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The pendant drop tensiometry is a preferred primary method for measuring interfacial tension (IFT) or surface tension. In a typical experimental setup, Gravitational pull opposes an interfacial tension forces to cause a liquid drop to develop into a pendant-like shape. The process comprises quantitatively modifying a theoretical profile generated through calculating the Young–Laplace capillarity formula to such an experimental pendant drop analysis and information by performing digital image analysis. However, specific parameters of this method lead to uncertainty in the obtained value of IFT. The present work aims to determine the role of factors such as evaporation of the hanging drop and its changing volume in interfacial tension, which is acting as major source of error in determining IFT of distilled water. This study details the measurement process and evaluation of standard errors with interfacial tension readings of distilled water by using pendant drop method with three calibrated needles of diameters, 0.9 mm (20 G), 1.27 mm (18 G) and 1.65 (16 G) mm. The needle's diameter influenced the evaporation rate and the value of IFT. Also, it is essential to consider these factors for further evaluating measurement uncertainties when determining the IFT of biological fluids. In addition, the present study makes an effort to determine the measurement error associated with drop volume and evaporation rate of drop in study. The distilled water used in the measurement had an absolute Interfacial tension of 72 mN/m, and the enlarged measurement errors were predicted to be in the range of 0.4–0.6%. It is critical to consider these associated measurement errors when determining the IFT of liquids for metrological application such as establishing primary standards for measurement pertaining to biological fluids. [ABSTRACT FROM AUTHOR]

The article discusses the principle of operation and the main components of jet-drop optical measuring systems (JDOMS) based on the pendant drop method for monitoring electric field strength (EFS), magnetic field (MFS) and gravitational field (GFS). To make a pendant drop sensitive to electric or magnetic fields, it is proposed to charge it electrostatically or to create it from a magnetic fluid, and its mass allows you to feel changes in the gravitational field. The use of magnetic fluids as the basis of a pendant drop is the most multifunctional for measurements of EFS, MFS and GFS. The implementation of the zero measurement method using a pendant drop as a comparison device, a null organ that perceives the difference in the effects of the measured and reference field(s) at the level of quasi-zero three-coordinate displacements, is considered. They are measured by high-precision optical measurement methods. Different variants of modern optical measurement systems are analyzed.

The shape of a pendant drop is studied by employing free energy minimization. This free energy includes the gravitational potential energy and the interfacial surface energy. We employed the Lagrange multipliers method to minimize free energy while maintaining drop volume as constant. The differential equation for the shape of any pendant drop was established as a function of one dimensionless parameter only. This novel dimensionless parameter is defined as the shape factor. Around the origin of the chosen coordinate axis, an analytical solution to the differential equation was found. For a general solution, a numerical approach was followed to estimate drop shape. Furthermore, we calculated the detached volume from the bulk pendant drop. Comparison of the results with the experimental findings shows good agreements. A new Axisymmetric Drop Shape Analysis method is suggested, which can help users estimate any unknown of the problem if one geometrical data of the drop is known. [ABSTRACT FROM AUTHOR]

We report on the surface and interface tension measurements of the two ionic liquids (ILs) [C8C1Im][PF6] and [m(PEGn)2Im]I (n = 2, 4, 6) in a surface science approach. The measurements were performed in a newly developed and unique experimental setup, which allows for surface tension (ST) measurements using the pendant drop method and for contact angle measurements using the sessile drop method under the well-defined conditions of a high vacuum (from 10−7 mbar). The setup also allows for in vacuum transfer to an ultrahigh vacuum system for surface preparation and analysis, such as in angle-resolved X-ray photoelectron spectroscopy. For [C8C1Im][PF6], we observe a linear decrease in the surface tension with increasing temperature. The ST measured under high vacuum is consistently found to be larger than under ambient conditions, which is attributed to the influence of water uptake in air by the IL. For [m(PEGn)2Im]I (n = 2, 4, 6), we observe a decrease in the ST with increasing polyethylene glycol chain length in a vacuum, similar to very recent observations under 1 bar Argon. This decrease is attributed to an increasing enrichment of the PEG chains at the surface. The ST data obtained under these ultraclean conditions are essential for a fundamental understanding of the relevant parameters determining ST on the microscopic level and can serve as a benchmark for theoretical calculations, such as molecular dynamic simulations. In addition to the ST measurements, proof-of-principle data are presented for sessile drop measurements in HV, and a detailed description and characterization of the new setup is provided. [ABSTRACT FROM AUTHOR]

В работе рассмотрены математические модели осесимметричных капиллярных менисков — лежащая и висящая капли, развернутый мениск, учитывающие размерную зависимость поверхностного натяжения. Доказаны теоремы существования и единственности решений задач, описывающих равновесные поверхности менисков. Разработаны и протестированы эффективные численные методы, предназначенные для приближенного расчета профилей менисков. На языке «Wolfram Language» написана компьютерная программа, с помощью которой проведены масштабные вычислительные эксперименты по выявлению степени и характера влияния параметров моделей на равновесную форму каждого из рассматриваемых менисков

Three different kinds of liquid marbles were prepared using polytetrafluoroethylene nanoparticles (PTFE-NPs), polytetrafluoroethylene microparticles (PTFE-MPs), and polyvinylidene fluoride nanoparticles (PVDF-NPs) as the surface layers, respectively. The surface properties and stability of the different marbles were studied. The surface energy of PTFE-NPs, PTFE-MPs, and PVDF-NPs marbles are 0.061 N/m, 0.065 N/m, and 0.067 N/m respectively. It was revealed that the PTFE-NPs liquid marbles have the smallest surface energy and the highest stability. In addition, we prepared the phenolphthalein regent marbles with the PTFE-NPs, PTFE-MPs, PVDF-NPs. Among the same basicity gas, compared with the pure phenolphthalein liquid drop, the color of the phenolphthalein regent marbles changed more quickly. [ABSTRACT FROM AUTHOR]

[Display omitted] The drop deposition technique can impact contact angle measurements. We hypothesized that the drop pinch-off, during the traditionally used pendant drop technique, significantly alters the static contact angle. The capillary waves and dynamic wetting pressure generated during the pendant drop deposition are the source for forced spreading, which can be circumvented by alternative liquid-needle drop deposition techniques. To compare the role of drop pinch-off and resultant dynamic wetting pressure, we meticulously observed and quantified the entire drop deposition process using high speed imaging until the drop attains the static contact angle in both cases, namely pendant drop and liquid needle deposition technique. Conventionally used standard substrates are compared using both techniques and further compared using literature data. The capillary waves and corresponding drop shape variations are analysed for quantifying the dynamic wetting pressure by measuring drop base diameter, contact angle and centre of mass. We compared three parameters - drop pinch-off, spreading behaviour and respective static contact angles along with the resultant dynamic wetting pressure for both the techniques, i.e., pendant drop and liquid-needle. For the pendant drop technique we observed a pronounced drop volume dependency of these parameters even though the corresponding Bond numbers are less than unity. In contrast, for the liquid needle there is no such dependency. With a theoretical argument corroborating experimental observations, this work highlights the importance of a well controlled drop deposition, with a minimum wetting pressure, in order to guarantee contact angle data that is independent of drop deposition effects, thereby only reflecting the substrate properties. [ABSTRACT FROM AUTHOR]

[Display omitted] Hypothesis: The value of the maximal weight of a pendant drop formed at the end of a syringe needle is lower than the intensity of the corresponding capillary force. The balance of the external forces applied to the maximal pendant drop must be completed by the overpressure generated by the piston of the syringe. Inside the drop, the Laplace pressure corresponds to this overpressure. Experiments: Pendant drops are made with three liquids and five different needle diameters. The influence of Laplace pressure on the maximal weight is experimentally highlighted by modulating the drop curvatures thanks to glass beads placed at the apex of the pendant drop. Their maximal weight and curvatures are measured by image analysis. Findings: Experiments confirm that the balance of external forces must be completed by the force acting on the syringe piston. The overpressure on the piston has an impact on the drops via the Laplace pressure. A master curve between the mean curvature and the maximal volume of the pendant drops is observed. This result allows to validate an expression of the maximal weight which integrates the Laplace pressure. This work contributes to a better understanding of the maximal pendant drop properties and beyond, of the capillary phenomenon. [ABSTRACT FROM AUTHOR]

[Display omitted] When an aqueous colloidal drop dries on a solid substrate, the final pattern of the dried deposit can be manipulated through controlling the internal flow states of the drop. We report a strategy to control the dried patterns of aqueous colloidal drop by controlling the drop configurations and relative humidity. For this purpose, both sessile and pendant drops are studied. The capillary flow, which is responsible for coffee-ring, is suppressed by increasing the relative humidity. Then, surprisingly, the internal convection in the pendant drop is significantly stronger than that in the sessile drop. This phenomenon leads to the formation of the disc-like and spot-like dried patterns in the sessile and pendant drop, respectively, which are the results of different interactions between the Marangoni and (buoyancy-induced) natural convections in the sessile and pendant drops. In the sessile drop, the Marangoni and natural convections mutually restrain each other due to their opposite flow directions. In contrast, in the pendant drop, the two convections mutually enhance each other, due to their same flow directions. This new strategy offers a foreign-material-free and external-force-free means to control the dried patterns of the drop. [ABSTRACT FROM AUTHOR]

[Display omitted] High hydrostatic pressure treatment causes structural changes in interfacial-active β-lactoglobulin (β-lg). We hypothesized that the pressure-induced structural changes affect the intra- and intermolecular interactions which determine the interfacial activity of β-lg. The conducted experimental and numerical investigations could contribute to the mechanistic understanding of the adsorption behavior of proteins in food-related emulsions. We treated β-lg in water at pH 7 with high hydrostatic pressures up to 600 MPa for 10 min at 20 °C. The secondary structure was characterized with Fourier-transform infrared spectroscopy (FTIR) and circular dichroism (CD), the surface hydrophobicity and charge with fluorescence-spectroscopy and ζ-potential, and the quaternary structure with membrane-osmometry, analytical ultracentrifugation (AUC) and mass spectrometry (MS). Experimental analyses were supported through molecular dynamic (MD) simulations. The adsorption behavior was investigated with pendant drop analysis. MD simulation revealed a pressure-induced molten globule state of β-lg, confirmed by an unfolding of β-sheets with FTIR, a stabilization of α-helices with CD and loss in tertiary structure induced by an increase in surface hydrophobicity. Membrane-osmometry, AUC and MS indicated the formation of non-covalently linked dimers that migrated slower through the water phase, adsorbed more quickly due to hydrophobic interactions with the oil, and lowered the interfacial tension more strongly than reference β-lg. [ABSTRACT FROM AUTHOR]

Structural color, renowned for its enduring vibrancy, has been extensively developed and applied in the fields of display and anticounterfeiting. However, its limitations in brightness and saturation hinder further application in these areas. Herein, we propose a pendant evaporation self-assembly method to address these challenges simultaneously. By leveraging natural convection and Marangoni flow synchronization, the self-assembly process enhances the dynamics and duration of colloidal nanoparticles, thereby enhancing the orderliness of colloidal photonic crystals. On average, this technique boosts the brightness of structural color by 20% and its saturation by 35%. Moreover, pendant evaporation self-assembly is simple and convenient to operate, making it suitable for industrial production. We anticipate that its adoption will remarkably advance the industrialization of structural color, facilitating its engineering applications across various fields, such as display technology and anticounterfeiting identification.

We developed a pendant-drop Raman spectrometer and observed cavity enhanced Raman scattered light. Further enhancement of the Raman scattered light was demonstrated by controlling the droplet shape, which was performed by applying vibrations to the pendant drop. We found that (1) stimulated Raman scattering proceeds and enhances its intensity at the surface of the pendant droplet, (2) a horizontally elongated droplet gives further enhancement by light confinement, and (3) only s-polarized light resonates in the pendant drop. This study facilitates spectroscopic studies on the liquid surface. [ABSTRACT FROM AUTHOR]

Phosphine sulfides are the pentavalent derivatives of organophosphines and are usually used as synthetic intermediates of organophosphines for their isolation as protected analogs because of the stability toward oxidation. Recently, several applications of phosphine sulfides have been disclosed as functional materials, organocatalysts, ligands, and extractants. In addition, several synthetic uses of phosphine sulfides have also developed, in which the thiophosphinyl group plays an essential role. This Highlight Review covers recent applications and syntheses of organophosphine sulfides. [ABSTRACT FROM AUTHOR]

Minimum Miscibility Pressure (MMP) is regarded as one of the foremost parameters required to be measured in a CO2 injection process. Therefore, a reasonable approximation of the MMP can be useful for better development of injection conditions as well as planning surface facilities. In this study, the impact of asphaltene content ranging from 3.84% to 16% on CO2/heavy oil MMP is evaluated. In this respect, slim tube miscibility and Vanishing Interfacial Tension (VIT) tests are used. Regarding the VIT test, the Interfacial Tension (IFT) is measured by means of two methods including pendant drop and capillary apparatuses, and thereafter the MMP measurement error between slim tube and VIT methods are calculated. Based on the results, by increasing the asphaltene content, the measured MMP by slim tube method increases linearly while that by VIT follows no clear trend. The results also indicate that there is an asphaltene content range within which the MMP error between slim tube and VIT tests is minimized. IFT measurement by pendant drop and Capillary Glass Tube (CGT) methods show that by increasing asphaltene content up to 10.15%, IFT declines, whereas for further increase in content, IFT increases because of the irregular dispersion of asphaltene in oil droplets. [ABSTRACT FROM AUTHOR]

The surface tension of blood plays an important role not only in the birth and decompression sickness but also in other functionality of the organism. It also provides capillary action during blood flow process. In this article, a simple and low-cost device is designed and fabricated for measuring the surface tension of blood by pendant drop method. In this device, a droplet of blood is formed in a closed chamber on tip of an 18-gauge blunt needle and it is photographed by a camera in very humid conditions (RH = 99%) to minimise the evaporation. A wetted wick is provided at the bottom of the chamber for maintaining constant relative humidity in chamber. Surface tension of blood is inferred using drop shape factor method and image analysis technique at various experimental conditions. This device is validated and calibrated with surface tension measurements of water and silicone oil. Its measurements are in good agreement against data reported in literature. Post-validation, surface tensions of blood samples with and without anticoagulant of healthy persons at various temperatures (range from 20 to 40 °C) was measured. It was found that the surface tension of normal blood samples strongly correlates with blood temperature. The surface tension of female blood was remarkably different from same of male blood. However, the effect of age (21-60 year) on the surface tension was negligible for all practical purposes. Increased percentage of anticoagulant in blood increases its surface tension. This research specifies a baseline for surface tension of normal blood samples at various conditions which in turn provides new insights to pathologists in identifying various disease conditions. [ABSTRACT FROM AUTHOR]

As the complex component in crude oil, asphaltenes adsorb at the interface and exhibit unique interfacial properties, ruling the emulsifying properties and emulsion stability of produced fluid. Hence, this study deals with the adsorption behavior and interfacial dilational properties of asphaltenes. To investigate the adsorption behavior of asphaltenes, the dynamic interfacial tension is obtained with the pendant drop shape method at various concentrations and temperatures. Based on the adsorption kinetic equation, the adsorption behavior is found to be controlled by different mechanisms at different stages. The initial stage is dominated by the diffusion mechanism. Then, the adsorption process is gradually affected by diffusion and random sequential adsorption simultaneously. Finally, the controlling factor changes to the random sequential adsorption of adsorbed asphaltenes. A higher concentration of asphaltenes can accelerate the diffusion coefficient by increasing the concentration gradient, while the effect of temperature is weaker than concentration. Based on the experimental results, a new method is developed to quantify the influences of different mechanisms. After that, the droplet dilation experiment is carried out to probe the structural properties of the formed interfacial layer. By fitting the experimental data with the Langmuir equation of state, the average interfacial area occupied per asphaltene molecule is calculated. Finally, the emulsion stability experiments are carried out to support the conclusions in the interfacial experiments. This study enriches the adsorption kinetics theory of asphaltenes. It also contributes to the safe and economical transportation of produced fluid and rises the efficiency of demulsification. [ABSTRACT FROM AUTHOR]

The objective of this work is to understand the effect of asphaltene on surface properties including interfacial tension (IFT) and reach a specific surfactant in order to inhibit asphaltene precipitation in heavy oils. IFT experiments are implemented by Pendant Drop method and effective mechanisms causing IFT fluctuations are interpreted. The uniform trend of IFT for water/synthetic oil diminished at 25 volume percent of n-heptane which is attributed to asphaltene precipitation onset. Repetition of the experiment with 31 volume percent of n-heptane and real oil sample of field “S” resulted in asphaltene precipitation onset point. This change is justified by different density of synthetic and real oil samples. In addition, three surfactants of L-DBSA (linear dodecyl-benzene-sulfonic acid), B-DBSA (branched dodecyl-benzene-sulfonic acid), and DR (dodecyl resorcinol) are used to retard the asphaltene precipitation onset. Due to special chemical structure and polar group of DR, it is a weaker surfactant than DBSA in terms of inhibition of asphaltene precipitation. Furthermore, chemical structure and spatial avoidance of branches in B-DBSA causes little adsorption to asphaltene molecules and insignificant stability that eventually postpones the onset of asphaltene precipitation at lower degree compared to L-DBSA. When surfactants are introduced, the thin film of asphaltene formed on water-oil interface exhibits enhanced stability with the order of L-DBSA>B-DBSA>DR. Results show 0.25 wt% of L-DBSA can considerably delay the onset point that remedies the problem of asphaltene precipitation in heavy oil fields to great extent. [ABSTRACT FROM AUTHOR]

This paper focuses on an experimental study of the surface tension of nanofluids based on ethylene glycol with various types of nitride nanoparticles. Samples were prepared using a two-step method with mass content between 1 and 5% of particles. Nanofluids contain three types of nitride nanoparticles: aluminum nitride, silicon nitride and titanium nitride with various particle average sizes. Surface tension of nanofluids was investigated at a constant temperature of 298.15 K with two different techniques: du Noüy ring method and pendant drop method. It is presented that experimental values obtained with both methods are in good agreement with each other. Also, results obtained during this study show that the addition of this type of nanoparticles does not have a significant impact on the surface tension of base fluid for the concentrations and diameters of nitride nanoparticles considered. [ABSTRACT FROM AUTHOR]

• 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]

Graphical abstract Abstract The effect of glycerol with sodium chloride (NaCl) on the phase behaviour of sodium dodecyl sulfate (SDS) near the Krafft point was studied by surface tension analysis using the pendant drop method. The critical micelle concentration (CMC) and Krafft Temperature (T K) of SDS in water: glycerol mixtures, across the full composition range, and in NaCl solutions within 0.005–0.1 M were obtained. The pendant drop method successfully allowed us to determine the Krafft point of SDS in high glycerol systems where other traditional methods (e.g. conductivity) have been ineffective. Overall the addition of glycerol increases the CMC and the T K , thus shifting the Krafft point of SDS to higher temperatures (increasing crystallisation temperatures) and higher SDS content in the presence of glycerol, which is interpreted as a result of the reduction in solvent polarity which opposes micellization. The addition of NaCl to the SDS – water-glycerol systems brings the CMC back down, while having no significant effect on the T K. Our results establish a robust route for tuning the Krafft point of model surfactant SDS by adjusting solvent quality and salt content. [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]

Graphical abstract Highlights • Interfacial preoccupation results in slow interfacial stabilization process. • Conformational state and charge affect adsorption rate and structural rearrangement. • Molecular dynamic simulation confirms adsorption behaviour analysed via pendant drop. • Force maps reveal increase in repulsion with interfacial preoccupation. • Repulsive forces differ with conformational state and charge of β-lg. Abstract Amphiphilic properties enable proteins like β-lactoglobulin to stabilize oil/water-interfaces and provide stability in food-related emulsions. During emulsification, the protein undergoes three stages: (I) migration through bulk phase, (II) adsorption, and (III) interfacial rearrangement at the oil/water-interface – the kinetics of which require further research. Therefore, the aim of our study was the analytical and computational investigation of stage (I) and (II) as a function of the interfacial preoccupation, conformational state and charge of β-lactoglobulin. For this purpose, the adsorption of β-lactoglobulin (at pH 7, pH 7 containing 0.1 M NaCl, and pH 9) at increasingly preoccupied oil/water-interfaces has been compared through measuring interfacial tension and ζ-potential and through running molecular dynamics simulations. With increasing interfacial preoccupation, (I) the migration via lag time increased and (II) the adsorption rate decreased. The (II) adsorption rate was highest for β-lactoglobulin containing NaCl, due to dense packing and electrostatic screening. β-lactoglobulin at pH 7 reached a lower adsorption rate than the more negatively charged β-lactoglobulin at pH 9, due to exposure of hydrophobic regions that had a greater effect on adsorption rates than electrostatic repulsion. Our research contributes to a profound understanding of the interfacial stabilization mechanism of proteins at oil/water-interfaces, necessary to characterise and control emulsification processes. [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]

We provide a C/C++ software for the shape analysis of deflated elastic capsules in a pendant capsule geometry, which is based on an elastic description of the capsule material as a quasi two-dimensional elastic membrane using shell theory. Pendant capsule elastometry provides a new in situ and non-contact method for interfacial rheology of elastic capsules that goes beyond determination of the Gibbs- or dilational modulus from area-dependent measurements of the surface tension using pendant drop tensiometry, which can only give a rough estimate of the elastic capsule properties as they are based on a purely liquid interface model. Given an elastic model of the capsule membrane, pendant capsule elastometry determines optimal elastic moduli by fitting numerically generated axisymmetric shapes optimally to an experimental image. For each digitized image of a deflated capsule elastic moduli can be determined, if another image of its undeformed reference shape is provided. Within this paper, we focus on nonlinear Hookean elasticity because of its low computational cost and its wide applicability, but also discuss and implement alternative constitutive laws. For Hookean elasticity, Young’s surface modulus (or, alternatively, area compression modulus) and Poisson’s ratio are determined; for Mooney-Rivlin elasticity, the Rivlin modulus and a dimensionless shape parameter are determined; for neo-Hookean elasticity, only the Rivlin modulus is determined, using a fixed dimensionless shape parameter. Comparing results for different models we find that nonlinear Hookean elasticity is adequate for most capsules. If series of images are available, these moduli can be evaluated as a function of the capsule volume to analyze hysteresis or aging effects depending on the deformation history or to detect viscoelastic effects for different volume change rates. An additional wrinkling wavelength measurement allows the user to determine the bending modulus, from which the layer thickness can be derived. We verify the method by analyzing several materials, compare the results to available rheological measurements, and review several applications. We make the software available under the GPL license at github.com/jhegemann/opencapsule. [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]