Axisymmetric Drop Shape Analysis Method Research Articles

In-situ measurement of interfacial tension: Further insights into effect of interfacial tension on the kinetics of CO2 hydrate formation

Hydrate-based CO2 capture and sequestration has been viewed as one of the most attractive technologies due to its environmental friendliness and relatively moderate temperature and pressure conditions. In current work, the kinetics of CO2 hydrate formation was investigated at pressures of 2.5–3.5 MPa and temperatures of 274.15–279.15 K and the effect of interfacial tension on CO2 hydrate formation rate was discussed. The interfacial tension between CO2 and aqueous solution during the hydrate formation was in-situ measured via an axisymmetric drop shape analysis method. Under the experimental conditions of this study, the interfacial tension decreased significantly with increasing pressure, while it did not vary obviously with temperature. As to SDS aqueous solutions, the interfacial tension at the gas-liquid interface decreased approximately 12% and 42% when the SDS concentration was 100 ppm and 500 ppm, respectively. In addition, CO2 hydrate formation rate ascended by 59% and 102% when the SDS concentration is 100 ppm and 500 ppm, respectively. Furthermore, a good linear relationship was observed between hydrate formation rate and ratio of driving force to interfacial tension (R2 = 0.9211). This work provides a new idea for rapid estimation of hydrate formation rate.

On the size-dependent behavior of drop contact angle in wettability alteration of reservoir rocks to preferentially gas wetting using nanofluid

Wettability alteration of rock surfaces toward gas wetting have been recognized as a practical approach for maximizing the production from the gas condensate reservoirs. Most of the reported work in this area applied the so called sessile drop contact angle measurement technique to examine the change in wetting state of a surface. However, the size-dependent wetting behavior of drop which could affect the exact determination of wettability and wettability changes was not well discussed in the previous studies. Therefore, in this work, the size dependency of contact angle for four different liquid-solid-gas systems; i.e., water-calcite-air, water-treated calcite-air (nanofluid treated calcite), oil-calcite-air, and oil-treated calcite-air, were investigated. To provide a better understanding of the displacement of reservoir gas and liquid following the wettability alteration process, a dynamic contact angle measurement approach was designed. The effect of drop size and surface roughness on the wetting state of calcite rock samples at the initial and altered condition of wettability was then investigated through experimental and modeling approaches. The surface roughness of calcite samples and drop contact angles were determined using Atomic Force Microscopy and Low bond axisymmetric drop shape analysis method, respectively. Analysis of results demonstrated that size dependency of contact angle imposed an error as high as 22.2° on the measurement of achieved wettability alteration in the case of water-calcite/treated calcite-air system. On the basis of calculated pseudo-line tension from the modeling schemes, it was also revealed that the surface roughness had a significant impact on wettability measurements. Both low and high pseudo-line tension values of order 10−9 and 10−6 N, respectively, could dictate some level of errors in contact angle measurements. However, high values were supposed to have a considerable degree of importance in gas condensate applications. The results of this study could provide a better understanding of the contact angle measurement experiments for petroleum engineers to evaluate the wettability alteration schemes.

Shape analysis of a sessile drop on a flat solid surface

ABSTRACTThe shape of a liquid drop on a flat solid surface is examined by using the minimization of free energy. This free energy consists of the contributions from the gravitational potential energy of the drop mass and the energy of the interfacial surfaces. The minimization of free energy is solved by the Lagrange multipliers method keeping the drop volume constant. The generic differential equation for the shape of the drop and a relation among the contact angles (Young equation) are determined as a result of the minimization via the variation of a functional and by imposing the condition for equilibrium of the drop. The differential equation is solved analytically around the origin of the selected coordinate axis and for two special cases. For the general solution, the differential equation is solved numerically to obtain the general shape of the sessile drop. Furthermore, a new Axisymmetric Drop Shape Analysis method is proposed for non–wetting sessile drop.

Surface tension measurement with a smartphone using a pendant drop

In this study, a novel powerful mobile surface tension instrument based on a smartphone (mobile-phone or handy) is developed. Axisymmetric drop shape analysis method was implemented on a smart-phone (mobile-phone) for surface or interfacial tension measurements. The novelty of this work is that we have been able to implement all the needed calculations on a smartphone, and used the smartphone hardware for image acquisition and display purposes. As such we have been able to create an instrument that is significantly more economical compared to current systems; also it is compact, and can be mobile for field work. It is shown that the accuracy of our method can be 0.001% with ideal synthetic drop profiles (750 synthetic droplets representing a wide range of surface tension values were used). The performance of this instrument was also compared with a high-end commercial surface tension measurement instrument. We used various liquids (from high to low surface tension), and show that our instrument and the developed methodology can provide surface tension measurements as precise and accurate as current commercial instruments.

Effects of temperature, pressure and fluid composition on hydrocarbon gas - oil interfacial tension (IFT): An experimental study using ADSA image analysis of pendant drop test method

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.

Interfacial Tension Effect on Cell Partition in Aqueous Two-Phase Systems

Aqueous two-phase systems (ATPS) provide a mild environment for the partition and separation of cells. We report a combined experimental and theoretical study on the effect of interfacial tension of polymeric ATPS on the partitioning of cells between two phases and their interface. Two-phase systems are generated using polyethylene glycol and dextran of specific properties as phase-forming polymers and culture media as the solvent component. Ultralow interfacial tensions of the solutions are precisely measured using an axisymmetric drop shape analysis method. Partition experiments show that two-phase systems with an interfacial tension of 30 μJ/m(2) result in distribution of majority of cells to the bottom dextran phase. An increase in the interfacial tension results in a distribution of cells toward the interface. An independent cancer cell spheroid formation assay confirms these observations: a drop of the dextran phase containing cancer cells is dispensed into the immersion polyethylene glycol phase to form a cell-containing drop. Only at very small interfacial tensions do cells remain within the drop to aggregate into a spheroid. We perform a thermodynamic modeling of cell partition to determine variations of free energy associated with displacement of cells in ATPS with respect to the ultralow interfacial tensions. This modeling corroborates with the experimental results and demonstrates that at the smallest interfacial tension of 30 μJ/m(2), the free energy is a minimum with cells in the bottom phase. Increasing the interfacial tension shifts the minimum energy and partition of cells toward the interfacial region of the two aqueous phases. Examining differences in the partition behavior and minimum free energy modeling of A431.H9 cancer cells and mouse embryonic stem cells shows that the surface properties of cells further modulate partition in ATPS. This combined approach provides a fundamental understanding of interfacial tension role on cell partition in ATPS and a framework for future studies.

Experimental study on the surface characteristics of polymer melts

With the dimensional reduction of micro-structure, the surface effects greatly influence the dimension precision and surface quality of micro-structure during micro-processing of thermoplastic materials. In this paper, the surface characteristics of several thermoplastic materials usually used in industry are investigated. The Axisymmetric Drop Shape Analysis (ADSA) method in nitrogen gas (N 2) environment is adopted to measure the surface tensions of polymethylmethacrylate (PMMA), polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE) melt under different temperatures. The theoretical relationships between surface tension and temperature of polymer melts are obtained. The comparisons are made among different polymer melts according to the relationship of molecular dynamics. The static contact angles between polymer melts (PMMA, PS) and mold materials (silicon, nickel) under different temperatures are also measured in N 2 environment using the ADSA method. The results indicate that the contact angle decreases with the increase of temperature and/or the increase surface energy of the mold material. This study provides relevant data to support for micro-injection molding and to optimize processing parameters.

On the use of liquid bridges as tensiometers

The measurement of the surface tension associated with the interface between two immiscible liquids requires the use of non-trivial experimental procedures. The Axisymmetric Drop Shape Analysis (ADSA) method has been used by several laboratories because it provides good results over a wide range of situations. In the ADSA method, the theoretical prediction for a drop equilibrium shape is fitted to experimental contours to find the surface tension value. Based on this idea, in the present paper the advantages of using liquid bridges instead of drops are discussed. The sensitivity of both the drop and the liquid bridge equilibrium shapes to the surface tension value is studied from the numerical solution of the Young-Laplace equation. The results show that the use of liquid bridges could lead to better results, especially if non-axisymmetric configurations are considered. The optimal values of the experimental parameters are found, and a numerical algorithm developed to calculate the surface tension value from a liquid bridge photograph is described.

Effect of Surfactants on Wetting of Super-Hydrophobic Surfaces

The effect of surfactants on wetting behavior of super-hydrophobic surfaces was investigated. Super-hydrophobic surfaces were prepared of alkylketene dimer (AKD) by casting the AKD melt in a specially designed mold. Time-dependent studies were carried out, using the axisymmetric drop shape analysis method for contact angle measurement of pure water on AKD surfaces. The results show that both advancing and receding contact angles of water on the AKD surfaces increase over time ( approximately 3 days) and reach the values of about 164 and 147 degrees , respectively. The increase of contact angles is due to the development of a prickly structure on the surface (verified by scanning electron microscopy), which is responsible for its super-hydrophobicity. Aqueous solutions of sodium acetate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and n-decanoyl-n-methylglucamine were used to investigate the wetting of AKD surfaces. Advancing and receding contact angles for various concentrations of different surfactant solutions were measured. The contact angle results were compared to those of a number of pure liquids with surface tensions similar to those of surfactant solutions. It was found that although the surface tensions of pure liquids and surfactant solutions at high concentrations are similar, the contact angles are very different. Furthermore, the usual behavior of super-hydrophobic surfaces that turn super-hydrophilic when the intrinsic contact angle of liquid on a smooth surface (of identical material) is below 90 degrees was not observed in the presence of surfactants. The difference in the results for pure liquids and surfactant solutions is explained using an adsorption hypothesis.

The solid surface free energy calculation: II. The limits of the Zisman and of the “equation-of-state” approaches

This paper follows the “defense” of the Good–van Oss–Chaudhury (GvOC) acid–base approach made in Part I and carries out a detailed analysis of the Zisman critical surface energy and, mainly, of the Neumann equation-of-state (EQS) theory. The analysis is made on both a “practical” and a theoretical basis, trying to highlight the acceptable fitting results of axisymmetric drop shape analysis (ADSA) methods and their independence of the assumed thermodynamic foundations of EQS. Some new and original criticisms of the EQS approach are raised and it is shown that other purely semiempirical models, represented by different fitting equations with the same number of parameters, can represent the data measured by ADSA method with the same goodness as EQS. The equation of state appears as one of many semiempirical approaches for the evaluation of surface free energy of solids. Independent of the previous analysis, the criteria used in ADSA measurements are evaluated and some comments made on them.