Axisymmetric Drop Shape Analysis Research Articles

Measurement of surface tension and contact angle of solar salt at high temperature with axisymmetric drop-shape analysis

As interfacial properties of common materials enabled many practical applications, those for molten salts would do the same. While their properties are important for the safety and design of thermal energy storage and modular nuclear reactors, they are hard to obtain because of the measurements done in confined spaces inside a furnace at high temperatures. Moreover, a small amount is preferred due to the high cost of salts. To overcome these limitations, an accurate method to measure surface tension, γ, and contact angles, θ, of molten salts were sought. As a steppingstone toward using more complex salts, solar salt was used because of safety and availability. Because molten salts typically show low θ, they provide large errors by incorrectly predicting volume and/or geometry. Therefore, various solid materials on which solar salts show high θ were sought and boron nitride was identified to provide θ > 60°. Later, solar salt sessile droplets were analyzed with the axisymmetric drop-shape analysis (ADSA) method to measure γ over a temperature range, which showed a good agreement with literature. Thus, this paper sets an experimental protocol that can be possibly used for other complex and hazardous salts that are scarcer and hard to handle. This paper extends the current knowledge by identifying solid surfaces on which the salts show low wettability for accurate measurements. Compared to other accurate methods, such as maximum bubble pressure method, this method used a small amount of salt (< 100 μL) while keeping a high accuracy.

Surface Roughness, Dynamic Wettability, and Interphase of Modified Melamine Formaldehyde-Based Adhesives on Jabon Wood.

The surface roughness and wettability of wood are critical aspects to consider when producing laminated wood products with adhesive applications. This study aims to investigate the surface roughness and dynamic wettability of Jabon wood in the presence of melamine formaldehyde (MF)-based adhesives. Commercial MF adhesives (MF-0) and modified MF adhesives (MF-1) were applied to Jabon wood, which includes tangential (T), radial (R), and semi-radial (T/R) surfaces. The surface roughness of Jabon wood was assessed using a portable stylus-type profilometer. The low-bond axisymmetric drop shape analysis (LB-ADSA) method was employed to identify the contact angle (θ) of the MF-based adhesives on Jabon wood. The wettability was determined by evaluating the constant contact angle change rate (K value) using the Shi and Gardner (S/G) model. Dynamic mechanical analysis (DMA) was employed to investigate the viscoelastic characteristics of the interphase analysis of the wood and MF-based adhesives. The roughness level (Ra) of the Jabon board ranged from 5.62 to 6.94 µm, with the T/R having a higher level of roughness than the R and T. MF-0 exhibited a higher K value (0.262-0.331) than MF-1 (0.136-0.212), indicating that MF-0 wets the surface of Jabon wood more easily than MF-1. The wood-MF-0 interphase reached a maximum stiffness of 957 N/m at 123.0 °C, while the wood-MF-1 had a maximum stiffness of 2734 N/m at 110.5 °C. In addition, the wood-MF-0 had a maximum storage modulus of 12,650 MPa at a temperature of 128.9 °C, while the wood-MF-1 had a maximum storage modulus of 22,950 MPa at 113.5 °C.

Are all yeast biosurfactants really capable of lowering surface tension below 30 mN/m ?

The study discusses pitfalls in attempts to determine reliable surface tension values for the culture media and their extracts for two biosurfactant-producing yeast strains: Rhodotorula graminis and Rhodotorula babjevae. The values obtained from an Axisymmetric Drop Shape Analysis (ADSA) tensiometer showed systematically more and more shallow dynamic surface tension decays, suggesting a deterioration of their surface activity. The rate of this apparent surface activity loss was shown to depend on the sample history, with slower changes observed in vigorously shaken samples. On the other hand, the force-based Wilhelmy plate method provided apparently stable surface tension values of the order of 30 mN/m, in accordance with numerous previous literature reports on similar yeast biosurfactants. Both observations can be justified by the presence of an oil emulsified by biosurfactants produced by the yeast. We show that the odd (apparent) surface tension results are in fact the measurement artifacts resulting from slow demulsification and subsequent oil-spreading assisted by the yeast biosurfactants. The apparent surface tension reduction is thus indeed caused by the presence of biosurfactants, but its value does not represent their real adsorption in a thermodynamic sense. Consequently, the often reported in the literature very low surface tension values for the yeast culture media, of the order of 30±5 mN/m, should be treated with caution, especially if the emulsion stabilized with the biosurfactant had not been fully destabilized prior to the measurement.

The shape of things to come: Axisymmetric drop shape analysis using deep learning

HypothesisIn the traditional approach to Axisymmetric Drop Shape Analysis (ADSA), the determination of surface tension or interfacial tension is constrained by computational speed and image quality. By implementing a machine learning-based approach, particularly using a convolutional neural network (CNN), it is posited that analysis of pendant drop images can be both faster and more accurate. ExperimentsA CNN model was trained and used to predict the surface tension of drop images. The performance of our CNN model was compared to the traditional ADSA, i.e. direct numerical integration, in terms of precision, computational speed, and robustness in dealing with images of varying quality. Additionally, the ability of the CNN model to predict other drop properties such as Volume and Surface Area was evaluated. FindingsOur CNN demonstrated a significant enhancement in experimental fit precision, predicting surface tension with an accuracy of (+/-) 1.22×10−1 mN/m and at a speed of 1.50 ms−1, outpacing the traditional method by more than 5×103 times. The model maintained an average surface tension error of 2.42×10−1 mN/m even for experimental images with challenges such as misalignment and poor focus. The CNN model also demonstrated showcased a high degree of accuracy in determining other drop properties.

Prediction of multi-bead profile of robotic wire and arc additive manufactured components recursively using axisymmetric drop shape analysis

ABSTRACT Dimension prediction of robotic wire and arc additive manufacturing (WAAM) part is fundamentally dependent on the modelling accuracy of single-bead profile and its subsequent overlapping ones. Current multi-bead overlapping models are still not capable of describing the flatten valley area of WAAM parts. This paper proposes a new recursive model, based on coordinate transformation and axisymmetric drop shape analysis (ADSA), to predict multi-bead overlapping profiles. First, a single-bead profile model for WAAM is established based on ADSA, followed by detailed description of conventional and proposed modified recursive ADSA profile model. The properties of developed recursive ADSA model are then investigated to reveal the effects of overlapping ratio and single-bead aspect ratio. Finally, multi-bead overlapping deposition experiment is carried out to validate the model feasibility. The results show that the modified recursive ADSA model is more accurate than the conventional one for its better accountability of valley areas. It is also indicated that the modified recursive ADSA model is suitable for the robotic WAAM process. The research outcome is beneficial to improving the forming accuracy of WAAM parts and geometry prediction of other additive manufactured products.

Adsorption kinetics and dilatational rheology of plant protein concentrates at the air- and oil-water interfaces

Adsorption kinetics and dilatational rheology of plant protein concentrates at the air- and oil-water interfaces were investigated at pH 7.0 in 100 mM NaCl. Three interfaces (air-water, triglyceride-water and terpene-water) and four protein concentrates (soy, pea, mung bean and rice) were examined. The dynamic interfacial properties were monitored by axisymmetric drop shape analysis. Kinetic modelling of the early and advanced stages of protein adsorption was carried out using the Ward-Tordai and Graham-Philips thermodynamic approaches. Construction of surface pressure master curves revealed a pseudo equilibrium plateau for legume proteins of ∼20, 12, and 22 mN/m at the air, triglyceride and terpene interfaces, respectively. In contrast, rice proteins have a lower capacity to increase the surface pressure at the oil interfaces (<15 mN/m). Data modelling revealed that diffusion is mostly independent of the protein composition, but protein rearrangement at the interfaces was faster at the oil than at the air interfaces. Dilatational rheological measurements revealed more elastic films at the air than at oil interfaces, with the dilatational storage modulus reaching values up to 37 mN/m. The least elastic films were formed at the terpene interfaces, with storage moduli being <25 mN/m for all isolates investigated. Lissajous plot construction revealed a strain-hardening behaviour of films upon compression and strain-softening on extension, the magnitude of which follows the order air > terpene > triglyceride. Overall, results show that botanical source and subphase composition are critical in selecting the optimum stabilisation strategy in multiphasic foods using plant proteins.

A New Method for Determining Interfacial Tension: Verification and Validation

Surface tension is a meaningful parameter influencing boiling and condensation in macroscopic scale, in confined spaces, or for nanofluids; it further affects boiling with surfactants. Surface, or interfacial, tension is an important property in the research into increasing heat transfer, enhancing efficiency of photovoltaic systems, improving engine operation, or forming drugs or polymers. It is often determined using axisymmetric drop shape analysis based on the differential equations system formulated by Bashforth and Adams. The closed-form expression of the interface shape states the radii defining the bubbles are the negative numbers, which causes the temperature profile drops along the heat transfer direction, e.g., in the Wiśniewski formulas for the temperature in the vapor bubbles; moreover, the drop, or bubble, possesses only one main radius of curvature, which may reduce the number of the unknowns and equations in the Bashforth and Adams algorithm. An alternative method applying the closed-form expression for the droplet shape is validated for the water (denser) drop flowing down in octane (the lighter liquid); its spare equation is used for verifying the outcomes.

A novel experimental-theoretical method to improve MMP estimation using VIT technique

In the gas injection process for enhanced oil recovery (EOR), the interfacial tension (IFT) and the minimum miscibility pressure (MMP) are two key parameters for evaluating the miscibility condition. The vanishing interfacial tension (VIT) method is an efficient method for measuring the interfacial tension of oil and gas versus pressure and can estimate the minimum miscibility pressure. A problem in the VIT test of a hydrocarbon system is that during this test, oil and gas densities vary due to the composition variation of both phases. But in the conventional interfacial measurement, this issue is ignored, and the initial and non-equilibrium densities are utilized for estimating the interfacial tension. In this study, it is tried to nominate a novel method for estimating the most accurate value of IFT. Hence, for this purpose, first a VIT test has been performed for a hydrocarbon gas and a live oil system and using the axisymmetric drop shape analysis (ADSA), the oil droplet has been analyzed, and the geometry factor, which has been defined in this paper, was calculated. After that, the volume of oil droplet and the gas have been measured and were used for vapor-liquid-equilibrium (VLE) or flash calculation using Peng-Robinson equation of state (PR-EOS). By VLE calculation fulfillment, the final composition of two phases is determined. Eventually, the densities of two phases have been calculated for VIT modification. Finally, the IFT results were plotted versus the pressure steps, and the MMP were estimated. Furthermore, to evaluate the results of VIT test and its modification, slim tube displacement (STD) test, which is known as the most reliable laboratory method for measuring the MMP in the oil industry, has been performed. The results show that the values obtained for MMP values for the VIT test with the corrected densities which are lower than the values obtained for the VIT test for not-corrected densities and the suggested experimental-theoretical method has been estimated the MMP more accurately. The MMP obtained by VIT test and modified VIT method was equal to 3858.72 and 3722.0 psi, respectively. Meanwhile, the slim tube test has measured the MMP equal to 3667.13 psi.

Quantifying the spontaneous emulsification of a heavy hydrocarbon with the presence of a strong surfactant

The rate of spontaneous emulsification phenomenon of heavy hydrocarbon in a non-ionic surfactant solution was studied experimentally. The size of a diminishing oil droplet was monitored photographically with the presence of an electrolyte. The obtained data were analysed using Axisymmetric Drop Shape Analysis (ADSA) software to measure the flux of emulsion droplets formed spontaneously. In contrast to all previous reports, a stable layer confined between the oil and aqueous solution was observed with a relatively constant thickness, ∼ 100 µm. The result shows that the rate of emulsification is reduced with the presence of electrolytes and is inversely correlated to its concentration. Considerable variation of the influence of electrolyte type on the kinetic of the spontaneous emulsification process for a given condition has been observed. Consequently, a new diffusion-control mechanism has been proposed, creating an extendable model to describe the process theoretically. The experimental data of emulsifying hexadecane in Triton X-100 micellar solution with the presence of potassium chloride and sodium chloride was well fitted to the proposed model. The results have important implications for surfactant-based cleaning processes.

Interfacial Dilational Rheology of Molecular Films in DC Electric Fields.

We present a novel technique to measure the interfacial dilational rheology of a molecular film adsorbed at a water-oil interface in a direct-current (dc) electric field. A film of a highly polar subfraction of asphaltenes was allowed to adsorb at a water drop interface, surrounded by an organic phase and subjected to a dc electric field. The measurements involved calculations of the dynamic interfacial tension (IFT), while the drop was sinusoidally oscillated, using our in-house axisymmetric drop shape analysis (ADSA) algorithm adapted for electric fields. The amplitude of the IFT waveform over equilibrium IFT and the phase difference from the applied area oscillations were used in the estimation of surface moduli. The asphaltene films were found to become more elastic on increasing bulk concentrations and electric field strengths. However, the effect was not monotonous and observed to be governed by combinations of these parameters. The Lucassen-van den Tempel (LVDT) model was used to further elucidate the experimentally obtained interfacial dilational moduli.

Field Application and Experimental Investigation of Interfacial Characteristics of Surfactant and CO2 Alternative Injection.

CO2 injection and water alternating gas (WAG) injection are crucial to improve the oil recovery method and have optimized development in numerous oil fields. Many issues, such as gas channeling, water clogging, and a shortage of gas injection capacity, are addressed in the studies. Considering these conflicts, we suggest in this work a unique method of surfactant alternating gas (SAG) injection. Additionally, axisymmetric drop shape analysis and other approaches are utilized to explore the interface properties of a variety of systems, including CO2/carbonated water/water/surfactant/oil systems. SAG injection combines the advantages of surfactant and WAG injection. Although CO2 molecules have an effect on surfactant aggregation at the oil–water interface in the SAG system, carbonated water has little effect on surfactant performance in lowering oil–water interface tension. Pilot studies reveal that a SAG ratio of 3:2 at 74 °C and 0.5 wt% concentration significantly improves oil recovery.

Use of a Geometric Parameter for Characterizing Rigid Films at Oil-Water Interfaces.

Interfacial tension and dilatational rheology are often used to characterize the mechanical response of a liquid interface using axisymmetric drop shape analysis (ADSA). It is important to note that for systems dominated by adsorption/desorption of surfactants, the contributions of extra mechanical stresses are negligible; thus, the Young-Laplace equation remains valid. However, for interfaces dominated by extra stresses, as in the case of particle monolayers or asphaltenes that clearly exhibit a skin (a rigid film), the nature of the elastic response is fundamentally different and the validity of the equation is questionable. Calculation of the interfacial tension and dilatational elasticity using drop shape analysis depends critically on the drop shape following the Young-Laplace equation. If the interface becomes more like a solid, the drop shape will deviate from being purely Laplacian. Indeed, the drop will exhibit a wrinkled surface as collapse continues. The geometric parameter RV/A, defined as the ratio (dV/V)/(dA/A) with V is the volume of the drop and A is the area of the interface), allows one to measure the deviation of the drop shape from purely Laplacian. For a simple interface (pure liquids or surfactant solutions), RV/A is quite close to the theoretical value of 1.5 of a perfect sphere. Nevertheless, if the molecules adsorbed at the interface begin to interact strongly, the ratio can vary. In the limit of long-time-scale experiments, RV/A of some drops approaches 2. We studied the evolution of the parameter RV/A for different systems, from simple to complex, as a function of oscillation frequencies and amplitudes of drop volume. The results obtained were compared to the values of the interfacial moduli and drop shape behavior to better characterize the regime change.

The role of supercritical carbon dioxide in modifying the phase and interfacial properties of multiphase systems relevant to combined EOR-CCS

Deep knowledge of interfacial properties and phase behavior is decisive for designing processes of oil recovery and carbon storage. The current study aims at investigating the impact of CO2 on relevant properties in binary and ternary model systems. Experimental density data of aqueous and hydrocarbon phases along with their CO2-saturated solutions are reported at pressures up to 250 bar, representing a feasible range of CO2 utilization in EOR and CCS. The interfacial tension (IFT) is determined in binary CO2-oil systems and ternary CO2-water/brine-oil systems as a function of pressure at 60 °C using the Axisymmetric Drop Shape Analysis (ADSA) technique. It is confirmed that IFT is a decreasing function of pressure in both systems. In ternary systems, CO2 acts as an amphiphilic molecule between the coexisting immiscible phases, reducing IFT as the pressure is increased. The impact of salt concentration on the IFT in ternary systems has also been investigated. The IFT is found to acquire its highest value at high salt concentrations. Additionally, a method for quantifying the real volumetric expansion (swelling) of the oil is proposed, and a systematic investigation of volumetric expansion as a function of pressure and aqueous phase composition is performed. It is shown that the swelling increases with pressure and decreases with salinity. Our findings confirm that the solubility of CO2 in the aqueous phase impacts both IFT and volumetric expansion. Lastly, a new approach to infer CO2 solubility in the oleic phase, based on volumetric expansion and density data, is developed and presented.

Experimental investigation of interfacial tension and oil swelling for asphaltenic crude oil/carbonated water system

The phenomenon of oil swelling at the oil-carbonated water (CW) system could be an important mechanism during the water alternating gas (WAG) injection process. Nevertheless, the study of the main mechanisms during water flooding (WF) is a complex topic that has not been well revealed so far, especially for asphaltenic crude oil (ACO) systems. Hence, the main goal of this experimental work is to determine the influence of carbon dioxide (CO2) within the water phase in the interfacial tension (IFT) between water and crude oil for an extensive range of pressures between 400 psi and 2000 psi (i.e. 2.76–13.79 MPa), under two temperatures of 313.15 and 323.15 K (i.e. 40 and 50 °C) by axisymmetric drop shape analysis (ADSA) method. The experimental results demonstrate that the water/ CW and crude oil IFTs decline with time. The value of dynamic IFT (DIFT) between CW and crude oil decreased about 6 mN/m in comparison with the oil–water DIFT. As a result of the CO2 solubility, the crude oil droplet swells with increasing pressure. When the temperature rises, the effects of increasing entropy phenomena and decline of liquids density is dominant compared to the solubility of CO2. Thus, the volume of oil droplet increases with temperature, unexpectedly. In addition, as thetemperature increases the water/CW-Oil IFT is slightly reduced over a wide range of pressure evaluated. Nevertheless, there is a slight increase as the pressure increases for the water–oil system. According to the predicted results, interfacial tension of the CW-oil system declines with increasing pressure until the solubility of CO2 is reached to a maximum value and then approximately remains changeless.

In English

Wettability is of pivotal importance in many areas of science and technology, ranging from the extractive industry to development of advanced functional materials and biomedicine problems. An increasing interest to wetting-related phenomena stimulates impetuous growth of research activity in this field. The presented review is aimed at the cumulative coverage of issues related to wettability and its investigation. It outlines basic concepts of wetting as a physical phenomenon, methods for its characterisation (with the emphasis on sessile drop techniques), and performances of contemporary instrumentation for wettability measurements. In the first section, physics of wettability is considered. The intermolecular interactions related to wetting are classified as dependent on their nature. Thus, discussion of interactions involving polar molecules covers permanent dipole - permanent dipole interactions and freely rotating permanent dipoles. Consideration of interactions resulting from the polarization of molecules includes interactions between ions and uncharged molecules, Debye interactions, and London dispersion interactions. Hydrogen bonds are discussed separately. The second section deals with the issues related to surface tension and its effect on shaping the surface of a liquid brought in contact with a solid body. The relationship between the surface tension and the contact angle as well as equations that quantify this relationship are discussed. The Young–Laplace equation governing the shape of the drop resting on the surface is analysed. The third section is devoted to the experimental characterization of surface wettability and the underlying theoretical analysis. Particular attention is paid to the method known as the Axisymmetric Drop Shape Analysis (ADSA). Principles of automated determination of relevant physical values from experimental data are briefly discussed. Basics of numerical techniques intended for analysing the digitized image of the drop and extracting information on surface tension and contact angle are outlined. In the fourth section, an overview of commercially available instrumentation for studying wettability and the contact angle measurements is presented. The prototype contact angle analyser designed and manufactured at the ISP NASU is introduced.

Refractive Index Effects in Pendant Drop Tensiometry

An optical model is established to investigate the effects of refractive index changes on the measurement of interfacial tension by the pendant drop method with axisymmetric drop shape analysis. In such measurements, light passes from the pendant drop through a surrounding bulk phase, an optical window and air to reach the lens of the camera system. The relation between object and image size is typically determined by calibration and, if the refractive indices of any of the materials in the optical path change between calibration and measurement, a correction should be made. The simple model derived in this paper allows corrections to be calculated along with the corresponding contribution to the overall uncertainty of the interfacial tension. The model was verified by measurements of the interfacial tension between decane and water under two different calibration conditions. Neglect of the correction was shown to cause errors of up to 6 % when the bulk phase changed from air (during calibration) to water (during measurements) and of about 9 % when the system was calibrated without the optical window used for the final measurements. The refraction changes due to high pressures and supercritical fluid states can also lead to measurement errors. The proposed model can facilitate more accurate interfacial tension measurements and reduce the amount of repetitive calibration work required.

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.

Axisymmetric Drop Shape Analysis using a low-cost home-made setup

Surface science and engineering gained an increasing importance in recent years, due to the potential benefit they can offer in many applications. An important parameter in this field is the surface wettability, that is in general evaluated by the contact angle. One of the most accurate techniques to measure this quantity is the axisymmetric drop shape analysis, based on the fitting of the theoretical Laplace-Young profile to the contour of experimental drops. In this paper the performance of a simple, low-cost setup – that can be built “at home” – to apply this technique is assessed, also including a detailed analysis about the influence of the most important parameters to set in the procedure. The latter aspect was evaluated by using computer-generated drop profiles and pictures, to have “realistic” images, but for drops of known properties. The experimental setup was built using a desk, a table lamp, a medical syringe, a support for the samples and a “bridge” camera. Measurements were performed on smooth and rough textile surfaces and the results were compared with previous measurements taken with a professional setup. From the comparison it can be affirmed that the performance of the home-made setup is very satisfactory.

Microfluidic protein detection and quantification using droplet morphology

Sensitive, inline detection of proteins is required for post-chromatographic analyses in proteomics, cell-based assays, and drug discovery workflows. Among the common inline methods, post-column derivatization requires chemical labels, while label-free methods are either expensive (mass spectrometry) or have limited sensitivity at small length scales (UV–Vis). This paper presents a label-free detection technique based on the concept that dissolved proteins can function as surfactants and decrease the dynamic interfacial tension (IFT) of an immiscible (water–oil) interface. Existing methods for measuring IFT, such as axisymmetric drop shape analysis (ADSA), operate in batch mode and are not suitable for continuous detection. Here we show that a microfluidic flow-focusing droplet generator operating at a frequency of > 100 Hz can track IFT changes continuously, with high temporal resolution and small detection volumes. Variations in protein concentration alter the size and shape of the drops/plugs formed, and these changes can be quantified in time using a high-speed camera and in-house image processing software. Moreover, the continuously refreshing interface alleviates issues related to surface aging. Two applications are demonstrated: (1) direct injection of a single protein into a microfluidic chip. (2) post-column detection of protein mixtures separated by high performance size exclusion chromatography (SEC HPLC). Of interest, the dynamic range of protein (bovine serum albumin, BSA) was 50 -104 μg/ml without using HPLC unit. The lowest limit of detection without HPLC unit was ~ 1 μg/ml of thyroglobulin protein in a 1 nl droplet, which equates to 1 fg of total protein. When used as a detector, the aforementioned detection method offered a sensitivity of six orders of magnitude higher than conventional UV–VIS detectors.

A machine learning approach for estimating surface tension based on pendant drop images

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.