Two-liquid Systems Research Articles

Droplet impact on liquid films: Bouncing-to-merging transitions for two-liquid systems

The dynamics of a liquid droplet impacting a liquid film of different compositions is critical for many industrial processes, including additive manufacturing and bio-printing. In this work we present an exposition of droplet impact on liquid films investigating the effects of mismatch in their properties on bouncing-to-merging transitions. Experiments are conducted for two sets of liquid combinations, namely, alkanes and silicon oils. The regime maps for impact outcomes (bouncing vs merging) are created from detailed experiments with various single- and two-liquid systems. The results highlight that the two-liquid systems exhibit an additional merging regime, which is not observed for single-liquid systems. Subsequently, the scaling analyses for transitional boundaries between various regimes are revisited, and new scaling laws are proposed to include the effects of asymmetry in the droplet and film properties. Finally, the experimental results are used to assess the performance of the proposed scaling laws.

Effects of surface wettability on contact line motion in liquid–liquid displacement

The behaviors of oil–water–solid interfaces are of basic and applied importance for many fields, such as energy, geochemistry, and biomedical sciences, yet molecular mechanisms are still poorly known. In particular, the traditional adsorption–desorption mechanism based on a single compound does not work to describe the contact line for liquid–liquid–solid systems where the two liquids have similar interactions with the solid surface. Here, we present a series of experiments of water spreading in alkanes, i.e., n-decane, n-dodecane, and n-hexadecane, under various surface wettabilities. The contact line motion is captured by the high-speed camera, allowing us to analyze the friction coefficient, displacement distance and frequency, and activation free energy during the contact line motion via the molecular kinetic theory. We further develop a friction model of contact line motion in liquid–liquid–solid systems by establishing the activation free energy of liquid–liquid–solid systems as a function of the activation free energies of separate one-liquid systems. Results show that the contact line friction coefficients increase with the increasing hydrophobicity (e.g., for n-decane, the coefficient changes from 0.3072 to 2.1967 Pa·s), explained by the increase in the displacement distance and sharp decrease in the displacement frequency of the molecules. Additionally, we find a non-monotonic dependence of the contribution of the surface part to the activation free energy on the wettability, with a maximum percentage of 70%–80% at the intermediate wettability, where the reduced collisions of liquid molecules weaken the molecular adsorption/desorption process. Finally, we find that the correlation coefficient n in our model is affected by the surface wettability, with a maximum at the intermediate wettability, showing a good prediction of the results in the literature. The reason can be attributed to the non-monotonic effects of wettability on the surface part of the activation free energy in two-liquid systems.

A generalized examination of capillary force balance at contact line: On rough surfaces or in two-liquid systems

We investigate the capillary force balance at the contact line on rough solid surfaces and in two-liquid systems. Our results confirm that solid-liquid interactions perpendicular to the interface have a significant influence on the lateral component of the capillary force exerted on the contact line. Surface roughness of the solid substrate reduces the mobility of liquid and alters how the perpendicular solid-liquid interactions transfer into a force acting parallel to the interface. A quantitative relation between surface roughness and the transfer strategy is proposed. Moreover, when a liquid is in coexistence with another immiscible liquid on a solid, the capillary forces exerted on liquids of both sides are involved in our theoretical model. The contact angle can be predicted by calculating three interfacial tensions. These arguments are then verified by molecular dynamics simulations. Our findings set up the generalized theoretical framework for the capillary force balance at the contact line and broaden its application in more realistic scenarios.

Prediction and measurement of the fastest-growing mode in two-liquid systems

The paper is concerned with the capillary instability of a liquid thread surrounded by another immiscible liquid. We study the occurrence of the dominant mode of perturbation for different combinations of Newtonian and viscoelastic fluids. When a viscoelastic fluid is present, in the limit of small deformations, the dispersion relation is obtained via a Maxwell type approximation of the viscosity curve which can be directly incorporated into an existing model of thread instability. Measurements of the fastest-growing mode are then performed and compared with theoretical predictions. A satisfactory agreement between the theory and experiment is found in terms of the wavenumber of the fastest-growing mode.

Wetting dynamics in two-liquid systems: Effect of the surrounding phase viscosity.

This paper reports the experimental results of a water droplet spreading on a glass substrate submerged in an oil phase. The radius of the wetted area grows exponentially over time forming two distinct regimes. The early time dynamics of wetting is characterized with the time exponent of 1, referred to as the viscous regime, which is ultimately transitioned to the Tanner's regime with the time exponent of 0.1. It is revealed that an increase in the ambient phase viscosity over three decades considerably slows down the rate of three-phase contact line movement. A scaling law is developed where the three-phase contact line velocity is a function of both spreading radius and mean viscosity, close to the geometric mean of the droplet and ambient fluids' viscosities. Using the proposed scaling and mean viscosity, all plots of spreading radius for different viscosity ratios collapse to a master curve. Furthermore, several cases with multiple rupture and spreading points, i.e., wetting in a nonideal system, are considered. The growth of an equivalent wetting radius in a multiple point spreading system is predicted by the developed scaling law.

Two-fluid wetting behavior of a hydrophobic silicon nanowire array.

The two-fluid wetting behavior of surfaces textured by an array of silicon nanowires is investigated systematically. The Si nanowire array is produced by a combination of colloidal patterning and metal-catalyzed etching, with control over its roughness depending upon the wire length. The nanowires are made hydrophobic and oleophobic by treatment with hydrocarbon and fluorinated self-assembled monolayers, respectively. Static, advancing, and receding contact angles are measured with water, hexadecane, and perfluorotripentylamine in both single-fluid (droplet on solid in an air environment) and two-fluid (droplet on solid in a liquid environment) configurations. The single-fluid measurements show wetting behavior similar to that expected by the Wenzel and Cassie-Baxter models, where the wetting or non-wetting behaviors are amplified with increasing roughness. The two-fluid systems on the rough surface exhibit more complex configurations because either the droplet or the environment fluid can penetrate the asperities depending upon the wettability of each fluid. It is observed that, when the Young contact angles are significantly increased or reduced from single-liquid to two-liquid systems, the effect of roughness is relatively minimal. However, when the Young contact angles are similar, roughness has almost identical influence on apparent contact angles in single- and two-liquid systems. The Wenzel and Cassie-Baxter models are modified to describe various two-fluid wetting states. In cases where metastable behavior is observed for the droplet, advancing and receding measurements are performed to suggest the equilibrium state of the droplet.

Two-Dimensional Materials as Emulsion Stabilizers: Interfacial Thermodynamics and Molecular Barrier Properties

A new application for two-dimensional (2D) materials is emulsification, where they can serve as ultrathin platelike interfacial stabilizers in two-liquid systems. We present a first detailed thermodynamic analysis of atomically thin 2D materials at organic-aqueous liquid-liquid interfaces and derive expressions for the transfer free energies of emulsion stabilization that account for material geometry, van der Waals transparency or opacity, and variable hydrophobicity. High mass potency is shown to be an intrinsic property of the 2D geometry, which at the atomically thin limit places every atom in contact with both liquid phases, resulting in unit atom efficiency. The thermodynamic model successfully predicts that graphene oxide but not pristine graphene has a favorable hydrophobic-hydrophilic balance for oil-water emulsion stabilization. Multilayer tiling is predicted to occur by the passivation of droplet surface patches left uncovered by packing inefficiencies in the first monolayer, and complete multilayer coverage is confirmed by cryogenic scanning electron microscopy. The molecular barrier function of graphene interfacial films causes a significant suppression of dispersed-phase evaporation rates with potential applications in controlled release. Finally, these emulsions can be used as templates for creating solid graphene foams or graphene microsacks filled with lipophilic cargos. Emerging 2D materials are promising as dispersants or emulsifiers where high mass potency and multifunctional properties are desired.

Steroid biotransformations in biphasic systems with Yarrowia lipolytica expressing human liver cytochrome P450 genes

BackgroundYarrowia lipolytica efficiently metabolizes and assimilates hydrophobic compounds such as n-alkanes and fatty acids. Efficient substrate uptake is enabled by naturally secreted emulsifiers and a modified cell surface hydrophobicity and protrusions formed by this yeast. We were examining the potential of recombinant Y. lipolytica as a biocatalyst for the oxidation of hardly soluble hydrophobic steroids. Furthermore, two-liquid biphasic culture systems were evaluated to increase substrate availability. While cells, together with water soluble nutrients, are maintained in the aqueous phase, substrates and most of the products are contained in a second water-immiscible organic solvent phase.ResultsFor the first time we have co-expressed the human cytochromes P450 2D6 and 3A4 genes in Y. lipolytica together with human cytochrome P450 reductase (hCPR) or Y. lipolytica cytochrome P450 reductase (YlCPR). These whole-cell biocatalysts were used for the conversion of poorly soluble steroids in biphasic systems.Employing a biphasic system with the organic solvent and Y. lipolytica carbon source ethyl oleate for the whole-cell bioconversion of progesterone, the initial specific hydroxylation rate in a 1.5 L stirred tank bioreactor was further increased 2-fold. Furthermore, the product formation was significantly prolonged as compared to the aqueous system.Co-expression of the human CPR gene led to a 4-10-fold higher specific activity, compared to the co-overexpression of the native Y. lipolytica CPR gene. Multicopy transformants showed a 50-70-fold increase of activity as compared to single copy strains.ConclusionsAlkane-assimilating yeast Y. lipolytica, coupled with the described expression strategies, demonstrated its high potential for biotransformations of hydrophobic substrates in two-liquid biphasic systems. Especially organic solvents which can be efficiently taken up and/or metabolized by the cell might enable more efficient bioconversion as compared to aqueous systems and even enable simple, continuous or at least high yield long time processes.

Predicting the Wetting Dynamics of a Two-Liquid System

We propose a new theoretical model of dynamic wetting for systems comprising two immiscible liquids, in which one liquid displaces another from the surface of a solid. Such systems are important in many industrial processes and the natural world. The new model is an extension of the molecular-kinetic theory of wetting and offers a way to predict the dynamics of a two-liquid system from the individual wetting dynamics of its parent liquids. We also present the results of large-scale molecular dynamics simulations for one- and two-liquid systems and show them to be in good agreement with the new model. Finally, we show that the new model is consistent with the limited data currently available from experiment.

Using a biphasic ionic liquid/water reaction system to improve oxygenase-catalysed biotransformation with whole cells

Whole cells are usually used for oxygenase-catalysed biotransformations to ensure efficient cofactor recycling and to avoid problems with purification and stability of these complex, multi-subunit enzymes. Productivity in oxygenase-catalysed biotransformations is frequently restricted by toxicity of the substrates to the cells, but attempting to solve this problem using two-liquid phase reaction systems with conventional solvents provides only modest improvements in productivity. Therefore, we developed new, two phase systems using water-immiscible ionic liquids. Even though [NMeOct3][NTf2] and [P6,6,6,14][NTf2], inhibited growth of Escherichia coli by 39% and 23% respectively, both ionic liquids could be used to improve toluene dioxygenase-catalysed conversion of toluene to toluenecis-glycol using recombinant E. colicells. The two-liquid phase reaction system improved resistance to toluene toxicity by 8-fold, and product concentrations increased by 2.5-fold in shake flask cultures. Product concentrations and specific product yields were improved by 200% and 238% respectively in bioreactors with an unrestricted oxygen supply.

Enhancement of Biodesulfurization in Two-Liquid Systems by Heterogeneous Expression of Vitreoscilla Hemoglobin

The vgb gene, encoding Vitreoscilla hemoglobin (VHb), was introduced into a specific desulfurization bacterium, Rhodococcus erythropolis LSSE8-1. The VHb-specific spectrum was observed for the recombinant. Compared to the wild type, the strain bearing vgb showed a higher biomass yield and desulfurizing activity.

Multiple effects of water on solvent-free enzymatic esterifications

Enhancement of the catalytic activity of lipases has commonly been performed by biocatalyst modification (immobilization on hydrophobic supports, activatiFon with interfaces, pretreatment with amphiphiles, etc.). In this work, the enhancement of the reaction rates obtained in lipase-catalyzed esterifications is achieved through modification of the number of liquid phases present in the medium in which the reaction takes place. The liquid medium may be constituted by either just one liquid phase or two liquid phases. The split of the liquid phase can be controlled by manipulating the water initially added to the system. As a result of the study of the role of water on the solvent-free enzymatic esterification of oleic acid with ethanol, the addition of relatively high amounts of water showed an unexpected beneficial effect: ester yields increased in systems with high amounts of water initially added. The addition of high amounts of water to the mixture of substrates led to the generation of two-liquid phases systems, in which the fatty acid and the produced ester remained in the organic phase, the added and reaction-generated water migrated to the aqueous phase, and the ethanol partitioned between both phases. The existence of a second aqueous phase from the beginning of the reaction favored the extraction of the water generated in the reaction, with an important reduction of the water content in the organic reactive phase. As a consequence, the increase in the global water content of the reaction medium not only did not favor hydrolysis, but also increased the fatty acid conversion in the first hours of reaction. Together with the lipase hydration and equilibrium shift, effects commonly considered in the literature of lipase-catalyzed esterifications, this manuscript emphasizes another effect of water related to the formation of a two-liquid phases system. Experimental and modeling data from reactions catalyzed with up to 15 biocatalysts (native and immobilized lipases) are presented to analyze the role of water on the rate of solvent-free enzymatic esterifications.

Modeling of droplet formation in stratified immiscible liquid-liquid flows

This study concerns prediction of the extent and location of mixing due to turbulence in flow of stratified, essentially immiscible two-liquid systems (such as that of diesel fuel and water) using computational fluid dynamics (CFD). The theoretical basis and calibration procedure for an empirical droplet formation (DF) model is presented. Then this new DF model is implemented into two different commercial CFD codes, namely CFX-4, developed by AEA Technology and FLOW-3D, developed by Flow Sciences, Inc.®, by way of a drift-flux model. The results of validation studies are presented in comparison to relevant experimental data. The validated model is then applied to various cases to study the mechanisms of mixing. Both steady and unsteady two-dimensional, as well as three-dimensional simulations are considered. The results are assessed with regards to their implications of what constitutes real mixing. Comparative results from two different codes using similar mathematical models are also presented.

Chrysotile as a support for the immobilisation of Mycobacterium sp. NRRL B-3805 cells for the bioconversion of β-sitosterol in an organic–aqueous two-liquid phase system

The use of chrysotile, a plentiful and low-cost inorganic material, as an effective immobilisation matrix for mycobacterial cells was evaluated. The side-chain cleavage of β-sitosterol to 4-androstane-3,17-dione (AD), in an organic–aqueous two-liquid phase system, was chosen as model system. Bis(2-ethylhexyl) phthalate (BEHP), a biocompatible organic solvent, was used as organic phase. The biotransformation was carried out with either growing or previously grown cells. Mycobacterial cells were shown to adsorb onto chrysotile in both aqueous buffered system and in organic–aqueous two-liquid phase systems. Cells tend to aggregate among the chrysotile fibres during and after growth. The formed chrysotile/cells complex retained catalytic activity under bioconversion conditions for 1 week, at least. No mass transfer hindrances were observed when the immobilised form of the biocatalyst was used, as compared to the free form. However, among the several approaches to produce the most efficient immobilised biocatalyst, the use of previously grown cells in contact with chrysotile led to the highest product yields as well as highest bioconversion rates. The feasibility of this approach was further evidenced since it allowed the implementation of a continuous mode of operation. A constant product yield of 50% was observed for a 1-week period with this experimental set-up. This work clearly highlights the ability of chrysotile to provide an efficient immobilisation matrix for whole cells to be used as biocatalysts in organic–aqueous two-liquid phase bioconversion systems.

Protein adsorption at liquid/liquid interface with low interfacial tension

Adsorption of proteins at the interface of two-liquid systems composed of aqueous ammonium sulfate solution and tert-butanol by phase separation was investigated by drop shape tensiometry. The change of interfacial tension with time and protein concentration as well as upon compression of the adsorbed layer were compared for bovine serum albumin, ovalbumin, β-lactoglobulin, lysozyme, trypsin and horse radish peroxidase. A correlation between the dilatational moduli of various protein films and the partitioning of proteins in the two-liquid system was found, which provides evidence to the role of emulsion stability in protein separation by three-phase partitioning.

Liquid-Liquid Solubilities for the Binary Systems of Methanol with Octane and 2,2,4-Trimethylpentane.

A laser light scattering technique was used for the determination of the mutual solubilities in two-liquid systems at moderate pressures. The solubility data and the upper critical solution temperatures are reported for the binary systems of methanol with octane and 2, 2, 4-trimethylpentane. The experimentally determined cloud points were satisfactorily correlated by the NRTL equation and Tsuboka–Katayama’s modification of the Wilson equation.

Integrated two-liquid phase bioconversion and product-recovery processes for the oxidation of alkanes: process design and economic evaluation

Pseudomonas oleovorans and recombinant strains containing the alkane oxidation genes can produce alkane oxidation products in two-liquid phase bioreactor systems. In these bioprocesses the cells, which grow in the aqueous phase, oxidize apolar, non-water soluble substrates. The apolar products typically accumulate in the emulsified apolar phase. We have studied both the bioconversion systems and several downstream processing systems to separate and purify alkanols from these two-liquid phase media. Based on the information generated in these studies, we have now designed bioconversion and downstream processing systems for the production of 1-alkanols from n-alkanes on a 10 kiloton/yr scale, taking the conversion of n-octane to 1-octanol as a model system. Here, we describe overall designs of fed-batch and continuous-fermentation processes for the oxidation of octane to 1-octanol by Pseudomonas oleovorans, and we discuss the economics of these processes. In both systems the two-liquid phase system consists of an apolar phase with hexadecene as the apolar carrier solvent into which n-octane is dissolved, while the cells are present in the aqueous phase. In one system, multiple-batch fermentations are followed by continuous processing of the product from the separated apolar phase. The second system is based on alkane oxidation by continuously growing cultures, again followed by continuous processing of the product. Fewer fermentors were required and a higher space-time-yield was possible for production of 1-octanol in a continuous process. The overall performance of each of these two systems has been modeled with Aspen software. Investment and operating costs were estimated with input from equipment manufacturers and bulk-material suppliers. Based on this study, the production cost of 1-octanol is about 7 US$kg-1 when produced in the fed-batch process, and 8 US$kg-1 when produced continuously. The comparison of upstream and downstream capital costs and production costs showed significantly higher upstream costs for the fed-batch process and slightly higher upstream costs for continuous fermentation. The largest cost contribution was due to variable production costs, mainly resulting from media costs. The organisms used in these systems are P. putida alk+ recombinants which oxidize alkanes, but cannot oxidize the resulting alkanols further. Hence, such cells need a second carbon source, which in these systems is glucose. Although the continuous process is about 10% more expensive than the fed-batch process, improvements to reduce overall cost can be achieved more easily for continuous than for fed-batch fermentation by decreasing the dilution rate while maintaining near constant productivity. Improvements relevant to both processes can be achieved by increasing the biocatalyst performance, which results in improved overall efficiency, decreased capital investment, and hence, decreased production cost. Copyright 1999 John Wiley & Sons, Inc.

Development of equipment and procedures for the safe operation of aerobic bacterial bioprocesses in the presence of bulk amounts of flammable organic solvents

Many pseudomonads and other bacteria can grow on aliphatic and aromatic hydrocarbons that occur in the environment. We are examining the potential of such organisms as biocatalysts for the oxidation of a variety of substituted aliphatic and aromatic compounds. In order to attain a high production rate of oxidation products via such biotransformations, we have focused on two-liquid phase culture systems. In these systems cells are grown in liquid media consisting of an aqueous phase containing water soluble growth substrates and droplets of a water immiscible organic solvent containing bioconversion substrates and products.

Developments toward large-scale bacterial bioprocesses in the presence of bulk amounts of organic solvents.

Many pseudomonads and other bacteria can grow on aliphatic and aromatic hydrocarbons that occur in the environment. We are examining the potential of such organisms as biocatalysts for the oxidation of a variety of substituted aliphatic and aromatic compounds. To attain a high production rate of oxidation products via such biotransformations, we have focused on two-liquid phase culture systems. In these systems, cells are grown in liquid media consisting of an aqueous phase containing water-soluble growth substrates and droplets of a water-immicible organic solvent containing bioconversion substrates and products. For industrial applications of such two-liquid phase processes, several questions remain. What are the maximum rates at which apolar compounds can be transferred from the apolar phase to cells growing in the aqueous phase, i.e., what are the maximum space-time yields attainable in two-liquid phase fermentations under practical conditions? What does an efficient downstream processing of two-liquid phase medium involve? What safety regimes should be considered in working with flammable organic solvents? Can elevated pressure be used to increase oxygen transfer? Based on answers to these questions, we have recently developed a high-pressure, explosion-proof bioreactor system with Bioengineering AG (Wald, Switzerland), which will be installed in our pilot plant and used to explore two-liquid phase bioconversions at a pilot scale.

SINGLE-SOURCE GAMMA RADIATION PROCEDURES FOR IMPROVED CALIBRATION AND MEASUREMENTS IN POROUS MEDIA

When dual-energy gamma radiation systems are employed for measurements in porous media, count rates from both sources are often used to compute parameter values. However, for several applications, the count rates of just one source are sufficient. These applications include the determination of volumetric liquid content values in two-liquid systems and salt concentration values in water-saturated porous media. Single-energy gamma radiation procedures for three applications are described in this paper. Through an error analysis, single-source procedures are shown to reduce the probable error in the determinations considerably. Example calculations and simple column experiments were conducted for each application to compare the performance of the new single-source and standard dual-source methods. In all cases, the single-source methods provided more reliable data than the traditional dual-source methods. In addition, a single-source calibration procedure is proposed to determine incident count rates indirectly. This procedure, which requires packing under saturated conditions, can be used in all single- and dual-source applications and yields accurate porosity and dry bulk density values.