applied field, which promote different patterns and processes. Furthermore, they can reproduce some of the dynamic assemblies displayed in bulk or form new ones, that take advantage of the interfacial phenomena or of the symmetry breaking introduce by the confining boundary. Magnetic colloids are also widely used for unraveling the guiding principles of 2D dynamic self-assembly, in designs devised for producing interface transport, as tiny probes for assessing interfacial rheological properties, neglecting the bulk and inertia contributions, as well as actuated stabilizing agents in foams and emulsions.

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.

Measuring Interfacial Tension of Emulsions in Situ by Microfluidics

Interfacial tension is a key parameter affecting industrially relevant properties of emulsions, such as morphology and stability. Although several methods are available to measure interfacial tension, they are based on generation of droplets starting from separate emulsion components and cannot directly probe the interfacial tension of an emulsion as such. Here, a novel microfluidic tensiometry device to measure interfacial tension of a water-in-oil emulsion in situ as a function of surfactant concentration is presented. In our approach, interfacial tension is obtained from a quantitative analysis of the deformation of individual emulsion droplets under steady state shear flow in microfluidic channels. The technique is validated by comparing the results with experimental data obtained by the pendant drop method in a broad range of interfacial tension values. A very good agreement is found, and an estimate of the surfactant critical micellar concentration (CMC) is also obtained. The proposed microfluidic setup can be used even at high surfactant concentrations, where the measurement is made more challenging by sample viscoelasticity, thus providing a powerful tool to determine the interfacial tension of complex systems in an extended concentration range. The technique could be also used for in-line monitoring of emulsion processing.

Magnetocapillary self-assemblies: Locomotion and micromanipulation along a liquid interface

This paper presents an overview and discussion of magnetocapillary self-assemblies. New results are presented, in particular concerning the possible development of future applications. These self-organizing structures possess the notable ability to move along an interface when powered by an oscillatory, uniform magnetic field. The system is constructed as follows. Soft magnetic particles are placed on a liquid interface, and submitted to a magnetic induction field. An attractive force due to the curvature of the interface around the particles competes with an interaction between magnetic dipoles. Ordered structures can spontaneously emerge from these conditions. Furthermore, time-dependent magnetic fields can produce a wide range of dynamic behaviours, including non-time-reversible deformation sequences that produce translational motion at low Reynolds number. In other words, due to a spontaneous breaking of time-reversal symmetry, the assembly can turn into a surface microswimmer. Trajectories have been shown to be precisely controllable. As a consequence, this system offers a way to produce microrobots able to perform different tasks. This is illustrated in this paper by the capture, transport and release of a floating cargo, and the controlled mixing of fluids at low Reynolds number.

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.

Temperature controlled tensiometry using droplet microfluidics

We develop a temperature controllable microfluidic device for the accurate measurement of temperature dependent interfacial tensions between two immiscible liquids. A localized temperature control system is integrated with the microfluidic platform to maintain an accurate temperature inside the device. The temperature uniformity and sensitivity are verified by both simulation and experimental results. Temperature dependent interfacial tensions are measured dynamically and rapidly, relying on quantitative analysis of the deformation and retraction dynamics of droplets under extensional flow. Our microfluidic tensiometry offers the capability of measuring temperature dependent interfacial tensions with precise and systematic temperature control in the range of room temperature to 70 °C, which is valuable for studying transient interfacial dynamics, interfacial reactions, and the surfactant adsorption process.

Nanopore Sensing in Aqueous Two-Phase System: Simultaneous Enhancement of Signal and Translocation Time via Conformal Coating

Nanofluidic resistive pulse sensing (RPS) has been extensively used to measure the size, concentration, and surface charge of nanoparticles in electrically conducting solutions. Although various methods have been explored for improving detection performances, intrinsic problems including the extremely low particle-to-pore volume ratio (<0.01%) and fast nanoparticle translocation (10-1000 µs) still induce difficulties in detection, such as low signal magnitudes and short translocation times. Herein, we present an aqueous two-phase system (ATPS) in a nanofluidic RPS for amplifying translocation signals and decreasing translocation speeds simultaneously. Two immiscible aqueous liquids build a liquid-liquid interface inside nanopores. As particles translocate from a high-affinity liquid phase into a lower-affinity one, the high-affinity liquid forms a conformal coating on the particles, which increases the effective particle size and amplifies the current-blockage signal. The translocation time is also increased, as the ATPS interface impedes the particle translocation. For 20 nm particles, 7.92-fold and 5.82-fold enhancements of signal magnitude and translocation time can be achieved. To our knowledge, this is the first attempt to improve nanofluidic RPS by treating an interface of solution reservoirs for manipulating target particles rather than nanopores. This direct particle manipulation allows us to solve the two intrinsic problems all at once.

On-Chip Magnetic Particle-Based Immunoassays Using Multilaminar Flow for Clinical Diagnostics

Magnetic particles have become popular in recent years for immunoassays due to their high surface-to-volume ratio and the ease of their manipulation. However, such assays also require multiple reaction and washing steps that are both time-consuming and manually laborious. Here, we describe a setup and methodology for performing rapid immunoassays on magnetic particles in continuous flow via their deflection through multiple laminar flow streams of reagents and washing solutions. In particular, we focus on the use of the microfluidic platform for a C-reactive protein (CRP) sandwich immunoassay in less than 60 s.

Spontaneous transfer of droplets across microfluidic laminar interfaces

The precise manipulation of droplets in microfluidics has revolutionized a myriad of drop-based technologies, such as multiple emulsion preparation, drop fusion, drop fission, drop trapping and drop sorting, which offer promising new opportunities in chemical and biological fields. In this paper, we present an interfacial-tension-directed strategy for the migration of droplets across liquid-liquid laminar streams. By carefully controlling the interfacial energies, droplets of phase A are able to pass across the laminar interfaces of two immiscible fluids from phase B to phase C due to a positive spreading coefficient of phase C over phase B. To demonstrate this, we successfully perform the transfer of water droplets across an oil-oil laminar interface and the transfer of oil droplets across an oil-water laminar interface. The whole transfer process is spontaneous and only takes about 50 ms. We find that the fluid dynamics have an impact on the transfer processes. Only if the flowrate ratios are well matched will the droplets pass through the laminar interface successfully. This interfacial-tension-directed transfer of droplets provides a versatile procedure to make new structures and control microreactions as exemplified by the fabrication of giant unilamellar vesicles and cell-laden microgels.

Interfacial rheometry of polymer at a water-oil interface by intra-pair magnetophoresis

We describe an interfacial rheometry technique based on pairs of micrometer-sized magnetic particles at a fluid-fluid interface. The particles are repeatedly attracted and repelled by well-controlled magnetic dipole-dipole forces, so-called interfacial rheometry by intra-pair magnetophoresis (IPM). From the forces (∼pN), displacements (∼μm) and velocities (∼μm s(-1)) of the particles we are able to quantify the interfacial drag coefficient of particles within a few seconds and over very long timescales. The use of local dipole-dipole forces makes the system insensitive to fluid flow and suited for simultaneously recording many particles in parallel over a long period of time. We apply IPM to study the time-dependent adsorption of an oil-soluble amino-modified silicone polymer at a water-oil interface using carboxylated magnetic particles. At low polymer concentration the carboxylated particles remain on the water side of the water-oil interface, while at high polymer concentrations the particles transit into the oil phase. Both conditions show a drag coefficient that does not depend on time. However, at intermediate polymer concentrations data show an increase of the interfacial drag coefficient as a function of time, with an increase over more than three orders of magnitude (10(-7) to 10(-4) N s m(-1)), pointing to a strong polymer-polymer interaction at the interface. The time-dependence of the interfacial drag appears to be highly sensitive to the polymer concentration and to the ionic strength of the aqueous phase. We foresee that IPM will be a very convenient technique to study fluid-fluid interfaces for a broad range of materials systems.

Mechanical Characterization of Ultralow Interfacial Tension Oil-in-Water Droplets by Thermal Capillary Wave Analysis in a Microfluidic Device

Measurements of the ultralow interfacial tension and surfactant film bending rigidity for micron-sized heptane droplets in bis(2-ethylhexyl) sodium sulfosuccinate-NaCl aqueous solutions were performed in a microfluidic device through the analysis of thermally driven droplet interface fluctuations. The Fourier spectrum of the stochastic droplet interface displacement was measured through bright-field video microscopy and a contour analysis technique. The droplet interfacial tension, together with the surfactant film bending rigidity, was obtained by fitting the experimental results to the prediction of a capillary wave model. Compared to existing methods for ultralow interfacial tension measurements, this contactless, nondestructive, all-optical approach has several advantages, such as fast measurement, easy implementation, cost-effectiveness, reduced amount of liquids, and integration into lab-on-a-chip devices.

Microfluidic magnetic self-assembly at liquid-liquid interfaces

We present a microfluidic method that controllably self-assembles microparticles into clusters at an aqueous two-phase liquid-liquid interface. The liquid-liquid interface is formed between converging flows of aqueous dextran and polyethylene glycol, in a microfluidic cross-slot device. We control the size of the self-assembled particle clusters as they pass through the liquid-liquid interface, by systematically varying the applied magnetic field gradient, and the interfacial tension of the liquid-liquid interface. We find that upon penetration through the interface, the number of particles within a cluster increases with increasing interfacial tension, and decreasing magnetic field gradient. We also develop a scaling model of the number of particles within a cluster, and observe an inverse scaling of the number of particles within a cluster with the dimensionless magnetic Bond number. Upon cluster penetration across the liquid-liquid interface, we find magnetic Bond number regimes where the fluid coating drains away from the surface of the cluster, and where the clusters are encapsulated inside a thin film coating layer. This self-assembly technique may find application in controlling the size of microscale self-assemblies, and coating such assemblies; for example, in clustering and coating of cells for immunoisolated cell transplants.

Interfacial tension of oil/water emulsions with mixed non-ionic surfactants: comparison between experiments and molecular simulations

Smaller Span molecules occupy the free spaces between bulkier Tween molecules thus lowering interfacial tension as compared to those obtained for single surfactant systems.

Composite microdiscs with a magnetic belt: preparation, chaining properties, and use as switchable catalyst carriers

We describe an emulsion-based preparation of patchy composite particles (diameter of 100-500 μm) consisting of a disclike epoxy core and a belt of porous polystyrene particles (diameter of 30 μm) with magnetite within the pores. Compared to the magnetically uniform polystyrene particles, the spontaneous aggregation of composite particles is suppressed when dispersed into liquid, which is attributed to the increased particle size, reduced magnetic susceptibility, and the shape of the magnetic domain distribution within the particles (spherical versus a belt). When the composite particles are coated by platinum-palladium layer we demonstrate they can be employed as switchable catalyst carriers, moving from one liquid phase to another when controlled by an external magnetic field.

Diamagnetic repulsion of particles for multilaminar flow assays

A continuous multilaminar flow reaction was performed on functionalised polymer particlesviadiamagnetic repulsion forces, using a simple, inexpensive setup.

Microfluidic conformal coating of non-spherical magnetic particles

We present the conformal coating of non-spherical magnetic particles in a co-laminar flow microfluidic system. Whereas in the previous reports spherical particles had been coated with thin films that formed spheres around the particles; in this article, we show the coating of non-spherical particles with coating layers that are approximately uniform in thickness. The novelty of our work is that while liquid-liquid interfacial tension tends to minimize the surface area of interfaces-for example, to form spherical droplets that encapsulate spherical particles-in our experiments, the thin film that coats non-spherical particles has a non-minimal interfacial area. We first make bullet-shaped magnetic microparticles using a stop-flow lithography method that was previously demonstrated. We then suspend the bullet-shaped microparticles in an aqueous solution and flow the particle suspension with a co-flow of a non-aqueous mixture. A magnetic field gradient from a permanent magnet pulls the microparticles in the transverse direction to the fluid flow, until the particles reach the interface between the immiscible fluids. We observe that upon crossing the oil-water interface, the microparticles become coated by a thin film of the aqueous fluid. When we increase the two-fluid interfacial tension by reducing surfactant concentration, we observe that the particles become trapped at the interface, and we use this observation to extract an approximate magnetic susceptibility of the manufactured non-spherical microparticles. Finally, using fluorescence imaging, we confirm the uniformity of the thin film coating along the entire curved surface of the bullet-shaped particles. To the best of our knowledge, this is the first demonstration of conformal coating of non-spherical particles using microfluidics.

3D printed chip for electrochemical detection of influenza virus labeled with CdS quantum dots

In this study, we report a new three-dimensional (3D), bead-based microfluidic chip developed for rapid, sensitive and specific detection of influenza hemagglutinin. The principle of microfluidic chip is based on implementation of two-step procedure that includes isolation based on paramagnetic beads and electrochemical detection. As a platform for isolation process, streptavidin-modified MPs, which were conjugated via biotinylated glycan (through streptavidin-biotin affinity) followed by linkage of hemagglutinin to glycan, were used. Vaccine hemagglutinin (HA vaxi) was labeled with CdS quantum dots (QDs) at first. Detection of the isolation product by voltammetry was the end point of the procedure. The suggested and developed method can be used also for detection of other specific substances that are important for control, diagnosis or therapy of infectious diseases.

Analytical model for the deformation of a fluid-fluid interface beneath an AFM probe

We present an analytical solution for the shape of a fluid-fluid interface near a nanoscale solid sphere, which is a configuration motivated by common measurements with an atomic force microscope. The forces considered are surface tension, gravity, and the van der Waals attraction. The nonlinear governing equation has been solved previously using the method of matched asymptotic expansions, and this requires that the surface tension forces far exceed those of gravity, i.e., the Bond number is much less than one. We first present this method using a physically relevant scaling of the equations, then offer a new analytical solution valid for all Bond numbers. We show that one configuration with a large effective Bond number, and thus one requiring our new solution, is a nanothick liquid film spread over a solid substrate. The scaling implications of both analytical methods are considered, and both are compared with numerical solutions of the full equation.