Microfluidic Droplet-Based Tool To Determine Phase Behavior of a Fluid System with High Composition Resolution.

The goal of this work is to develop a simple microfluidic approach to characterizing liquid-liquid phase behavior in complex aqueous mixtures of organics and salts. We take advantage of the permeability of inexpensive microfluidic devices to concentrate aqueous solutions on chip. We demonstrate a technique that allows phase boundaries to be identified with high compositional resolution and small sample volumes. Droplets of single phase samples are produced on-chip and concentrated in the device beyond the phase boundary line to map system phase behavior. Results are demonstrated on ammonium sulfate and organic (poly(ethylene oxide)) aqueous solutions and compared with macroscopic and literature results.

Formation and elasticity of membranes of the class II hydrophobin Cerato-ulmin at oil-water interfaces.

Protein surfactants show great potential to stabilize foams, bubbles, and emulsions. An important family of surface active proteins, the hydrophobins, is secreted by filamentous fungi. Two hydrophobin classes have been recognized, with Class II exhibiting slightly better solubility than Class I, although neither is very soluble in water. Hydrophobins are small proteins (8-14 kDa), but they are larger and more rigid than typical surfactants such as sodium dodecyl sulfate. This rigidity seems to be manifested in the strength of adsorbed hydrophobin layers on oil droplets or air bubbles. A particular Class II hydrophobin, Cerato-ulmin, was characterized at the oil-water interface (the oil was squalane). The results are compared to measurements at the air-water interface, newly extended to lower Cerato-ulmin concentrations. For both oil-water and air-water interfaces, static and dynamic properties were measured during the evolution of the membrane structure. The static measurements reveal that dilute Cerato-ulmin solution efficiently decreases the interfacial tension, whether at oil-water or air-water interfaces. The reduction in surface tension requires several hours. Interfacial mechanics were characterized too, and the dilatational modulus was found to reach large values at both types of interfaces: 339 ±â€¯19 mN/m at the squalane-water interface and at least 764 ±â€¯45 mN/m at the air-water interface. Both values well exceed those typical of small-molecule surfactants, but come closer to those expected of particulate-loaded interfaces. Circular dichroism provides some insight to adsorption-induced molecular rearrangements, which seem to be more prevalent at the oil-water interface than at the air-water interface.

Effect of surfactant tail length and ionic strength on the interfacial properties of nanoparticle-surfactant complexes.

Mixed nanoparticle-surfactant systems are effective foam stabilizing agents, but the lack of colloidal stability of the bulk dispersions makes interfacial characterization challenging. This study investigates the adsorption of CnTAB/SiO2 complexes at air/water interfaces through surface tension and interfacial rheology measurements. The effects of surfactant tail length, ionic strength, and interfacial processing on the surface properties are measured utilizing a bulk reservoir exchange methodology to avoid bulk destabilization. The surfactant structure controls the surface tension of the system, but has minimal impact on particle surface coverage or interfacial mechanics. Once adsorbed, nanoparticles remain pinned at the surface, while the surfactant is able to desorb upon bulk exchange with deionized water. Particle packing on the interface governs the interfacial mechanics, which can be modified by increasing the ionic strength of the bulk solution. Fully rigid interfaces can be generated at low particle coverages by controlling the ionic strength and interfacial processing. These findings contribute to the understanding of mixed particle-surfactant systems and inform formulation and process design to achieve the desired interfacial mechanical properties.

Droplet-based characterization of surfactant efficacy in colloidal stabilization of carbon black in nonpolar solvents.

Development of an electrostatic stabilization mechanism for colloidal suspensions in nonpolar fluids requires an improved understanding of the interactions between the inverse micelles and particles as well as the roles that steric and electrostatic effects play. A droplet-based millifluidic device is designed and used to investigate the stabilization effects of surfactants on colloidal suspensions. A system containing carbon black and the surfactant OLOA 11000 suspended in dodecane is chosen as a well-characterized system to study sedimentation quantitatively. This device takes advantage of sub-millimeter optical path lengths to characterize sedimentation at concentrations at which sedimentation is not observable in the bulk and to achieve higher resolution in composition. A simple image analysis algorithm has been developed and applied to quantify sedimentation. Conductivity measurements using electrochemical impedance spectroscopy (EIS) are coupled with the sedimentation experiments to identify the concentration ranges in which steric and electrostatic effects are dominant. A more gradual transition is observed than previously reported.

Insoluble layer deposition and dilatational rheology at a microscale spherical cap interface.

The dilatational properties of insoluble monolayers are important for understanding the mechanics of biological systems and consumer products, but isolating the dilatational response of an interface is challenging due to the difficulties in separating dilatation from shear and other deformation modes. Oscillations of a microscale bubble radius are useful for generating purely dilatational flows, but the current deposition methods for insoluble layers onto fluid interfaces are not easily scaled down. In this paper, we describe a miscible solvent exchange procedure for generating insoluble layers at an air-water interface pinned at the tip of a capillary tens of micrometers in diameter. We show that the amount of surfactant adsorbed at the interface can be controlled by the initial concentration dissolved in isopropanol (the starting solvent) and the volumetric flow rate of solvent exchange. Surface pressure-area isotherms and dilatational moduli are measured concurrently for three insoluble surfactants: palmitic acid (PA), dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). The isotherms measured on the microscale interface compare well with previous experiments performed on a Langmuir trough. However, the magnitudes of the dilatational moduli differ from those measured on either Langmuir trough or pendant drop apparatuses. Several possible reasons for the observed differences are discussed. A comparison of the dilatational modulus with the Gibbs elasticity is used to determine the presence of dilatational extra stresses at the interface. The isotherm and dilatational modulus of the insoluble component of the industrial surfactant Tween 80 are measured using this approach. The methods developed here also open the possibility for future study of the important role of finite size effects on microstructure formation and the resulting interfacial mechanics.

Formation of a Rigid Hydrophobin Film and Disruption by an Anionic Surfactant at an Air/Water Interface.

Hydrophobins are amphiphilic proteins produced by fungi. Cerato-ulmin (CU) is a hydrophobin that has been associated with Dutch elm disease. Like other hydrophobins, CU stabilizes air bubbles and oil droplets through the formation of a persistent protein film at the interface. The behavior of hydrophobins at surfaces has raised interest in their potential applications, including use in surface coatings, food foams, and emulsions and as dispersants. The practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, alone and in the presence of added surfactants. In this study, the adsorption behavior of CU at air/water interfaces is characterized by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to adsorb irreversibly at air/water interfaces. The magnitude of the dilatational modulus increases with adsorption time and surface pressure until CU eventually forms a rigid film. The persistence of this film is tested through the sequential addition of strong surfactant sodium dodecyl sulfate (SDS) to the bulk liquid adjacent to the interface. SDS is found to coadsorb to interfaces precoated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU by SDS. Sequential adsorption results in mixed layers with properties not observed in interfaces generated from complexes formed in the bulk. These results lend insight to the complex interfacial interactions between hydrophobins and surfactants.

Stability of a compound sessile drop at the axisymmetric configuration.

The equilibrium configuration of compound sessile drops has been calculated previously in the absence of gravity. Using the Laplace equations, we establish seven dimensionless parameters describing the axisymmetric configuration in the presence of gravity. The equilibrium axisymmetric configuration can be either stable or unstable depending on the fluid properties. A stability criterion is established by calculating forces on a perturbed Laplacian shape. In the zero Bond number limit, the stability criterion depends on the density ratio, two ratios of interfacial tensions, the volume ratio of the two drops, and the contact angle. We use Surface Evolver to examine the stability of compound sessile drops at small and large Bond numbers and compare with the zero Bond number approximation. Experimentally, we realize a stable axisymmetric compound sessile drop in air, where the buoyancy force exerted by the air is negligible. Finally, using a pair of fluids in which the density ratio can be tuned nearly independently of the interfacial tensions, the stability transition is verified for the axisymmetric configuration. Even though the perturbations are different for the theory, simulations and experiments, both simulations and experiments agree closely with the zero Bond number approximation, exhibiting a small discrepancy at large Bond number.

Sequential adsorption of an irreversibly adsorbed nonionic surfactant and an anionic surfactant at an oil/aqueous interface.

Aerosol-OT (AOT) and Tween 80 are two of the main surfactants in commercial dispersants used in response to oil spills. Understanding how multicomponent surfactant systems interact at oil/aqueous interfaces is crucial for improving both dispersant design and application efficacy. This is true of many multicomponent formulations; a lack of understanding of competition for the oil/water interface hinders formulation optimization. In this study, we have characterized the sequential adsorption behavior of AOT on squalane/aqueous interfaces that have been precoated with Tween 80. A microtensiometer is used to measure the dynamic interfacial tension of the system. Tween 80 either partially or completely irreversibly adsorbs to squalane/aqueous interfaces when rinsed with deionized water. These Tween 80 coated interfaces are then exposed to AOT. AOT adsorption increases with AOT concentration for all Tween 80 coverages, and the resulting steady-state interfacial tension values are interpreted using a Langmuir isotherm model. In the presence of 0.5 M NaCl, AOT adsorption significantly increases due to counterion charge screening of the negatively charged head groups. The presence of Tween 80 on the interface inhibits AOT adsorption, reducing the maximum surface coverage as compared to a clean interface. Tween 80 persists on the interface even after exposure to high concentrations of AOT.

Controlling thread formation during tipstreaming through an active feedback control loop.

Microscale tipstreaming is a hydrodynamic phenomenon capable of producing submicron sized droplets within a microfluidic device. The tipstreaming process results in the drawing of a thin thread from a highly curved interface and occurs as a result of interfacial surfactant concentration gradients that develop due to elongational flows generated within flow focusing geometries. However, in conventional microfluidic devices, the thread formation is periodically interrupted by the formation of larger primary droplets. This study presents an active feedback control loop capable of eliminating the production of primary droplets and producing a continuous thread, and therefore a continuous droplet stream. A proportional controller is used to successfully control the position of the interface and generate a continuous thread. A derivative component is incorporated in an attempt to increase controller stability, but this component is found to be ineffective. Analysis of the tip position as a function of time is performed to determine the optimal proportional gain constant and set point value for the proportional controller that minimize fluctuations in the produced droplet sizes. The generation of a continuous thread facilitates the use of tipstreaming in several applications, including nanoparticle synthesis, chemical detection, and enzyme activity studies.

Tuning bubbly structures in microchannels.

Foams have many useful applications that arise from the structure and size distribution of the bubbles within them. Microfluidics allows for the rapid formation of uniform bubbles, where bubble size and volume fraction are functions of the input gas pressure, liquid flow rate, and device geometry. After formation, the microchannel confines the bubbles and determines the resulting foam structure. Bubbly structures can vary from a single row ("dripping"), to multiple rows ("alternating"), to densely packed bubbles ("bamboo" and dry foams). We show that each configuration arises in a distinct region of the operating space defined by bubble volume and volume fraction. We describe the boundaries between these regions using geometric arguments and show that the boundaries are functions of the channel aspect ratio. We compare these geometric arguments with foam structures observed in experiments using flow-focusing, T-junction, and co-flow designs to generate stable nitrogen bubbles in aqueous surfactant solution and stable droplets in oil containing dissolved surfactant. The outcome of this work is a set of design parameters that can be used to achieve desired foam structures as a function of device geometry and experimental control parameters.

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces.

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.

Formation and ordering of topological defect arrays produced by dilatational strain and shear flow in smectic-A liquid crystals.

A microscale shear cell is used to study the formation of parabolic focal conic defects in the thermotropic smectic-A liquid crystal 8CB (4-octyl-4'-cyanobiphenyl). Defects are produced by four distinct methods: by the application of dilatational strain alone, by shear flow alone, by dilatational strain and subsequent shear flow, and by the simultaneous application of dilatational strain and shear flow. We confirm that defects originate within the bulk, consistent with the previously suggested undulation instability mechanism. In the presence of a shear flow, we observe that defect formation requires micrometer-level dilatations, whose magnitude depends on the sample thickness. The size and ordering of both disordered and ordered defect arrays is quantified using a pair distribution function. Deviations from the predictions of linear stability theory are observed that have not been reported previously. For example, defects form a square array with greater ordering in the principal flow direction. Ordering due to shear flow does not change the average defect size. It has been shown previously that the principal defect sizes of ordered defects scale differently with sample thickness than the wavelength of the small amplitude undulations. We find that disordered defects show a similar deviation from this predicted wavelength.

Using bulk convection in a microtensiometer to approach kinetic-limited surfactant dynamics at fluid-fluid interfaces.

The impact of transport of surfactants to fluid-fluid interfaces is complex to assess and model, as many processes are in the regime where kinetics, diffusion and convection are comparable. Using the principle that the timescale for diffusion decreases with increasing curvature, we previously developed a microtensiometer to accurately measure fundamental transport coefficients via dynamic surface tension at spherical microscale liquid-fluid interfaces. In the present study, we use a low Reynolds number flow in the bulk solution to further increase the rate of diffusion. Dynamic surface tension is measured as a function of Peclet number and the results are compared with a simplified convection-diffusion model. Although a transition from diffusion to kinetic-limited transport is not observed experimentally for the surfactants considered, lower bounds on the adsorption and desorption rate constants are determined that are much larger than previously reported rate constants. The results show that the details of the flow field do not need to be controlled as long as the local Reynolds number is low. Aside from other pragmatic advantages, this experimental tool and analysis allows the governing mechanisms of surfactant transport at liquid-fluid interfaces to be quantified using flow near the interface to decrease the length scale for diffusion, separating the relevant timescales.

The effect of alkane tail length of CiE8 surfactants on transport to the silicone oil-water interface.

Detailed surfactant transport studies have typically been restricted to the air-water interface. This is mainly due to the lack of experimental devices and techniques available to study liquid-liquid interfaces. As a result, there is a lack of relevant data and understanding of surfactant behavior in microfluidic studies and emulsion applications. Using a novel shape fitting algorithm for a pendant drop capable of handling fluids of similar densities, i.e. low Bond numbers, we measure the dynamic surface tension as a function of bulk surfactant concentration at the silicone oil-water interface for a homologous series of C(i)E(8) nonionic surfactants. We show that the isotherms governing equilibrium at the oil-water and air-water interfaces are very different. Using a scaling analysis comparing two governing mass transport timescales, we demonstrate that there exists a transition from diffusion-limited to kinetic-limited dynamics at the silicone oil-water interface. Adsorption rate constants are determined from a one parameter nonlinear fit to dynamic surface tension data. These results demonstrate that the dynamics of interfacial transport are highly dependent on the immiscible fluids that form the interface.

Diffusion-limited adsorption to a spherical geometry: the impact of curvature and competitive time scales.

Although the time scale governing diffusion-limited transport of soluble species from solution onto a planar interface is well understood, the time scale governing transport onto a spherical interface is not. The time scales that have been proposed in the literature for spherical interfaces do not capture the correct asymptotic behavior for increasing bubble radius and do not capture previously reported experimental observations of the effect of concentration. This paper develops a diffusion-limited time scale that is dependent on an intrinsic length scale termed the spherical depletion depth. The time scale is determined by considering a specific example of diffusion-limited transport of surfactant species to a water-air interface and is verified using numerical simulations and experiments. This newly derived diffusion time scale will have a significant impact on our understanding of fundamental phenomena at spherical fluid-fluid and fluid-solid interfaces, especially those involving micrometer and nanometer length scales.

A microtensiometer to probe the effect of radius of curvature on surfactant transport to a spherical interface.

Diffusion of surfactant to a spherical interface depends on the radius of curvature of the interface; the smaller the radius of curvature is, the faster the dynamics. This paper presents and validates an experimental apparatus, denoted a "microtensiometer", to study the dependence of surfactant dynamics on radius of curvature. Dynamic surface tension is monitored for a range of bubble radii from 17 to 150 microm, and the dynamics are compared with those obtained using the classic pendant drop experiment for a nonionic surfactant at the air-water interface. Experiments reveal that dynamic surface tension follows a diffusion-limited scaling, in which radius of curvature is a key parameter. Despite the clear scaling behavior of the experimental equilibration time, the full dynamic curve for an initially clean interface cannot be predicted by a diffusion-limited transport model using the molecular diffusion coefficient and a single isotherm. However, the same model is shown to correctly predict compression-expansion experiments. Aside from elucidation of surfactant transport, this device provides a tool for rapid measurements of interfacial properties using a significantly lower volume of sample than current methods.

A non-gradient based algorithm for the determination of surface tension from a pendant drop: application to low Bond number drop shapes.

The pendant drop method is one of the most widely used techniques to measure the surface tension between gas-liquid and liquid-liquid interfaces. The method consists of fitting the Young-Laplace equation to the digitized shape of a drop suspended from the end of a capillary tube. The first use of digital computers to solve this problem utilized nonlinear least squares fitting and since then numerous subroutines and algorithms have been reported for improving efficiency and accuracy. However, current algorithms which rely on gradient based methods have difficulty converging for almost spherical drop shapes (i.e. low Bond numbers). We present a non-gradient based algorithm based on the Nelder-Mead simplex method to solve the least squares problem. The main advantage of using a non-gradient based fitting routine is that it is robust against poor initial guesses and works for almost spherical bubble shapes. We have tested the algorithm against theoretical and experimental drop shapes to demonstrate both the efficiency and the accuracy of the fitting routine for a wide range of Bond numbers. Our study shows that this algorithm allows for surface tension measurements corresponding to Bond numbers previously shown to be ill suited for pendant drop measurements.

Experimental observations of the squeezing-to-dripping transition in T-shaped microfluidic junctions.

An experimental study of droplet breakup in T-shaped microfluidic junctions is presented in which the capillary number and flow rate ratio are varied over a wide range for several different viscosity ratios and several different ratios of the inlet channel widths. The range of conditions corresponds to the region in which both the squeezing pressure that arises when the emerging interface obstructs the channel and the viscous shear stress on the emerging interface strongly influence the process. In this regime, the droplet volume depends on the capillary number, the flow rate ratio, and the ratio of inlet channel widths, which controls the degree of confinement of the droplets. The viscosity ratio influences the droplet volume only when the viscosities are similar. When there is a large viscosity contrast in which the dispersed-phase liquid is at least 50 times smaller than the continuous-phase liquid, the resulting size is independent of the viscosity ratio and no transition to a purely squeezing regime appears. In this case, both the droplet volume and the droplet production frequency obey power-law behavior with the capillary number, consistent with expectations based on mass conservation of the dispersed-phase liquid. Finally, scaling arguments are presented that result in predicted droplet volumes that depend on the capillary number, flow rate ratio, and width ratio in a qualitatively similar way to that observed in experiments.

Role of surface anchoring and geometric confinement on focal conic textures in smectic-A liquid crystals.

A high surface area-to-volume ratio in microchannels increases the importance of surface interactions within them. In layered liquids, such as smectic liquid crystals, surface interactions play an important role in the formation of defect textures. We use 8CB liquid crystal, which is in the smectic-A phase at room temperature, as a model layered liquid. PDMS surfaces can be tuned to be hydrophilic or hydrophobic, and due to the nature of liquid crystalline molecules, we show that this results in planar or homeotropic anchoring conditions, respectively. In a confined system, contrary to the bulk, generated defects cannot grow freely. In the present work, we show that the confinement offered by PDMS microchannels along with the capability of creating mixed anchoring conditions within them results in the formation of particular ordered defect textures through increased surface interactions in smectic-A liquid crystals. Our observations imply that microscale confinement is useful for controlling the size, size distribution, and packing structure of microscale defect structures within these materials. In addition, we show that by placing a droplet of smectic-A liquid crystal on a PDMS surface containing microscale parallel cracks, ordered focal conic defects form between two adjacent cracks. The distance between two adjacent cracks dictates the size of the defects. These observations could lead to useful ideas for exploring new technologies for flexible optical devices or displays that utilize smectic-A liquid crystals.