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• The CA relaxation profile was used to determine the advancing CA of sessile drop. • Sessile drop of surfactant solution was formed from an equilibrated pendant drop. • Two advancing contact angles were detected for TX-100 and C 12 E 6 solution drops. • Only one advancing CA was detected for the sessile drop of SDS solution. • Pre-equilibration of the drop before deposition affected the CA relaxation profile. This study investigated the uniqueness of advancing contact angle (CA) by examining the evaporation process of a sessile drop of surfactant solution [sodium dodecyl sulfate (SDS), hexaethylene glycol monododecyl ether (C 12 E 6) and Triton X-100 (TX-100)] on parafilm. The sessile drop was formed from a pendant drop that had reached its equilibrium surface tension in an atmosphere with ∼100% humidity. The relaxation of the drop profile and wetting diameter were recorded from the moment the drop touched the parafilm, and the data were then analyzed to determine the advancing CA. The results showed that for SDS solution drops, only one advancing CA was observed at each SDS concentration. However, the experimental data of TX-100 and C 12 E 6 solution drops indicated the presence of two distinct regions that showed a near-constant CA with a quasi-equilibrium advance of the triple line; hence matching the definition of an advancing CA. The possibility of the existence of two advancing CA was further explored and the presence of two advancing CA for the sessile drops of TX-100 and C 12 E 6 solution was verified by closely examining the advance of the triple line. [ABSTRACT FROM AUTHOR]

Highlights • The formation of protein films can be followed by interfacial shear measurements. • The properties of the film formed strongly depends on the pH value. • Surface charges and protein interactions determine rheological properties. • The Wagner-I model can properly describe the behaviour of O/W interfaces. • The model can reproduce interfacial linear, non-linear viscoelastic and flow results. Abstract The objective of this work is the evaluation of chickpea protein adsorption at oil/water (O/W) interface as a function of protein concentration and pH value (2.5, 5.0 and 7.5). To assess molecular interpretation, interfacial tension is determined as a function of concentration with a Wilhelmy plate, whereas interfacial small amplitude oscillatory shear (i-SAOS) properties are determined using a double wall-ring (DWR) geometry controlled by a DHR3 rheometer (TA Instruments) and a pendant drop tensiometer (IT Concept) is used to determine linear viscoelastic dilatational measurements. This work provides a model which could predict both the linear and non-linear viscoelastic behavior of complex fluid-fluid interfacial layers. To this end, relaxation tests using the DWR device are carried out at the interface under the linear and non-linear regimes. Steady state viscosity values are also obtained to check the ability of the model to predict the interfacial flow behaviour. Results show that the Wagner-I model can reproduce fairly well the steady state flow behaviour of chickpea protein-adsorbed interfaces. This model is based on the use of a memory function calculated from the generalized Maxwell (obtained from i-SAOS measurements) and a damping function obtained by the Laun model from linear and non-linear relaxation tests. [ABSTRACT FROM AUTHOR]

Highlights • The accuracy of the Oldroyd-B model is examined experimentally. • The model drastically overestimates the damping rate of an oscillating drop. • Possible causes of this failure are discussed. Abstract We study both theoretically and experimentally the forced and free oscillations of a viscoelastic pendant drop in the linear regime. The Jeffreys model is numerically solved using the zero-shear viscosity of the steady shear flow in a rotational rheometer, and the stress relaxation time measured with a filament stretching elongational rheometer. The damping factor characterizing the first oscillation mode is one order of magnitude larger than that measured in the experiments. Three possible conclusions can be drawn from this study: (i) the Jeffreys model severely underestimates the viscosity decrease due to elasticity; (ii) the zero-shear viscosity measured in the rotational rheometer is much larger than the effective viscosity responsible for energy dissipation in the pendant droplet oscillations; and (iii) the relaxation time measured with the filament stretching elongational rheometer is much smaller than that characterizing those oscillations. The failure of the Jeffreys model necessarily entails that of all nonlinear constitutive relationships which the Jeffreys model can be derived from, like the commonly used Oldroyd-B model. [ABSTRACT FROM AUTHOR]

When a viscous liquid empties from an initially full, wide vertical tube, the drainage behaviour changes from a filament to a regime in which individual drops are shed by a lens formed at the end of the tube: liquid drains down the wall and the lens grows until it becomes unstable. This drop shedding regime was investigated for four Newtonian liquids (rapeseed oil, glycerol, honey and golden syrup) in three tube sizes and two tube materials (Bond number based on tube i.d. >1 in all cases). The drop mass increased modestly with flow rate and the equivalent sphere diameter, d , was strongly related to the capillary length L c  ≡ ( γ / ρg ) 1/2 rather than the tube diameter. The results were fitted to a correlation of the form d/L c  =  f (Bond, Reynolds, Morton, sine of the contact angle) derived from dimensional analysis. The data were compared with existing models for drop formation from filled narrow capillaries and a new, simple model based on a quasi-static model of the lens. Agreement with these models was poor, particularly for larger tubes, indicating the need for more detailed analysis. Insights into the dynamics, generated by video analysis of the lens shape, are presented. [ABSTRACT FROM AUTHOR]

A significant amount of water used during hydraulic fracturing operation in shale reservoirs remains trapped in the formation mainly due to capillary forces. In this paper, the effect of Alpha Foamer ® surfactant in enhancing the efficiency of the fracturing fluid was investigated in the presence and absence of Betain C60 as a co-surfactant at various fracturing water salinities. The performance of Alpha Foamer ® surfactant was examined by measuring the surface tension and contact angle between the surfactant solutions and methane gas at different pressures and temperatures. The result indicates that Alpha Foamer ® has significantly reduced the surface tension between water and methane gas from 69 mN/m to 33.2 mN/m at critical micelle concentration of 0.25 wt%. The addition of Betain C60 as a co-surfactant has slightly reduced the surface tension to 26.6 mN/m. It has also been observed that the increase in the pressure and temperature would be an advantage with additional reduction in the surface tension. In addition, the selected surfactant and co-surfactant concentrations have shown a significant reduction in the contact angle that makes the shale formation strongly water wet. The shifting of the wetting phase due to receding contact angle would aid in desorption of methane gas from shale matrix. The desorption is further enhanced with the reduction of surface tension between fracking fluid and methane gas by surfactant. The findings from this research can be used to optimize fracturing fluid, and to improve the overall sweep efficiency and shale gas productivity as well as water flowback. [ABSTRACT FROM AUTHOR]

• The dynamic light scattering method was applied to characterize the diffusion of nanoparticles in nanofluids in form of a pendant drop and a levitated droplet. • The particle diffusivities in a pendant drop of a silica nanofluid can be determined by the method in combination with a heterodyne detection scheme. During the evaporation of the pendant drop, crust formation on the surface of the drop and its time of appearance can be identified by the method. The particle diffusivities in a pendant drop decrease with increasing evaporation time, which can be attributed to a combined influence of the concentration dependence of particle diffusivity and particle aggregation or agglomeration. • It was demonstrated that the dynamics of the particle number density fluctuations in a levitated droplet of a silica nanofluid can be characterized by the dynamic light scattering method. • In both cases, the evolution of the characteristic time reflecting the diffusive process of nanoparticles can be determined until crust formation. The diffusion of nanoparticles in nanofluids in form of pendant drops and levitated droplets is characterized by dynamic light scattering. In combination with a heterodyne detection scheme, particle diffusivity in pendant drops can be determined by this method. The diffusivities therein correspond well to the values measured inside a cuvette. During evaporation, crust formation in the drop can also be identified by the method. Dynamic light scattering experiments for levitated droplets are performed with an ultrasonic acoustic levitator. Again, the dynamics of the particle number density fluctuations inside the droplet can be characterized, but the derivation of particle diffusivities is not yet possible. In both cases, the evolution of the characteristic time reflecting the diffusive process of nanoparticles can be determined until the point of crust formation. In summary, the applicability and limitation of dynamic light scattering for the characterization of diffusion of particles in evaporating drops are identified. [ABSTRACT FROM AUTHOR]

The purpose of this communication is to draw attention to a puzzling inconsistency in the published data for the interfacial tension between methane and water at high-pressure and high-temperature conditions. The published data sets fall into two categories – those exhibiting linear dependence of interfacial tension on temperature, in agreement with the phenomenological Eötvös rule, and others exhibiting a peculiar and seemingly reproducible non-linear trend. The inconsistency seems to have gone largely unnoticed. That is somewhat disconcerting considering that the methane/water system is a commonly used reference system for evaluation of fluid interfacial tension models. The literature data are reviewed, discussed, and compared with a novel set of interfacial tension data for the methane/water system obtained in this study. [ABSTRACT FROM AUTHOR]

Interfacial dynamics of a pendant drop suspended from a needle support was studied via a new experimental method, without external excitation, to accurately quantify the impact of size and surface properties on the natural frequency. For drops ranging between 3.0 and 40.2 mm 3 , the natural frequency of vibration varied proportionally to the volume raised to the exponent −3/4, V −3/4 . A direct relationship between the frequency and the water drops was obtained f(Hz) ∝ S − 3/4 t c −1 , in which S is the dimensionless size parameter. Previous works performed employing external excitation suggested a direct impact of the supporting size, which might be explained by the acting force. The current experimental results do not follow this prediction, indicating insignificant influence of the support base on the recorded frequency. The results indicate a significant role of surface tension, which is taken into account by size factor S , on the vibration of the pendant drop. [ABSTRACT FROM AUTHOR]

Drop-shape analysis using pendant or sessile drops is a well-established experimental technique for measuring the interfacial or surface tension, and changes thereof. The method relies on deforming a drop by either gravity or buoyancy and fitting the Young–Laplace equation to the drop shape. Alternatively one can prescribe the shape and measure the pressure inside the drop or bubble using pressure tensiometry. However, when an interface with a complex microstructure is present, extra and anisotropic interfacial stresses may develop due to lateral interactions between the surface-active moieties, leading to deviations of the drop shape or even a wrinkling of the interface. To extract surface-material properties of these complex interfaces using drop-shape analysis or pressure tensiometry, the Young–Laplace law needs to be generalized in order to account for the extra and anisotropic stresses at the interface. In the present work, we review the different approaches that have been proposed to date to extract the surface tension as the thermodynamic state variable, as well as other rheological material properties such as the compression and the shear modulus. To evaluate the intrinsic performance of the methods, computer generated drops are subjected to step-area changes and then subjected to analysis using the different methods. Shape-fitting methods, now combined with an adequate constitutive method, do however perform rather poorly in determining the elastic stresses, especially at small area strains. An additional measurement o f the pressure or capillary-pressure tensiometry is required to improve the sensitivity. However, pressure-based methods still require the knowledge of the undeformed reference state, which may be difficult to achieve in practice. Moreover, it is not straightforward to judge from what point onwards one needs to go beyond the Young–Laplace equation. To overcome these limitations, a method based on stress fitting, which uses a local force balance method, is introduced here. One aspect of this new method is the use of the Chebyshev transform to numerically describe the contour shape of the drop interface. For all methods we present a detailed error analysis to evaluate if, and with what precision, surface material parameters can be extracted. Depending on the desired information, different ideal experimental conditions and most suitable methods are discussed, in addition to having a criterion to investigate if extra and anisotropic stresses matter. [ABSTRACT FROM AUTHOR]

The present study is concerned with the formation and detachment of polymer solutions pendant drops initially attached to immersed needles in Newtonian mineral oil. Drop formation, pinch-off and detachment is studied in two cases: (i) one single needle placed vertically and (ii) two needles placed horizontally opposite to each other. In the second case, the interface meniscus presents an elongated shape near the two needles and oscillations of this interface are observed. These are associated with the drop detachment. [ABSTRACT FROM AUTHOR]

In this paper, we study the stability of pendant drops suspended from a syringe needle tip using theory and experiments. We apply an eigenvalue analysis to the Young-Laplace equation for pendant drops to predict the instability type and critical drop volume that detaches and falls from a syringe needle tip due to gravity. We performed pendant drop experiments using different liquids and syringe needle sizes for a wide range of Bond numbers, 0. 2 < B o ∗ < 1. 2. Experimental results that included pinched-off drop volumes were compared with values computed at their stability limit from the eigenvalue analysis. The stability analysis was capable of predicting axisymmetric instabilities, and the agreement between volumes at pinch-off found through numerical and experimental analysis depended on the Bond number. [Display omitted] [ABSTRACT FROM AUTHOR]

We study the evolution of the growing volume of a droplet formed out of a capillary tube driven at a constant applied pressure by optical volume detection. A particular application aims at modeling the dynamic of the drop in the case of the injection of drugs in the middle-ear cavity using a small target drug delivery system. Various dispense tips such as a steel needle or silicone catheters and different t -BuOH aqueous solutions were used. A physical model based on the simplified Laplace–Young equation for a spherical droplet was developed to interpret the experimental results and model the observed deviation of the mean flow rate from the Poiseuille law when the drop radius is smaller than the capillary length. We emphasize the large influence of the surface tension which introduces a highly nonlinear effect at low applied pressure just above the capillary threshold pressure. The model fits well the evolution of the growing volume at various applied pressure up to a maximum diameter corresponding to the drop size close to the capillary length of the fluid, l c = γ / ρ g , based on the liquid density ρ , gravity g and surface tension γ. We demonstrate that the model can also be applied to non spherical pendant droplets providing the apex elongation over the drop height of a spherical cap of the same volume is kept in a 30% range. • Parametric growing drop model and mean flow rate of the drip at constant pressure. • Simplified Laplace-Young equation extended to non spherical pendant droplets. • Measurement of liquid surface tension by dripping with a camera and a capillary tip. • Dispense tips for drop injection in the middle-ear cavity and drug delivery systems. • Spherical drop shape modeling by video observations applied to elongated droplets. [ABSTRACT FROM AUTHOR]

The interfacial tension (IFT) at vapor/liquid equilibrium was measured for the pure water system following the saturation line (further named as water (l)/water (v) or pure water) as well as for the binary water/nitrogen system (further named water/nitrogen). In order to obtain vapor/liquid IFT data, the pendant drop method was applied in a high-pressure-high temperature view cell. The vapor/liquid IFT of the binary water/nitrogen system was investigated at 10, 15 and 18 MPa and for temperatures in the range of 323–598 K. For the water (l)/water (v) system, the temperature ranges from 423 K to 623 K at the respective saturation pressure. The IFT of the binary water/nitrogen system is in a similar range compared to the IFT of the pure water system up to temperatures of 500 K although showing an influence of the nitrogen pressure due to solubility and adsorption effects. At temperatures above 500 K, the IFT starts to considerably differ as a function of temperature, reflecting the respective phase behaviour of both systems. The parachor method following Macleod-Sugden was used to correlate the IFT of the binary water/nitrogen system, as well as the pure water system. The equation is in good agreement with the experimental results, showing an AAD of 4.97% for the water/nitrogen system and 3.02% for the pure water system. The obtained experimental results are further in good agreement with comparable literature data. [Display omitted] • New data on interfacial tension of water near critical point via pendant drop method. • Data for two systems: water liquid/vapor system and water/nitrogen system. • Densities obtained by PC SAFT. • Correlation of IFT using Macleod-Sugden approach. • Influence of nitrogen correlated qualitatively with phase behaviour. [ABSTRACT FROM AUTHOR]

An attempt was done to construct a reliable framework for measurement of hydrocarbon gas-oil interfacial tension (IFT) for live crude oil systems at reservoir conditions. We utilized a state-of-the-art experimental apparatus and robust experimental procedure to measure precisely gas/oil IFT using pendant drop technique and analyze the images based on Axisymmetric Drop Shape Analysis method. Initially, all phases were brought into equilibrium at experimental conditions and the density of each phase was measured. To measure IFT for each pair of fluids, an IFT cell was almost filled with the gas phase, and the live crude oil was then injected to create a pendant drop. The image was captured, using a microscope attached to a digital camera, and analyzed with Drop Analysis System software to determine the interfacial tensions. The densitometer and the IFT measurement apparatuses were all calibrated using reference fluids. We tried to spotlight the effects of pressure, temperature and composition on interfacial tensions. The findings show that effects of pressure are more profound on IFT rather than temperature. As pressure increases, particularly at low pressure conditions, the gas-oil interfacial tension declines with a descending trend. This behavior continues even at higher pressure with smaller rates. Increases in temperature increase the interfacial tensions as well. The effects of pressure and temperature are linked to the changes in density contrast, mutual gas-oil solubility, and miscibility development. The IFT variation can also be affected by the oil composition and therefore the trends and/or IFT values might change from one fluid system to another. The effects of composition are found to have significant effects on IFT, which depend on the phase envelope of the sample. [ABSTRACT FROM AUTHOR]

UV–vis spectroscopy is a powerful tool to investigate surface phenomena. Surface tension measurements coupled to spectroscopic techniques can help to elucidate how the interface organization influences the electronic properties of molecules. However, appreciable sample volumes are usually necessary to achieve strong signals during conduction of experiments. This study reports on the simultaneous acquisition of surface tension data and UV–vis spectra by axisymmetric drop shape analysis (ADSA) coupled to diffuse reflectance (DRUV) spectrophotometry using a pendant microliter-drop that requires small sample volumes and low analyte concentrations. Three example systems gave evidence of the applicability of this technique: (a) disaggregation of an organic dye driven by surfactant as a function of the surface tension and alterations in the UV–vis spectra, (b) activity of a glycosylphosphatidylinositol anchored enzyme estimated from formation of a colored product, and (c) interaction between this enzyme and biomimetic membrane systems consisting of dipalmitoylphosphaditylcholine and cholestenone. Apart from using smaller sample volume, this coupled technique allowed to investigate interfacial organization in the light of electronic spectra obtained in loco within a shorter acquisition time. This procedure provided precise interfacial information about static and dynamic systems. This has been the first study describing the kinetic activity of an enzyme in the presence of phospholipid monolayers through simultaneous determination of the surface tension and UV–vis spectra. [ABSTRACT FROM AUTHOR]

Motivated by contaminant remediation, we study the volume of oil (oleic acid) removed from a liquid lens by a falling particle. When a spherical particle is dropped from a fixed height into an oil lens that floats on top of a water surface, a portion of the oil adheres to the sphere. Once the sphere comes to rest at the subsurface, the oil forms an equilibrium pendant drop that remains attached to the sphere. We find in experiments with spheres of different sizes and materials, that the pendant drop volume is an increasing function of sphere mass for each material and a decreasing function of sphere density. By contrast, the normalized droplet volume in all of our experiments scales with sphere mass following V oil / V sphere ∼ M −0.544 . Thus, for a given size, lighter spheres capture more oil relative to their own volume than do heavier spheres and are more efficient at removing oil from a water surface in our experiments. Estimates for the upper bound of the normalized droplet volume, determined from the continuous family of solutions of the Young–Laplace equation, show the same qualitative dependence on the sphere mass. [ABSTRACT FROM AUTHOR]

In this manuscript we introduce algorithms based on the pattern search method that are used to estimate equilibrium surface tension and surfactant transport parameters from pendant drops. The pattern search method is an efficient minimization technique for estimating multiple unknown parameters. To introduce and validate the method for measuring surface tension and transport parameters we use axisymmetric drop shape analysis (ADSA) for pendant drops of aqueous sodium oleate (SO) and aqueous sodium dodecyl sulfate (SDS) in mineral oil, along with several other classical data sets from the literature. The data show good agreement with other studies suggesting that the pattern search method may be a robust alternative to gradient based search methods. [ABSTRACT FROM AUTHOR]

Introduction The development of the coaxial double capillary 15 years ago opened up the possibility to undertake accurate desorption and penetration studies of interfacial layers in the pendant drop technique. Drop and bubble methods offer several advantages with respect to other interfacial techniques. They allow a more stringent control of the environmental conditions, use smaller amounts of material and provide a much higher interface/volume ratio than in conventional Langmuir Troughs. Experimental The coaxial capillary was developed 15 years ago at the University of Granada as an accessory for the pendant drop surface film balance. It allows exchanging the subphase of the drop without disturbing the surface film and preserving the drop volume throughout the subphase exchange. Hence, this methodology enables one to carry out a great variety of interfacial studies well beyond the usual adsorption profiles. Penetration studies, sequential adsorption measurements, desorption kinetics, reversibility of adsorption and testing of enzymatic treatments on interfacial layers are amongst the principal applications. The coaxial capillary has been recently upgraded to a multi-exchange device which has boosted its applicability. It can be now used to address multilayer formation, create soft interfacial nano-composites such as membranes, polyelectrolyte assemblies and simulate in vitro digestion in a single droplet. Applications This review aims to compile the experimental work done, using the pendant drop subphase exchange in the last decade, and how its use has provided new insights into the surface/interfacial properties of many different materials. Special emphasis is placed on recent work regarding simulation of in vitro digestion in order to address issues relating to metabolism degradation profiles. The use of this methodology when dealing with interfacial studies allows setting the foundations of interfacial engineering technology. Based on subphase exchange experiments, we aim to develop models for competitive adsorption of different compounds at the interface and build up layer-by-layer interfacial structures. Future challenges comprise the design of finely adjusted nanoengineering systems, based on multilayer assemblies with tailored functionalities, to match the application demand. [ABSTRACT FROM AUTHOR]

Pendant bubble and drop devices are invaluable tools in understanding surfactant behavior at fluid–fluid interfaces. The simple instrumentation and analysis are used widely to determine adsorption isotherms, transport parameters, and interfacial rheology. However, much of the analysis performed is developed for planar interfaces. The application of a planar analysis to drops and bubbles (curved interfaces) can lead to erroneous and unphysical results. We revisit this analysis for a well-studied surfactant system at air–water interfaces over a wide range of curvatures as applied to both expansion/contraction experiments and interfacial elasticity measurements. The impact of curvature and transport on measured properties is quantified and compared to other scaling relationships in the literature. The results provide tools to design interfacial experiments for accurate determination of isotherm, transport and elastic properties. [ABSTRACT FROM AUTHOR]

Amplification and detection of nucleic acid sequences within integrated microsystems is routinely conducted using the technique of droplet PCR, wherein the polymerase chain reaction (PCR) is performed in microscale water-in-oil droplets (nanoliter to picoliter volumes). During droplet PCR, interactions at the interface of the droplet tend to dominate. Specifically, adsorption of the polymerase at the droplet interface leads to inefficient amplification. To reduce polymerase adsorption, surfactants such as the silicone-based ABIL EM90 have been commonly used. However, these surfactants have been selected largely through trial and error, and have been only somewhat effective. For example, when using ABIL EM90, 8 times (8×) the manufacturer prescribed concentration of polymerase was necessary for amplification. In this report, we use the pendant drop technique to measure adsorption and loss of enzyme at droplet interfaces for various surfactant-oil combinations. Dynamic interfacial tension and surface pressure measurements showed that significant polymerase adsorption occurs when using ABIL EM90. In contrast, much lower polymerase adsorption is observed when using Brij L4, a nonionic surfactant with a C 12 tail and an oxyethylene headgroup, which has not yet been reported for droplet PCR. These results correlate strongly with droplet PCR efficiency. Brij L4 enables highly efficient PCR at 2× polymerase concentration, and still enables effective PCR at 1× polymerase concentration. Overall, this work introduces a methodology for quantitatively assessing surfactants for use with droplet microreactors, and it demonstrates the practical value of this new approach by identifying a surfactant that can dramatically improve the efficiency of droplet PCR. [ABSTRACT FROM AUTHOR]

In this work, the experimental surface tensions for (CH 4 + H 2 O), (C 2 H 6 + H 2 O), (CO 2 + H 2 O) and (C 3 H 8 + H 2 O) binary systems have been measured with the pendant drop tensiometer at temperatures ranged (284.15–312.15) K and pressures ranged (1–60) bar. Subsequently, the combination of gradient theory (GT) and Chen–Kreglewski Statistical Association Fluid Theory equation of state (CK-SAFT EOS) is applied for modeling these experimental surface tensions and determining the density profiles of interface. Also, a proposed influence parameter as a function of the densities of the bulk phases is applied for these systems. The binary interaction parameters for these systems are determined according to the bulk densities and surface tensions. Subsequently, the density profiles for the surface layer are determined. The surface tensions determined with the gradient theory (GT) agree well with experimental surface tensions for these systems (overall AAD ∼ 1.36 and 1.37 for constant and the proposed influence parameters, respectively). [ABSTRACT FROM AUTHOR]

This work focuses on the effect of 2,2,2-trifluoroethanol (TFE) on certain micellar properties of Triton X-165 (TX-165) in aqueous solutions. The results obtained from various experimental techniques are used to interpret the interactions in the systems studied. The added TFE has strong effect on lowering the cloud point (CP) in the solutions of TX-165. This effect is consistent with partial dehydration of the oxyethylene groups in the micelle. The decrease of both the critical micelle concentration (CMC) and the CP with the increase of TFE concentration is justified by the interaction between TFE molecule and the oxyethylene group of TX-165, by the low hydrogen bond ability of the hydroxyl oxygen atom of TFE and by the water structure maker effect of TFE. The micelle aggregation number determined by fluorescence decreases initially with the increase of TFE concentration and then increases for the highest TFE concentrations. Fluorescence quenching data indicate that about 45% of pyrene probe is accessible to the quencher (TFE). The results of viscosity are consistent with ellipsoidal shape of TX-165 micelles. The changes in the intrinsic viscosity with increasing TFE concentration may reflect changes in the solvation of micelles and conformational changes in the polar head groups in the micelle. These factors affect the packing of the monomers and the micelle aggregation number. The TFE cosolvent confers hydrophobicity to the micelles of TX-165. [ABSTRACT FROM AUTHOR]

The oscillation dynamics of bubbles and drops that are attached to the tip of a capillary are experimentally investigated to test the applicability of simplified linear analyses to real-world situations. The experiments are performed with 0.7-2.6mm air bubbles in pure water (without surfactants) and with 0.7-1.3mm drops of pure water. The bubbles or drops are excited by the vertical motion of the capillary. The consequent shape oscillations, which are recorded by a high-speed camera, are processed to determine the oscillation eigenmodes (their frequencies, shapes, and damping rates). If the bubbles or drops are sufficiently small to preserve the spherical static shapes, the measured characteristics of the eigenmodes will correspond to the analyses. If the bubble or drop size is sufficiently large to form a neck, the eigenmode frequency decreases and the shape oscillations will correspond to those of a free bubble. The reduction in eigenmode frequency is correlated with the ratio of the actual volume of the bubble to the detachment volume of the bubble. [ABSTRACT FROM AUTHOR]

A laboratory study was conducted to investigate the effects of alcohols on interfacial tension (γ) between crude oil and carbon dioxide by the pendant drop method over pressures up to 16.55 MPa and at 375.15 K. Simultaneously, the density of fluids was measured as the data were required for the calculation of γ. The linear, branched, and mixed alcohols were used at concentrations (0, 2.5, 5, and 10)% by volume. Analysis of results revealed that moving from linear to branched alcohols results in higher γ reduction. However, branching of alcohols for reducing the γ was only effective up to a certain carbon chain length after which, increasing the carbon chain length or branches have no effect on γ reduction. Furthermore, it was revealed that γ reduction for different concentrations of alcohols were approximately equal at high pressure (≈16.55 MPa), thus lower concentration of the alcohols was suggested to be used to reduce the project cost of enhanced oil recovery in high pressure reservoirs. [ABSTRACT FROM AUTHOR]

One of the procedures to measure the surface and interfacial tension (ST and IFT) is pendant drop tensiometry which has distinguished drawbacks to determine the ultra-low interfacial tension. The main drawbacks of previous pendant drop procedures were incapacitation for the drop formation at ultra-low IFT values, inability to provide a suitable photo for small needles, and drop formation with non-uniform discharges. Refer to the lack of a comprehensive method for measuring all ranges of IFT and ST from ultra-low to high values felt by considering the porous media limitation in many unique studies, such as enhanced oil recovery (EOR), and carbon capture and storage (CCS), to cover the fluid-fluid and gas-fluid interactions, improving the pendant drop method is to be crucial. In this study, a new procedure was introduced to measure the ultra-low IFT in order of 10−3 mN m−1 using different needles with an outer diameter (OD) of 31G (0.46 mm) to 34G. The needles were used in this study had OD from 34G (0.16 mm) to 14G (2.11 mm) to provide a wide range of IFT and ST values from ultra-low to high values. At last, a comprehensive table was made based on more than 450 IFT and ST results to assist the researchers in selecting the best type of needle for performing the pendant drop test according to the range of IFT and ST values by considering varied applications. [Display omitted] • Spinning drop tensiometry is not a suitable method for determining the fluid-fluid and gas-fluid interactions in porous media due to the presence of a significant convective mass transfer. • The best method for determination of liquid-liquid and gas-liquid interactions in porous media is pendant drop tensiometry. • As much as interfacial tension (IFT) and surface tension (ST) values are less, the researcher must use the needle with a lower diameter in pendant drop tensiometry. • The best needle sizes for measuring the ultra-low IFT and ST values (near 10−3 mN m−1) are from 34G (0.16 mm) to 31G (0.25 mm). • The appropriate needle sizes for measuring the IFT and ST values lower than 1 mN m−1 are from 34G (0.16 mm) to 26G (0.46 mm). [ABSTRACT FROM AUTHOR]

High-temperature alloy melts introduced into carbon/carbon (C/C) composites by reactive melt infiltration (RMI) to form ultra-high-temperature ceramics (UHTCs) is currently an important method for improving their ablation resistance. As a fundamental thermo-property, surface tension has an important impact on the infiltration of alloy melts into the C/C composites. However, the surface tension of binary alloy systems, such as Ti–Zr and Ti–Hf, used for RMI has been little studied. In this work, experimental measurements through the pendant drop method and theoretical calculations of the surface tension of Ti–Zr and Ti–Hf alloys with different compositions have been carried out. Meanwhile, inevitably, impurities are introduced during the determination of surface tension at high temperatures, mainly carbon and oxygen. The effect of impurities on the phase composition, distribution, and surface tension of the alloys is investigated. The results demonstrated that the experimental measurements are in good agreement with the theoretical values of Butler's equation. The surface tension of Ti–Zr and Ti–Hf alloys increases with the content of Zr or Hf at the same temperature. Small amounts of oxygen are mainly dissolved in the lattice and have no significant effect on the surface tension. Carbon slightly decreases the surface tension of the alloy. • Experimental measurement and theoretical calculation of surface tension of Ti–Zr and Ti–Hf alloys for the first time. • The surface tension of Ti–Zr and Ti–Hf alloys increases with the content of Zr or Hf at the same temperature. • Carbon decreases the surface tension of the alloy. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of two different families of ILs on IFT between crude oil and water was examined. [•] The used two different families of ILs were namely imidazolium and pyridinium. [•] Results revealed more successful IFT reduction using imidazolium compared with pyridinium. [•] Results revealed significant effect of salinity on reduction of IFT especially for imidazolium. [Copyright &y& Elsevier]

Abstract: Mass transport between organic solvents (S) and carbon dioxide (CO2) has been investigated at pressures below the mixture critical pressure (MCP). Acetone and dimethyl sulfoxide (DMSO) have been used as organic solvents. Profiles of the mixture composition have been measured through liquid solvent drops pendant at steady conditions in a pressurized CO2 environment. The drop has been kept steady by feeding exactly that amount of liquid through a feed glass capillary into the pendant drop that evaporated at the same time from the drop into the CO2 environment. The liquid fed into the pendant drop has been either pure organic solvent or organic solvent with some CO2 dissolved. Profiles of the mixture composition have been quantified applying an optical Raman line imaging technique and the respective involved mass transport mechanisms are discussed comprehensively. Evaporative cooling was shown to dominate the temperature change at the inner margin of the drop, while mixing enthalpies were shown to be of minor importance. By fitting a mass transport model to DMSO concentration profiles, calculated from the mass fraction composition profiles, the mass transport properties diffusivity D, Biot-number Bi and mass transfer coefficient β could be derived. When acetone has been used as solvent, no steady drops could be formed not allowing a model based quantification of the mass transport. [Copyright &y& Elsevier]

Highlights: [•] An experimental method is presented for the interfacial tension between two liquids. [•] The bubble contour is evaluated by image processing. [•] A force balance is used for the interfacial tension. [•] The Laplace equation relating internal pressure and curvature is not involved. [Copyright &y& Elsevier]

Abstract: The surface tension of boron trioxide was measured with uncertainty less than ±0.5mN/m in the temperature range between 500°C and 900°C, using the pendant drop method coupled with a video enhanced image digitization technology and a best-fitting algorithm. These accurate surface tension data showed a peculiar temperature dependence: a mild increase at temperatures between 500°C and 800°C followed by a strong increase at T >800°C. To explain this unusual temperature coefficient of the surface tension, a change in the boron oxide structure while raising the temperature was proposed in this study. The main component of the vitreous B2O3 network, the planar boroxol ring, breaks up when the temperature increases. However, hypothesizing an analogy with the superstructures that are present in the binary borate glasses, this breakdown is compensated by the formation of new groups characterized by four-fold coordinated borons and three-dimensional structure. The presence of these groups causes a general strengthening of the B2O3 structure, especially noticeable at the complete breakdown of the boroxol rings. This proposed model of the structure changes successfully interprets the singular behavior of the surface tension temperature coefficient of the boron trioxide. [Copyright &y& Elsevier]

Abstract: The drop shape (sessile or pendant) method is widely used to measure the surface tension of glass melts. The accuracy of surface tension measurement has been improved by applying the technologies of video-image digitization and best-fitting algorithms. An accurate surface tension can be determined using hundreds of edge points from drop profiles and the best-fit with the drop profiles from the Young–Laplace equation. In this study, we examine the accuracy of measuring the surface tension of glass melts using the sessile or pendant drop methods. Experimental data show that the accuracy of surface tension measurement using the pendant drop method (±3mN/m) is better than that obtained by using the sessile drop method (±9 to ±20mN/m, depending on the contact angle). [Copyright &y& Elsevier]

Abstract: Drop shape techniques are extensively used for surface tension measurement. The accuracy of surface tension measurements using a pendant drop setup depends mainly on the drop shape. As the pendant drop shape becomes close to spherical, the performance of drop shape techniques deteriorates. It is important to consider how to optimize the pendant drop design and how to quantify the accuracy of the surface tension measurements. In this paper, the design and accuracy of pendant drop methods is considered in detail. Dimensional analysis is used to describe similarity in pendant drop shapes and to express the problem using appropriate dimensionless groups. A quantitative criterion called shape parameter is then used to express quantitatively the difference in shape between a given experimental drop and a spherical shape. The shape parameter is found to depend only on two dimensionless groups: the dimensionless volume and the Bond number. A critical shape parameter (minimum value of the shape parameter that guarantees a specified accuracy) is shown to depend only on Bond number. A set of experiments were performed with pure liquids to illustrate the change of the critical shape parameter with the Bond number. Results show that high accuracy is indeed reachable independent of the algorithms used. [Copyright &y& Elsevier]

• The dynamic surface tensions of Silwet-laden DMF drops are measured in ambient and saturated DMF vapor conditions. • The pendant drop with the maximum volume falls off around 1 min at high Silwet concentrations. • The dynamic surface tensions decrease slowly for more than 10 min at low Silwet concentrations. • In saturated DMF vapor conditions, fall-off and never reaching equilibrium surface tensions are still observed. • A simple origin based on the weak solvophobic nature of few siloxane groups of Silwet in DMF is proposed. Silwet L-77 has the unique ability to spread aqueous solutions across surfaces to a greater extent than conventional surfactants. Moreover, the Silwet-laden N,N-dimethylformamide (DMF) droplet can exhibit self-propulsion on substrates. Although the Marangoni effect is responsible, the origin of the surface tension gradient along the droplet remains unclear. The time evolution of a hanging Silwet-laden DMF droplet below a substrate is observed. Then, the dynamic surface tensions of pendant drops are determined for both high and low Silwet concentrations in ambient and saturated DMF vapor conditions. For pendant drops with maximum volumes, the dynamic surface tensions are found to decrease slowly for more than 10 min at low Silwet concentrations (e.g., 1 wt%). However, at high Silwet concentrations (e.g., 10 wt%), the drop falls off around 1 min. As the initial drop volume is slightly less than its maximum value, surface tensions continue decaying at low Silwet concentrations but reach equilibrium values at high Silwet concentrations. Once the equilibrium surface tension is attained, the increment of the drop volume does not change the result. The peculiar interfacial dynamics is attributed to the weakly solvophobic nature of few siloxane groups of Silwet in DMF. [Display omitted]. [ABSTRACT FROM AUTHOR]

Abstract: The method employed to incorporate guest molecules onto phospholipid Langmuir monolayers plays an important role in the interaction between the monolayer and the guest molecules. In this paper, we show that for the interaction between horseradish peroxidase (HRP) and a monolayer of dipalmitoylphosphatidylglycerol (DPPG) does depend on the method of HRP incorporation. The surface pressure isotherms of the mixed DPPG/HRP monolayers, for instance, were less expanded when the two materials were co-spread than in the case where HRP was injected into the subphase. Therefore, the method for incorporation affected not only the penetration of HRP but also the changes in molecular packing caused to the DPPG monolayer. With experiments with the monolayer on a pendant drop, we observed that the incorporation of HRP affects the dynamic elasticity of the DPPG monolayer, on a way that varies with the surface pressure. At low pressures, HRP causes the monolayer to be more rigid, while the converse is true for surface pressures above 8mN/m. Taken all the results together, we conclude that HRP is more efficiently incorporated if injected into the subphase on which a DPPG monolayer had been spread and that the interaction between HRP and DPPG is maintained even at high surface pressures. This is promising for the possible transfer of mixed films onto solid substrates and for applications in biosensors and drug delivery systems. [Copyright &y& Elsevier]

Abstract: The effect of ultrasound on flow through a capillary using the pendant drop method was investigated. Water was injected into a 0.1mm Hastelloy C-276 capillary tube submersed into several mineral oils with different viscosity, and kerosene. The average drop rate per minute was measured at several ultrasonic intensities. We observed that there exists a peak drop rate at a characteristic intensity, which strongly depends on oil viscosity and the interfacial tension between water and the oil. The semi-quantitative results reveal that the remarkable change in the interfacial forces between oil and water could be the explanation to the enhancement of oil recovery when the ultrasonic waves are applied. [Copyright &y& Elsevier]

Abstract: This study focuses on how intact, hydrophobic bacteria in their stationary (i.e., non-dividing) phase could adsorb onto the hexadecane–water interface and alter its mechanical properties. The two strains of bacteria used in forming the interfacial films were Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c. Using the dynamic pendant drop technique, the film interfacial tension was monitored as the surface area was made to undergo transient changes. Under static conditions, both types of bacteria had no effect on the interfacial tension. When subjected to transient excitations, however, the two bacterial films exhibited clear and qualitatively similar rheological properties: they responded as two-dimensional Maxwellian materials when the interfacial areas were dilated suddenly, but appeared to be purely elastic upon rapid area compression. Such rheological behaviours are “non-linear” in that the responses of the tension to area dilation and contraction are not mirror images of one another. Despite their qualitative similarities, the two types of film had very distinct film elasticities and relaxation times. The most striking difference between the two bacterial films was revealed under continuous reduction of area, when the A. venetianus RAG-1 system displayed a “paper-like” interface, whereas the interface of the R. erythropolis 20S-E1-c system was “soap film-like”. These macroscopic observations could be explained by the surface ultrastructures of the two cell strains determined using transmission electron microscopy. [Copyright &y& Elsevier]

Abstract: Axisymmetry of the height versus radius data z(r) of pendant drops is used to simplify the integral equation of the first kind for the third derivative d3 z/dr 3. This equation is solved by Tikhonov regularization and the derivative is used to calculate the radius of curvature at the apex. This key property of pendant drops together with the droplet volume is then used to obtain the surface tension. This is done by referring to the relationship between dimensionless droplet volume and dimensionless apex curvature for different bond numbers. The performance of this route to surface tension is demonstrated by applying it to several sets of data taken from the literature. A key limitation of this method is identified and a number of simple extensions are described. [Copyright &y& Elsevier]

Abstract: Interfacial tension of water–CO2 interface was measured by pendant drop method in the presence of a surfactant of various concentrations. The surfactants used were three surfynols which are non-ionic blanched hydrocarbon with different length of the alkyl side chain. Prior to the interfacial tension measurements, the solubility of the surfynols in CO2 were determined from cloud point method. The measured interfacial tensions indicated that an addition of small amount surfactant did reduce the interfacial tension. The interfacial activities of surfactants were evaluated from the slope of the interfacial tension reduction curve against the surfactant concentration and rationalized in terms of the molecular natures such as hydrophobic alkyl chain length. [Copyright &y& Elsevier]

Abstract: A novel apparatus, PeDro, was used to measure interfacial tension in a two-component system made of water and compressed carbon dioxide at temperatures ranging 278–335K and pressures 0.1–20MPa. Our optimized experimental setup utilized the quasi-static pendant drop method and ensured experimental errors below 2%. The interfacial tension showed a pronounced dependence on pressure and temperature. A regression function was derived that allows to interpolate between the experimental data with high precision. Molecular dynamics simulations were performed for liquid–liquid and liquid–vapor interfaces between water and carbon dioxide at elevated pressures. The interfacial tension was obtained from long constant-volume production runs as the difference between normal and tangential pressure components. The results showed a good agreement with experimental data, with our model system reproducing faithfully the pressure–temperature dependence of the interfacial tension. [Copyright &y& Elsevier]

Abstract: The eyelid meibomian gland secretions form the outer layer of the tear film. That layer functions as a lubricant during a blink, and as a barrier against intrusion of foreign bodies. The lipid film is also exposed to proteins present in the aqueous phase that may adsorb there, and thus form an integral part of the surface of the tear film, or possibly, cause disruption to the outermost layer. Therefore, the adsorption of tear proteins to the meibomian lipid layer was object of the present investigation. A model tear was set up coating a pendant drop of saline with a film of meibomian lipids and measuring variations of the interfacial pressure after the injection of tear proteins into the aqueous subphase at their physiological concentration. All tear proteins adsorbed at the interface causing the initial surface pressure to increase. For each protein, a limiting surface pressure at which a given protein was no longer able to insert into the lipid layer was found. Among the proteins tested, lipocalin was the most surface active one and inserted into the lipid layer in the whole range of surface pressure exerted by the meibomian lipid mixture. Lactoferrin, lysozyme and IgA also interacted with the lipids whereas albumin interacted more weakly. The timescale of the protein insertion into the lipid layer was of the order of 102 s. It was hypothesized that protein adsorption at the interface could be associated with structural changes. Indeed, the enzymatic activity of lysozyme was maintained in the presence of an outermost meibomian lipid layer that prevented its denaturation while exposure at the air/aqueous interface induced significant lysozime degradation. meibomian lipid composition is therefore functional to maintain tear proteins activity. [Copyright &y& Elsevier]

Abstract: A quantitative criterion called “shape parameter” to evaluate the quality of surface tension measurement of Axisymmetric Drop Shape Analysis (ADSA) is presented. ADSA is a powerful technique for the measurement of interfacial tensions and contact angles of pendant drops, sessile drops, and bubbles. Despite the general success of ADSA, deficient results may be obtained for drops close to spherical shape. Therefore, the “shape parameter” was used to determine the range of drop shapes in which ADSA succeeds or fails. The “shape parameter” is a dimensionless parameter that expresses quantitatively the difference in shape between a given experimental profile and an inscribed circle. The surface tension measurements of ADSA were evaluated for both pendant drop and constrained sessile drop configurations using the shape parameter. Different shapes of the pendant drop were studied using different sizes and materials of holders. For each drop configuration, a “critical shape parameter” was defined based on the minimum value of the shape parameter that guarantees an error of less than ±0.1mJ/m2. Furthermore, the effects of the type of liquid and constellation on the “critical shape parameter” were studied. [Copyright &y& Elsevier]

Blockages in natural gas and oil pipelines can be caused by the aggregation of clathrate hydrate crystals which form at the oil-water-gas interfaces. Knowledge of interfacial tensions between components are crucial to understanding the interface physical chemistry and numerically simulating pipeline multiphase flow. This study measures the effect of the presence of natural gas components on the oil-water interfacial tension and its potential implications on flow assurance in oil-gas pipelines. Decane is used to mimic oil and is saturated with the gaseous mixture of methane, ethane, and propane as is consistent with simulations of multiphase dynamics. Water droplets were generated in decane and photographed through a window using pendant drop method in order to determine the oil-water interfacial tension. This study examined both mechanical and thermodynamic phenomena by considering interfacial tension under different pressure/temperature conditions covering the natural gas hydrate formation region. The effect of the gas mixture composition and pressure on the interfacial tension was determined at different temperatures in the range of pipeline operating conditions. The behavior was explained from both experimental and theoretical points of view. At lower pressures, the presence of the C2 and C3 natural gas components lowers the decane-water interfacial tension compared to the case of the decane-water-methane system. In particular, the pressure dependence of the interfacial tension was found to be almost five times higher than that in the absence of C2 and C3. By decreasing the interfacial tension, the presence of ethane and propane may facilitate the mixing of the water and oil phases in pipelines, therefore affecting the rate of hydrate formation. [Display omitted] • Interfacial tension data covering natural gas hydrate formation area. • Recreate a natural gas transportation pipeline and measure the interfacial tensions. • Measurement was conducted using a ternary system, natural gas-decane-water. • Effect of gas mixture and its each gas components on interfacial tension. • Understanding multiphase dynamics and gas hydrate formation in the pipeline. [ABSTRACT FROM AUTHOR]

• Prediction of surface tension (ST) value of pendant drops by machine learning (ML). • Convolutional neural network trained using pendant drop images of known ST. • Estimating ST for unknown pendant drops outside the range of training data. • ST prediction of any arbitrary pendant drop shape during transient drop generation. • ML model capable of ST prediction independent of fluid properties. An image-based machine learning (ML) method is introduced to predict the surface tension of an ethanol-water pendant drop made of unknown ethanol-water composition. In contrast to previous neural-network based surface tension predictors that rely on using either the simplified molecular-input line-entry system or experimentally measured surface tension values, the present image-based deep neural network model directly predicts surface tensions based on arbitrary pendant drop shapes at any stage before breakup. Using convolutional neural networks (CNNs), surface tension values are accurately obtained independent of the drop size and liquid properties. Two CNN architectures are presented that accurately predict the surface tension of a pendant drop for three different ML models. To improve the generality of the ML models, image data augmentation technique is used to generate more representatives from available data. Approximating the surface tension for unknown pendant drops outside the range of the given classes has also been demonstrated. The trained machine learning models have an overall accuracy of about 98% in predicting the surface tension of a pendant drop containing an unknown ethanol-water composition. Additionally, the ML models are tested on unknown methanol-water mixtures to demonstrate the generality and the results show good predicting accuracy. Besides the accuracy of CNN models, performance measures including Precision, Recall and F1-score are reported for each surface tension value in the dataset. Compared with the physics-based axisymmetric drop shape analysis techniques, the present method is not limited to equilibrium pendant drop images and is much faster and more versatile. This image-based ML method shows great promise in directly predicting the surface tension values based on vastly available pendant drop images. [ABSTRACT FROM AUTHOR]

Interfacial properties of the (CH 4 + CO 2 + H 2 O) system are of great importance in many geotechnical engineering applications. In this study, we report the first experimental measurements of the interfacial tensions in the (CH 4 + CO 2 + H 2 O) system under both three-phase (VLLE) and biphasic conditions. The measurements were made by the pendant drop method. The compositions of the coexisting phases were obtained from a previous study of the phase behavior and the phase densities were then calculated from an equation of state. IFTs along five isotherms in the VLLE region and along six isotherms in biphasic region are reported. In the VLLE region, the IFT between the water-rich liquid and the gas phase varied between (33 and 39) mN·m−1, with a sharp increase as the pressure increased along an isotherm towards the upper critical end point. In the same region, the IFT between the water-rich and CO 2 -rich liquids varied between (30 and 33) mN·m−1. The IFT between the gas phase and the CO 2 -rich liquid phase was too small to measure accurately but an approximate value was obtained which is consistent with Antonov's equality. In the biphasic region, measurements were made at temperatures up to 423 K and at pressures up to 30 MPa. As observed in other water-gas systems, the IFT declines monotonically along isotherms with increasing pressure and decreases with increasing temperature at constant pressure. Image 1 [ABSTRACT FROM AUTHOR]

• P. aeruginosa and S. epidermidis form stable biofilms at n -decane-water interfaces. • Biosurfactants or hydrophobicity drive bacterial adsorption on oil-water interfaces. • Monitoring interfacial rheology elucidates bacteria adhesion and biofilm formation. Bacterial adsorption to interfaces is the initial step in biofilm formation. The mechanism of biofilm formation at liquid-liquid interfaces differs from the process of biofilm formation on solid-liquid interfaces. Until now, the former is not well understood. We study the bacterial adsorption and biofilm formation of three different bacteria, P. aeruginosa , S. aureus, and S. epidermidis at the n -decane-water interface, with focus on the relationship between bacteria wettability, excretion of biosurfactants, and biofilm formation. The adhesion capacity of these bacteria to hydrocarbons was characterized using the bacterial adherence to hydrocarbons test. We monitored the interfacial rheology of bacterial adsorption and biofilm formation at the interface over time using a drop shape analyzer and imaged the formed biofilms by using fluorescence and scanning electron microscopy. P. aeruginosa showed high adhesion capacity to hydrocarbons, while the adhesion capacity of both staphylococci was negligible. P. aeruginosa also showed rapid adsorption to the n -decane-water interface as measured by the rapid decrease in interfacial tension for the pure bacteria suspension. However, S. epidermidis , with a negligible hydrophobicity value, showed the most substantial reduction in interfacial tension and the formation of the most elastic biofilms at the oil-water interface. S. epidermidis accomplishes this by the secretion of biosurfactants. S. aureus did not form biofilms at the n -decane-water interface, in contrast to P. aeruginosa and S. epidermidis. We conclude that bacterial adsorption and biofilm formation at oil-water interfaces, in general, are not simple functions of cell hydrophobicity. Biosurfactant modification and metabolism of the interface also play essential roles. [ABSTRACT FROM AUTHOR]

Solution of the Young-Laplace equation for fluid-fluid interface shapes is crucial in several applications related to colloids, capillarity, and surfaces. Despite significant progress in analysis and computation, the solution framework in literature is quite sparse with different norms for non-dimensionalisation and often limited to simplistic boundary conditions. These problems can be overcome and a unified solution framework can be developed if the relevant parameters are non-dimensionalised with respect to the capillary length, and collapsed onto a single graphical chart in the form of axes and contours. Here we use vector parameterized cubic splines to represent axisymmetric droplet profiles, which are then evolved quasi-statically to get to the equilibrium shapes using a novel thermodynamic free energy minimization-based algorithm. The generated dataset for a wide range of geometric and surface parameters is validated and then compiled in the form of a graphical chart. The chart traces the complete profile of axisymmetric pendant drops, predicts their evolution and stability on any surface, all without running any computer simulation. Most importantly, the contact angle contours on this chart offer the capability to incorporate the effect of contact angle hysteresis, a situation which is practically relevant, but has yet not been modelled. [ABSTRACT FROM AUTHOR]