Balance Of Interfacial Tensions Research Articles

Spontaneous Particle Ordering, Sorting, and Assembly on Soap Films.

Soap bubbles exhibit abundant fascinating phenomena throughout the entire life of evolution with different fundamental physics governing them. Nevertheless, the complicated dynamics of small objects in soap films are still unrevealed. Here, we report the first observation of spontaneous particle ordering in a complicated galaxy of soap films without any external energy. The balance of interfacial tension at two liquid-gas interfaces is theoretically predicted to govern belted wetted particles (BWPs) traveling along a specified path spontaneously. Such spontaneous particle path-finding is found to depend on the particle size and hydrophilic properties. Spontaneous particle sorting is directly realized via these discrete and distinctive paths for different particles. The deformation of the soap membrane facilitates 1D/2D particle organization along the path. This observation represents the discovery of a new spontaneous order phenomenon in soap film systems and provides a new energy-free approach for particle separation and soft colloidal crystal assembly.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Facile Monitoring of Water Hardness Levels Using Responsive Complex Emulsions.

The cationic content of water represents a major quality control parameter that needs to be followed by a rapid, on-site, and low-cost method. Herein, we report a novel method for a facile monitoring of the mineral content of drinking water by making use of responsive complex emulsions. The morphology of biphasic oil-in-water droplets solely depends on the balance of interfacial tensions, and we demonstrate that changes in the surfactant effectiveness, caused by variations in the mineral content inside the continuous phase, can be visualized by monitoring internal droplet shapes. An addition of metal cations can significantly influence the surfactant critical micelle concentrations and the surface excess values and therefore induce changes in the effectiveness of ionic surfactants, such as sodium dodecyl sulfate. The morphological response of Janus emulsions droplets was tracked via a simple microscopic setup. We observed that the extent of the droplet response was dependent on the salt concentration and valency, with divalent cations (responsive for water hardness), resulting in a more pronounced response. In this way, Ca2+ and Mg2+ levels could be quantitatively measured, which we showcased by determination of the mineral content of commercial water samples. The herein demonstrated device concept may provide a new alternative rapid monitoring of water hardness levels in a simple and cost-effective setup.

Responsive Janus and Cerberus emulsions via temperature-induced phase separation in aqueous polymer mixtures

Complex aqueous emulsions represent a promising material platform for the encapsulation of cells, pharmaceuticals, or nutrients, for the fabrication of structured particles, as well as for mimicking the barrier-free compartmentalization of biomolecules found in living cells. Herein, we report a novel, simple, and scalable method of creating multicomponent aqueous droplets with highly uniform internal droplet morphologies that can be controllably altered after emulsification by making use of a thermal phase separation approach. Specifically, temperature-induced phase separation inside as-formed emulsion droplets comprising aqueous mixtures of two or more hydrophilic polymers allows for the generation of Janus and Cerberus emulsion droplets with adjustable internal morphologies that are solely controlled by a balance of interfacial tensions. We demonstrate our approach by applying both, microfluidic and scalable batch production, and present a detailed model study with predictive capabilities that enables fine-tuning and dynamically altering the droplet morphology as a function of types, molecular weights, and hydrophilicities of the polymers as well as the surfactant hydrophilic-lipophilic balance. The ability to rationally design complex aqueous emulsion droplets with previously unattainable dynamic control over their morphologies after emulsification entails the potential to design new responsive soft materials with implications for a variety of applications beyond encapsulation, including the design of complex adaptive and self-regulating materials, e.g. for chemical and biological sensing applications.

Bubble Meets Droplet: Particle-Assisted Reconfiguration of Wetting Morphologies in Colloidal Multiphase Systems.

Wetting phenomena are ubiquitous in nature and play key functions in various industrial processes and products. When a gas bubble encounters an oil droplet in an aqueous medium, it can experience either partial wetting or complete engulfment by the oil. Each of these morphologies can have practical benefits, and controlling the morphology is desirable for applications ranging from particle synthesis to oil recovery and gas flotation. It is known that the wetting of two fluids within a fluid medium depends on the balance of interfacial tensions and can thus be modified with surfactant additives. It is reported that colloidal particles, too, can be used to promote both wetting and dewetting in multifluid systems. This study demonstrates the surfactant-free tuning and dynamic reconfiguration of bubble-droplet morphologies with the help of cellulosic particles. It further shows that the effect can be attributed to particle adsorption at the fluid interfaces, which can be probed by interfacial tensiometry, making particle-induced transitions in the wetting morphology predictable. Finally, particle adsorption at different rates to air-water and oil-water interfaces can even lead to slow, reentrant wetting behavior not familiar from particle-free systems.

Theoretical and experimental studies of drop size in membrane emulsification – Single pore studies of hydrodynamic detachment of droplets

Abstract Single pore set-up with the drop detachment process observed by high speed CCD camera is a convenient system to study basic processes governing formation of emulsions by cross-flow membrane emulsification. We studied dependence of the droplet size on the volumetric flows of continuous and dispersed phase in square glass cell (10 mm × 10 mm), where the droplets were formed at glass capillary (inner diameter ID = 0.075 mm) or steel needle (ID = 0.3 mm) for selected surfactants: anionic – sodium dodecyl sulphate (SDS), cationic – dodecyl trimethyl ammonium bromide (DDTAB), non-ionic – Tween 80 and oil soluble anionic – docusate sodium (AOT). As oil phases dodecane and cyclohexane were used. We found that the simplified model based on the balance of interfacial tension, buoyancy and hydrodynamic forces can correctly reproduce trends in the dependence of the droplet size on continuous and dispersed phase flow intensity and dynamic interfacial tension. However some effects not accounted for by the model, such as unexpected bubble shape deformation, were also observed and are discussed in the paper.

Quasi-Immiscible Spreading of Aqueous Surfactant Solutions on Entangled Aqueous Polymer Solution Subphases

Motivated by the possibility of enhancing aerosol drug delivery to mucus-obstructed lungs, the spreading of a drop of aqueous surfactant solution on a physically entangled aqueous poly(acrylamide) solution subphase that mimics lung airway surface liquid was investigated. Sodium dodecyl sulfate was used as the surfactant. To visualize spreading of the drop and mimic the inclusion of a drug substance, fluorescein, a hydrophilic and non-surface-active dye, was added to the surfactant solution. The spreading progresses through a series of events. Marangoni stresses initiate the convective spreading of the drop. Simultaneously, surfactant escapes across the drop's contact line within a second of deposition and causes a change in subphase surface tension outside the drop on the order of 1 mN/m. Convective spreading of the drop ends within 2-3 s of drop deposition, when a new interfacial tension balance is achieved. Surfactant escape depletes the drop of surfactant, and the residual drop takes the form of a static lens of nonzero contact angle. On longer time scales, the surfactant dissolves into the subphase. The lens formed by the water in the deposited drop persists for as long as 3 min after the convective spreading process ends due to the long diffusional time scales associated with the underlying entangled polymer solution. The persistence of the lens suggests that the drop phase behaves as if it were immiscible with the subphase during this time period. Whereas surfactant escapes the spreading drop and advances on the subphase/vapor interface, hydrophilic dye molecules in the drop do not escape but remain with the drop throughout the convective spreading. The quasi-immiscible nature of the spreading event suggests that the chemical properties of the surfactant and subphase are much less important than their physical properties, consistent with prior qualitative studies of spreading of different types of surfactants on entangled polymer subphases: the selection of surfactant for pulmonary delivery applications may be limited only by physical and toxicological considerations. Further, the escape of surfactant from individual drops may provide an additional spreading mechanism in the lung, as hydrodynamic and/or surface pressure repulsions may drive individual droplets apart after deposition.

Universal Deformation of Soft Substrates Near a Contact Line and the Direct Measurement of Solid Surface Stresses

Droplets deform soft substrates near their contact lines. Using confocal microscopy, we measure the deformation of silicone gel substrates due to glycerol and fluorinated-oil droplets for a range of droplet radii and substrate thicknesses. For all droplets, the substrate deformation takes a universal shape close to the contact line that depends on liquid composition, but is independent of droplet size and substrate thickness. This shape is determined by a balance of interfacial tensions at the contact line and provides a novel method for direct determination of the surface stresses of soft substrates. Moreover, we measure the change in contact angle with droplet radius and show that Young's law fails for small droplets when their radii approach an elastocapillary length scale. For larger droplets the macroscopic contact angle is constant, consistent with Young's law.

Behavior of thermal bubbles formed from a single nucleation site

Thermal bubble formation is a fundamental process in nucleate boiling heat transfer and in many microelectromechanical thermal systems. Here, we report an experimental study of the dynamic and thermal behavior of bubbles generated at a single site, that is, a microcavity filled with alumina particles. The thermal process associated with the bubble departure from the isolated cavity, in particular, was shown to be different from that of macroscale boiling. The bubble departure diameter remains constant in a low superheat (or Jakob number) regime which is solely determined by the balance of interfacial tension and buoyancy. In addition, the bubble departure frequency increases along the bubble size as the substrate temperature rises. The further-increased frequency of bubbles emerging from the cavity causes multiple bubbles to coalesce before the preceding bubble completely detaches from the substrate, thus, leading to the decrease of apparent departure frequency with the increase of substrate temperature.

Grain Growth Simulations Including Particle Pinning Using the Multiphase-field Concept

In this paper, the effect of particle pinning on grain boundary motion is investigated by phase-field modeling. In general, the kinetics of grain growth in multicrystalline materials is determined by the interplay of curvature driven grain boundary motion and the balance of interfacial tension at the vertices of a grain boundary network. A comprehensive way to treat both effects in one model is given by the phase-field approach. The specific feature of the multiphase-field model used for this investigation is its ability to treat each grain or phase boundary with its individual characteristics, together with a thermodynamic coupling which allows a sound treatment of phase transformation, e.g. the formation of precipitates of a second phase.The pinning effect itself is simulated on the nanometer scale resolving the interaction of individual inert or reactive precipitates with a curved grain boundary. From these simulations an effective pinning force is deduced, and a model for a driving force dependent grain boundary mobility is formulated accounting for the pinning effect in the grain growth simulation on the mesoscopic scale. These simulations demonstrate how particle pinning leads to much slower growth kinetics and a different grain morphology with higher boundary curvatures in the stationary state. Finally, an increase of the pinning force due to a changing particle density, e.g. during heat treatment, is shown to result in a transition between normal and abnormal grain growth before grain coarsening is inhibited completely.

Spreading Dynamics of Surfactant Solutions

The spreading behavior of microdrops of surfactant solutions at solid surfaces has been studied. The influence on the spreading of different factors, such as the drop lifetime prior to surface contact, surface tension dynamics, surface energy, and surfactant properties, was systematically investigated. The results obtained suggest the existence of two spreading regimes exhibiting different spreading characteristics: In the first, nondiffusive regime, the spreading is very rapid and controlled to different extents by inertia, gravity, and capillarity, depending on the drop size, impact energy, and interfacial tension balance. It is shown in this study that the initial drop surface tension, which is set by the surface tension decay rate and the drop lifetime prior to the surface impact, strongly influences the maximum spreading distance in the nondiffusive spreading regime. The second, diffusion-controlled regime, is characterized by slower concentration-dependent spreading rates. The spreading rate is, here, mainly controlled by the diffusive transport of surfactant to the expanding liquid−vapor interface. In this regime, the drop base radius exhibits an approximate rb2 ∝ t dependence on time. The spreading kinetics at hydrophobic surfaces has been discussed in the framework of a simplified theory. Depending on the assumptions regarding the drop shape, rb5/2 ∝ t to rb2 ∝ t spreading laws are obtained. This agrees reasonably with the experimentally observed relationship.

Bitumen spreading and formation of thin bitumen films at a water surface

Bitumen spreading at gas bubble surfaces was observed through a stereoscopic microscope for Whiterocks oil sand samples submerged in alkaline solutions. This phenomenon was also observed for model systems where air bubbles were placed at the surface of a bitumen-coated quartz slide. Finally, the film pressure for bitumen films spreading at the aqueous phase surface was measured as a function of pH with the Wihelmy plate technique. It was found that a balance of interfacial tensions in all these systems favors spreading of the bitumen at the gas bubble surface because of a positive spreading coefficient. Filming occurs by the initial advancement of a thin film, the “bitumen film precursor”. This bitumen film precursor reduces the surface tension of the aqueous phase, and as a consequence, affects the driving force (spreading coefficient) for spreading. At room temperature, the gas bubble surface is covered by the bitumen film precursor during the first few seconds of contact between the phases. Subsequently, in a matter of minutes the bubble is enveloped by the bitumen film. Although thermodynamic conditions favor envelopment of the gas bubble by bitumen, several discontinuities in the thickness of the thin films (bitumen film precursors) were observed with a microscope. For oil sand systems, fine mineral particles with hydrophobic properties further complicate the spreading of bitumen over the gas bubble surface and promote the formation of bitumen islands.

Development of Experimental Phase Equilibria For Miscible Hydrocarbon Enhanced Oil Recovery

Abstract A major factor in the success of a miscible hydrocarbon enhanced oil recovery process is the phase behaviour relationship between the injection and reservoir fluids. This paper describes the experimental procedures developed at Gulf Canada R&D to generate phase equilibrium data for specific reservoirs. The data have been applied in evaluating the potential for enhanced recovery in a number of Western Canadian carbonate reservoirs. The over-all procedure for a complete phase equilibrium study is described. This comprises field sampling, reservoir fluid characterization, injection fluid preparation, development of phase envelopes and correlation of phase behaviour. An integral part of the program involved the development Of a combination of analytical and high-pressure techniques to facilitate the rapid accumulation of vapour-liquid equilibrium data for reservoir-injection fluid systems. The Peng-Robinson equation of state and a new K value correlation were used successfully to rationalize the measured data. Experimental phase behaviour measurements in conjunction with appropriate correlation methods have proved valuable in tailoring the design of injection fluids to meet both miscibility requirements and economic constraints. Introduction Recent reports(1,2) have indicated that the use of enhanced recovery methods can increase Canada's available conventional oil reserves by approximately 2 to 3 billion barrels. Up to 95% of the additional oil with an API gravity greater than 25º could feasibly, and economically, be recovered using currently available miscible displacement techniques involving injection into the reservoir of either CO2 or hydrocarbon fluids. Realizing this potential, Gulf Canada Resources Inc. have been actively pursuing a program of initiating limited full-scale enhanced recovery projects and implementing pilot-scale floods(3) at various locations in Western Canada. To date, because of the types of reservoirs considered primarily carbonate and the local availability of suitable injection fluid components, attention has centred on the process of hydrocarbon miscible displacement. In support of these projects, the Gulf Canada R&D Department has developed an extensive experimental program to provide data both for preliminary screening of candidate reservoir and injection fluids and for detailed design of optimum injection fluids in specific reservoirs. This paper describes the experimental program that has evolved during application to such projects over the past five years. Miscible Displacement The residual oil saturation, after primary and secondary production of a reservoir, is a function of the formation geometry and the interfacial tensions of the fluids occupying the formation. The purpose of the injection fluid in enhanced recovery is to access the residual oil, and, by modifying the balance of interfacial tensions, to mobilize the residual oil to production. In miscible displacement processes, as the name implies, this purpose is achieved when miscibility is attained between the displacing fluid (injection fluid) and the residual oil, thereby eliminating the interfacial tension between the two. The miscibility of an injection fluid and reservoir oil is governed by the phase behaviour which is unique to each specific combination. The relationship between phase behaviour, miscibility and composition at reservoir temperature and pressure is illustrated on the pseudo-ternary Point R is the reservoir fluid composition and X represents the critical composition or plait point.