Interfacial aspects of water drop formation at micro-engineered orifices

Capillary pressure-based measurement of dynamic interfacial tension in a spontaneous microfluidic sensor

The size of droplets and bubbles, and the properties of emulsions and foams strongly depend on dynamic interfacial tension (γ_d) - a parameter that is often inaccessible due to the very short time scales for droplet and bubble formation, and the inaccessibility of (e.g., food) production lines. To solve this challenge, we developed a microfluidic tensiometer that can measure γ_d by monitoring the formation time of both droplets and bubbles. Our tensiometer is a pressure-driven microfluidic device that operates based on the principle of a pressure balance: the formation of a droplet (or a bubble) is initialized when the Laplace pressure of the interface is decreased below the externally applied pressure, and this decrease is caused by a reduction in γ_d that can be calculated from the applied pressure and the Young-Laplace equation. The decay of γ_d due to surfactant adsorption can be followed at the characteristic time scale, which is dependent on surfactant type and concentration. For 0.05-1% wt sodium dodecyl sulfate (SDS), we were able to measure γ_d at time scales down to 1 ms and 0.1 ms for droplet and bubble interfaces, respectively, at increasing applied pressures and SDS concentrations. Our tensiometer proves to be a simple, robust method that inherently allows access to nearly the full range of dynamic interfacial tension at relevant time scales.

Effects of dynamic adsorption on bubble formation and coalescence in partitioned-EDGE devices

HYPOTHESIS

Dynamic adsorption effects can play a crucial role in bubble formation and stabilization. We hypothesize that microfluidic tools provide direct insights to these effects, and that the final bubble size depends on the intersection of time scales for bubble formation versus adsorption of proteins.

EXPERIMENTS

We use a microfluidic device to study Laplace pressure-driven formation of bubbles that are stabilized by whey proteins. Bubble behavior is studied as a function of the pressure difference imposed across the pores (P_d^∗), and thus the bubble formation time (τ, ranging from μs to s), using highspeed recordings, quasi-static pressure arguments and a semi-empirical coalescence model.

FINDINGS

We observe two distinct bubble formation regimes, delimited by the pressure difference required to initiate bubble formation in pure water, P_d^∗= 1400 mbar. When P_d^∗<1400 mbar, protein adsorption is a requisite to lower the surface tension and initialize bubble formation. Individual bubbles (fixed d₀~ 25 μm) are formed slowly with τ≫1 ms. When P_d^∗ exceeds 1400 mbar, bubbles (fixed d₀~ 16 μm) experience no adsorption lag and thus are formed at steeply increasing frequency, with τ < 1 ms. Interaction between these bubbles causes finite coalescence to a diameter d_coal that increases for lower τ. A minimum time of 0.4 ms is needed to immediately stabilize individual bubbles. Our study provides a promising microfluidic tool to study bubble formation and coalescence dynamics simultaneously.

Molecular dynamics simulations of hyperbranched poly(ethylene imine)-graphene oxide nanocomposites as dye adsorbents for water purification

Atomistically detailed molecular dynamics simulations were employed to study the adsorption capacity of graphene-oxide-based (GO) aqueous systems for the methylene blue (MB) dye in the presence of branched poly(ethylene imine) (BPEI) polymers. The polymeric component was either freely mixed or chemically attached to GO. The main focus was the elucidation of the effects originating from the presence of BPEI molecules in the association of MB with the formed GO complexes. The effect of temperature was also examined. It was found that the presence of the cationic BPEI molecules results in the formation of a distinct microenvironment characterized by a polymer-mediated interconnected morphology which promotes the development of larger-sized MB clusters. These clusters were found to form in the vicinity of the GO flakes, increasing thus the adsorption capacity of the dye molecules in the polymer-containing systems. Particularly in the system with the BPEI-functionalized GO flakes, a persistent percolated structure is formed, which results in a more restricted diffusion of the MB molecules, increasing thus significantly their residence time close to the GO surface. The clustering behavior of MB was found to be temperature-dependent in the BPEI-based models, providing useful information regarding the conditions for optimal adsorption performance of such membranes, in nanofiltration processes.

Mapping Bubble Formation and Coalescence in a Tubular Cross-Flow Membrane Foaming System

Membrane foaming is a promising alternative to conventional foaming methods to produce uniform bubbles. In this study, we provide a fundamental study of a cross-flow membrane foaming (CFMF) system to understand and control bubble formation for various process conditions and fluid properties. Observations with high spatial and temporal resolution allowed us to study bubble formation and bubble coalescence processes simultaneously. Bubble formation time and the snap-off bubble size (D0) were primarily controlled by the continuous phase flow rate (Qc); they decreased as Qc increased, from 1.64 to 0.13 ms and from 125 to 49 µm. Coalescence resulted in an increase in bubble size (Dcoal>D0), which can be strongly reduced by increasing either continuous phase viscosity or protein concentration-factors that only slightly influence D0. Particularly, in a 2.5 wt % whey protein system, coalescence could be suppressed with a coefficient of variation below 20%. The stabilizing effect is ascribed to the convective transport of proteins and the intersection of timescales (i.e., μs to ms) of bubble formation and protein adsorption. Our study provides insights into the membrane foaming process at relevant (micro-) length and time scales and paves the way for its further development and application.

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.

Linking Findings in Microfluidics to Membrane Emulsification Process Design: The Importance of Wettability and Component Interactions with Interfaces

In microfluidics and other microstructured devices, wettability changes, as a result of component interactions with the solid wall, can have dramatic effects. In emulsion separation and emulsification applications, the desired behavior can even be completely lost. Wettability changes also occur in one phase systems, but the effect is much more far-reaching when using two-phase systems. For microfluidic emulsification devices, this can be elegantly demonstrated and quantified for EDGE (Edge-base Droplet GEneration) devices that have a specific behavior that allows us to distinguish between surfactant and liquid interactions with the solid surface. Based on these findings, design rules can be defined for emulsification with any micro-structured emulsification device, such as direct and premix membrane emulsification. In general, it can be concluded that mostly surface interactions increase the contact angle toward 90°, either through the surfactant, or the oil that is used. This leads to poor process stability, and very limited pressure ranges at which small droplets can be made in microfluidic systems, and cross-flow membrane emulsification. In a limited number of cases, surface interactions can also lead to lower contact angles, thereby increasing the operational stability. This paper concludes with a guideline that can be used to come to the appropriate combination of membrane construction material (or any micro-structured device), surfactants and liquids, in combination with process conditions.

Structured microparticles with tailored properties produced by membrane emulsification

This paper provides an overview of membrane emulsification routes for fabrication of structured microparticles with tailored properties for specific applications. Direct (bottom-up) and premix (top-down) membrane emulsification processes are discussed including operational, formulation and membrane factors that control the droplet size and droplet generation regimes. A special emphasis was put on different methods of controlled shear generation on membrane surface, such as cross flow on the membrane surface, swirl flow, forward and backward flow pulsations in the continuous phase and membrane oscillations and rotations. Droplets produced by membrane emulsification can be used for synthesis of particles with versatile morphology (solid and hollow, matrix and core/shell, spherical and non-spherical, porous and coherent, composite and homogeneous), which can be surface functionalised and coated or loaded with macromolecules, nanoparticles, quantum dots, drugs, phase change materials and high molecular weight gases to achieve controlled/targeted drug release and impart special optical, chemical, electrical, acoustic, thermal and magnetic properties. The template emulsions including metal-in-oil, solid-in-oil-in-water, oil-in-oil, multilayer, and Pickering emulsions can be produced with high encapsulation efficiency of encapsulated materials and narrow size distribution and transformed into structured particles using a variety of solidification processes, such as polymerisation (suspension, mini-emulsion, interfacial and in-situ), ionic gelation, chemical crosslinking, melt solidification, internal phase separation, layer-by-layer electrostatic deposition, particle self-assembly, complex coacervation, spray drying, sol-gel processing, and molecular imprinting. Particles fabricated from droplets produced by membrane emulsification include nanoclusters, colloidosomes, carbon aerogel particles, nanoshells, polymeric (molecularly imprinted, hypercrosslinked, Janus and core/shell) particles, solder metal powders and inorganic particles. Membrane emulsification devices operate under constant temperature due to low shear rates on the membrane surface, which range from (1-10)×10(3) s(-1) in a direct process to (1-10)×10(4) s(-1) in a premix process.

Fabrication of Microcapsules for Dye-Doped Polymer-Dispersed Liquid Crystal-Based Smart Windows

A dye-doped polymer-dispersed liquid crystal (PDLC) is an attractive material for application in smart windows. Smart windows using a PDLC can be operated simply and have a high contrast ratio compared to those of other devices that employed photochromic or thermochromic material. However, in conventional dye-doped PDLC methods, dye contamination can cause problems and has a limited degree of commercialization of electric smart windows. Here, we report on an approach to resolve dye-related problems by encapsulating the dye in monodispersed capsules. By encapsulation, a fabricated dye-doped PDLC had a contrast ratio of >120 at 600 nm. This fabrication method of encapsulating the dye in a core-shell structured microcapsule in a dye-doped PDLC device provides a practical platform for dye-doped PDLC-based smart windows.

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

This study extends our previous work on droplet generation in microfluidic T-junction generators to include dynamic interfacial tension effects created by the presence of surfactants. In Paper I [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016322 (2012)], we presented experimental findings regarding the formation process in the squeezing-to-transition regime, and in Paper II [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016323 (2012)] we developed a theoretical model that describes the performance of T-junction generators without surfactants. Here we study dynamic interfacial tension effects for two surfactants, one with a small molecular weight that adsorbs quickly, and the other with a large molecular weight that adsorbs slowly. Using the force balance developed in Paper II we extract the dynamic interfacial tension from high speed videos obtained during experiments. We then develop a theoretical model to predict the dynamic interfacial tension in microfluidic T-junction generators as a function of the surfactant properties, flow conditions, and generator design. This model is then incorporated into the overall model for generator performance to effectively predict the size of droplets produced when surfactants are present.

On-demand generation of monodisperse femtolitre droplets by shape-induced shear

We describe a method for creating discrete femtolitre-scale water-in-oil droplets on demand, based solely on a geometrically induced reduction in oil/water interfacial area at microfabricated junction orifices. This on-demand generation method is driven by self-shear of droplets due to interfacial tension induced forces resulting from a localized transition in microchannel height. The magnitudes of shear stresses involved appear to be significantly less than the shearing instabilities used to split off daughter droplets from aqueous mother plugs at microfabricated junctions in continuous water-in-oil segmented flows, which implies that this method may be better suited for studying biochemical reactions and reaction kinetics in droplets of decreased volume without loss of chemical reactivity due to redistribution of surfactant density used to passivate the oil/water interface. Predictable droplet generation rates under constant pressure conditions or the gated formation of one, two or more droplets at a time with fixed pressure pulses have been demonstrated in a similar manner to active on-demand droplet generation strategies, but with a simpler system not needing actuation and sensing equipment beyond a pressure regulator.

Microfluidic device for robust generation of two-component liquid-in-air slugs with individually controlled composition

Using liquid slugs as microreactors and microvessels enable precise control over the conditions of their contents on short-time scales for a wide variety of applications. Particularly for screening applications, there is a need for control of slug parameters such as size and composition. We describe a new microfluidic approach for creating slugs in air, each comprising a size and composition that can be selected individually for each slug. Two-component slugs are formed by first metering the desired volume of each reagent, merging the two volumes into an end-to-end slug, and propelling the slug to induce mixing. Volume control is achieved by a novel mechanism: two closed chambers on the chip are initially filled with air, and a valve in each is briefly opened to admit one of the reagents. The pressure of each reagent can be individually selected and determines the amount of air compression, and thus the amount of liquid that is admitted into each chamber. We describe the theory of operation, characterize the slug generation chip, and demonstrate the creation of slugs of different compositions. The use of microvalves in this approach enables robust operation with different liquids, and also enables one to work with extremely small samples, even down to a few slug volumes. The latter is important for applications involving precious reagents such as optimizing the reaction conditions for radiolabeling biological molecules as tracers for positron emission tomography. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10404-010-0617-0) contains supplementary material, which is available to authorized users.

Determination of dynamic interfacial tension and its effect on droplet formation in the T-shaped microdispersion process

Interfacial tension is an important physical property affecting the droplet formation process in microfluidic devices. This work presents the variation of dynamic interfacial tension caused by slow adsorption of surfactant, as well as its influence on the liquid/liquid microdispersion process in a T-shaped microchannel. Using hexane/water-Tween 20 as the working system, it was observed that the droplet size changed with the variation of surfactant concentration when the concentration of Tween 20 was lower than 10 mmol/L, but hardly changed at higher concentrations, which was caused by the unsaturated adsorption and saturated adsorption of surfactant, respectively. The saturated interfacial tension was measured with an interfacial tension meter, and the relationship between the interfacial tension and the droplet diameter was established. Accordingly, the dynamic interfacial tension with unsaturated adsorption of surfactant was determined. The main factors affecting the dynamic interfacial tension were discussed, and a semiempirical equation was established to characterize those effects.

Methods for the preparation and manufacture of polymeric nanoparticles

This review summarizes the different methods of preparation of polymer nanoparticles including nanospheres and nanocapsules. The first part summarizes the basic principle of each method of nanoparticle preparation. It presents the most recent innovations and progresses obtained over the last decade and which were not included in previous reviews on the subject. Strategies for the obtaining of nanoparticles with controlled in vivo fate are described in the second part of the review. A paragraph summarizing scaling up of nanoparticle production and presenting corresponding pilot set-up is considered in the third part of the review. Treatments of nanoparticles, applied after the synthesis, are described in the next part including purification, sterilization, lyophilization and concentration. Finally, methods to obtain labelled nanoparticles for in vitro and in vivo investigations are described in the last part of this review.