Drop Surface Tension Research Articles

Effect of curved surfaces on the impacting nano-droplets and their shape control: A molecular dynamics simulation study

Considerable non-flat solid substrates are widely-existed where liquid drops inevitably impact on their surfaces, precise control of shape evolution of these impacting drops on such surfaces is desired in nature and some advanced technologies. Herein, a common but less-concerned substrate with curvature is used to investigate the impact dynamic of liquid metals by performing molecular dynamics (MD) simulations, which exhibits anisotropic impact behavior with distinct spreading and retraction in the two perpendicular directions and results in an elongated strip-like shape during shape evolution. The effects of curvature diameter, liquid properties (drop size and surface tension), and impact velocity are deeply demonstrated by quantitatively comparing the value of length ratio that is defined to describe the drop asymmetrical impact behaviors. This work helps advance our understanding of how the liquid metal evolves after impact on the curved surface and provides some feasible strategies to control its deposition and shape for some potential applications, such as 3D printing techniques, special coating, as well as drop-casting process.

The influence of the drop formation rate at spreading over a microstructured surface on the contact angle

The article presents the experimental dependences of a macro-contact angle and the diameter of a distilled water drop spreading over solid microstructured surface on surface average roughness (Ra) and fluid flow rate (G). It has been found that at changing G from 0.005 to 0.02 ml/s, the contact angle decreases, and at a liquid flow rate over 0.02 ml/s, it increases. With small values of G (0.005−0.01 ml/s), the drop diameter grows throughout the spreading process. In the range of G from 0.02 to 0.16 ml/s at the final stage of spreading, the contact line pinning, i.e., the diam-eter constancy, has been detected. The hypothesis about the mechanism of the pinning process has been formulated: it is due to the zero sum of all forces acting on the drop (inertia, viscosity, friction, gravity, and surface tension).

Predictive Model of Supercooled Water Droplet Pinning/Repulsion Impacting a Superhydrophobic Surface: The Role of the Gas-Liquid Interface Temperature.

Dynamical analysis of an impacting liquid drop on superhydrophobic surfaces is mostly carried out by evaluating the droplet contact time and maximum spreading diameter. In this study, we present a general transient model of the droplet spreading diameter developed from the previously defined mass-spring model for bouncing drops. The effect of viscosity was also considered in the model by definition of a dash-pot term extracted from experiments on various viscous liquid droplets on a superhydrophobic surface. Furthermore, the resultant shear force of the stagnation air flow was also considered with the help of the classical Homann flow approach. It was clearly shown that the proposed model predicts the maximum spreading diameter and droplet contact time very well. On the other hand, where stagnation air flow is present in contradiction to the theoretical model, the droplet contact time was reduced as a function of both droplet Weber numbers and incoming air velocities. Indeed, the reduction in the droplet contact time (e.g., 35% at a droplet Weber number of up to 140) was justified by the presence of a formed thin air layer underneath the impacting drop on the superhydrophobic surface (i.e., full slip condition). Finally, the droplet wetting model was also further developed to account for low temperature through the incorporation of classical nucleation theory. Homogeneous ice nucleation was integrated into the model through the concept of the reduction of the supercooled water drop surface tension as a function of the gas-liquid interface temperature, which was directly correlated with the Nusselt number of incoming air flow. It was shown that the experimental results was qualitatively predicted by the proposed model under all supercooling conditions (i.e., from -10 to -30 °C).

Adsorption of C14EO8 at the interface between its aqueous solution drop and air saturated by different alkanes vapor.

The dynamic and equilibrium surface tension for drops of aqueous C14EO8 solutions at the interface to pure air or pentane, hexane, heptane and toluene saturated air, and the dynamic surface tension of pure water at these interfaces are presented. Two theoretical models were employed: both assuming a diffusion controlled adsorption of the surfactant, and either a diffusion or kinetic barrier governed adsorption of the alkanes. The experimental results are best described by the model which implies a diffusion control for the C14EO8 molecules and the existence of a kinetic barrier for the alkane molecules. The desorption of alkanes from the surface layer after equilibration and their subsequent removal from the measuring cell was studied as well. The desorption process was shown to be slow for heptane and hexane. However, for the pentane vapor the desorption is quite rapid, and after the desorption commences the surface tension becomes equal to that at the interface with pure air.

Tear fluid-eye drops compatibility assessment using surface tension

Objective: To evaluate the compatibility of commercially available eye drop surface tension with the tear film physiological range and to characterize commonly used ophthalmic excipients in terms of their surface activity under eye-biorelevant conditions.Significance: There are a number of quality requirements for the eye drops (e.g. tonicity, pH, viscosity, refractive index) that needs to comply with the physiological parameters of the eye surface. However, the adjustment of surface tension properties of the eye drops to the normal range of surface tension at the air/tear fluid interface (40–46 mN/m) has received rather less attention thus far. Yet, the surface tension at the air/tear fluid interface is of vital importance for the normal function of the eye surface.Methods: The surface tension compatibility of the isotonic aqueous solutions of commonly used ophthalmic excipients as well as 18 approved eye drops with the tear fluid have been evaluated using surface tension method.Results: Each ophthalmic ingredient including the preservatives, solubilizing agents and thickening agents can influence the surface tension of the final formulation. In case of complex ophthalmic formulations one should also consider the possible interactions among excipients and consequent impact on overall surface activity. Out of 18 evaluated eye drops, three samples were within, 12 samples were below and three samples were above the physiological range of the tear fluid surface tension.Conclusions: Our results provide a rationale for clinical studies aiming to assess the correlation between the eye drops surface tension and the tear film (in)stability.

Nanoemulsion Formulations of Fungicide Tebuconazole for Agricultural Applications.

Tebuconazole (TBZ) nanoemulsions (NEs) were formulated using a low energy method. TBZ composition directly affected the drop size and surface tension of the NE. Water fraction and the organic-to-surfactant-ratio (RO/S) were evaluated in the range of 1–90 and 1–10 wt %, respectively. The study was carried out with an organic phase (OP) consisting of an acetone/glycerol mixture containing TBZ at a concentration of 5.4 wt % and Tween 80 (TW80) as a nonionic and Agnique BL1754 (AG54) as a mixture of nonionic and anionic surfactants. The process involved a large dilution of a bicontinuous microemulsion (ME) into an aqueous phase (AP). Pseudo-ternary phase diagrams of the OP//TW80//AP and OP//AG54//AP systems at T = 25 °C were determined to map ME regions; these were in the range of 0.49–0.90, 0.01–0.23, and 0.07–0.49 of OP, AP, and surfactant, respectively. Optical microscope images helped confirm ME formation and system viscosity was measured in the range of 25–147 cP. NEs with drop sizes about 9 nm and 250 nm were achieved with TW80 and AG54, respectively. An innovative low-energy method was used to develop nanopesticide TBZ formulations based on nanoemulsion (NE) technology. The surface tension of the studied systems can be lowered 50% more than that of pure water. This study’s proposed low-energy NE formulations may prove useful in sustainable agriculture.

Mixed Protein/Hexane Adsorption Layers Formed at the Surface of Protein Solution Drops Surrounded by Hexane Vapor

The surface tension of aqueous solution drops of ß‐casein, ß‐lactoglobulin, and human serum albumin at the interface to air and hexane saturated air are measured by drop profile analysis tensiometry. The results indicate that the dynamic and equilibrium surface tension for a hexane vapor atmosphere are considerably lower as compared to the values at the interface with pure air. The experiments are performed with the initial adsorption of protein followed by hexane adsorption, and with subsequent removal of hexane from the measuring cell after equilibrium is attained. Two models are proposed, both of which assume the diffusion of protein in the solution and either diffusion‐governed or barrier‐governed hexane coadsorption. The new developed theoretical equilibrium model assumes a double layer adsorption in these systems, with the first layer composed of protein mixed with hexane and the second layer formed by hexane molecules only. The experimental and calculated equilibrium surface tension data are in a perfect agreement. Note the model parameters are exactly the same as those found for the individual compounds.

The significance of drop non-sphericity in sprays

This paper presents a new framework to model drop dynamics in Lagrangian sprays. The framework builds on the Taylor Analogy Breakup (TAB) model. Real-fluid (gas-liquid) thermodynamics applicable to multicomponent systems are combined with Gradient Theory to facilitate detailed calculations of drop surface tension forces, oscillations, and breakup processes. This is combined with a more detailed treatment of deforming drop dynamics to construct more accurate representations of the local interfacial exchanges of mass, momentum, and energy. The framework is derived using an energy balance equation that explicitly enforces drop momentum conservation during the breakup process. This facilitates development of a refined set of drop equations that address current shortcomings in the prediction of drop properties over a wide range of relevant breakup conditions. The resulting drag forces, evaporation, and heating rates deviate significantly from the predictions given by contemporary drop models used in modern simulations. These deviations are quantified using Large Eddy Simulation (LES) with a Lagrangian-Eulerian modeling approach. The analysis demonstrates how the model improvements in the new framework provides a more detailed representation of physical complexities that are largely neglected in modern studies.

Comparison of three different arc modes for laser-arc hybrid welding steel

Traditionally, Laser Arc-Hybrid Welding (LAHW) is made using a gas metal arc heat source in pulsed mode. In this study, welds made with three different arc modes under various conditions are compared. The arc modes compared for LAHW are: Standard, Pulsed, and Cold Metal Transfer (CMT). The pulsed mode, using power modulation, is more controlled than the “natural” Standard mode and offers globular drop transfer with reduced heat input to the work piece, enabling thinner materials to be welded. The CMT arc mode also uses power modulation as well as controlled wire feeding. This enables surface tension drop transfer, involving even less heat input and is also considered to generate less undercuts and spatter than the other two arc modes. The welds compared were made in 7 mm thick S420 laser cut steel sheets with two different welding speeds and gap sizes. They were made using close-to-production setup within the limitations of the CMT capabilities, i.e., low and medium wire feed rates. The weld caps and roots were studied optically and the structures were analyzed by making cross-sectional macrographs longitudinally and transversally. These cross sections were analyzed geometrically, structurally, and also hardness tested. The welds were also studied and analyzed by means of pre- and postweld scanning as well as high speed imaging. The study shows that the laser keyhole reduces the effect of the arc modes, enabling higher welding speeds than without the laser. The gouge formed in front of the keyhole determines occurrence of undercuts and bead uniformity. The material mixing of filler and base material, and laser penetration spiking is also influenced by the depth of the arc gouge. The material structure is also affected to some degrees for expected performances. Even though the short-arc technique CMT show promising results, the pros and cons for each arc technique are discussed.

Cyclodextrin–poloxamer aggregates as nanocarriers in eye drop formulations: dexamethasone and amphotericin B

In this present study cyclodextrin (CD)–poloxamer aggregates were characterized and developed as ophthalmic drug carriers. The combined effect of γCD/2-hydroxypropyl-γCD (HPγCD) mixtures and poloxamer on solubilization and permeability of two model drugs, dexamethasone (Dex) and amphotericin B (AmB), was investigated. The CD–poloxamer interaction and complex aggregation were examined by 1H nuclear magnetic resonance (1H-NMR), their solubilizing ability by high-performance liquid chromatography, and their particle size determined by dynamic light scattering and transmission electron microscopy. Formulations containing either 1.5% w/v Dex or 0.15% w/v AmB in eye drop suspensions containing various γCD/HPγCD ratios and poloxamer 407 (P407) were prepared. The solubility of the drugs, surface tension and hemolytic effect of the eye drops and drug permeation from selected formulations were determined. The 1H-NMR study showed that P407 formed inclusion complex with CDs by inserting its poly(propylene oxide) segment into the CD cavity. P407 and γCD interacted with each other to form nanosized aggregates, and the observed concentration of dissolved γCD and P407 progressively decreased with increasing γCD and P407 concentrations. Including a high proportion of HPγCD improved the drug solubilization and reduced the hemolytic effect. The surface tension of the formulations decreased with increasing P407 concentration. Furthermore, increasing P407 content in the formulations enhanced formation of complex aggregates with consequent slower drug release. It was concluded that the drug/γCD/HPγCD complex was stabilized by P407 through formation of multi-component aggregates. Thus, CD–poloxamer aggregates are self-assembled nanocarriers from which drug delivery characteristics can be adjusted by changing the γCD/HPγCD/P407 ratios.

Onsager's irreversible thermodynamics of the dynamics of transient pores in spherical lipid vesicles.

Onsager's irreversible thermodynamics is used to perform a systematic deduction of the kinetic equations governing the opening and collapse of transient pores in spherical vesicles. We show that the edge tension has to be determined from the initial stage of the pore relaxation and that in the final state the vesicle membrane is not completely relaxed, since the surface tension and the pressure difference are about 25% of its initial value. We also show that the pore life-time is controlled by the solution viscosity and its opening is driven by the solution leak-out and the surface tension drop. The final collapse is due to a non-linear interplay between the edge and the surface tensions together with the pressure difference. We also discuss the connection with previous models.

English

Biosurfactants are surface active agents with broad range commercial applications in various industries and have considerable advantages over their chemical counterparts. In this study, drop collapse assay, oil spreading assay, emulsification assay and surface tension measurement were compared for their reliability and ease of use to screen biosurfactant producing six extremely halophilic bacteria isolated from saline soil of Chott El Hodna-M’sila (Algeria), which is considered as a thalassohaline environment. Results from screening methods revealed that, CH2 and CH5 strains are potential candidates for biosurfactant production. Among these, there was a very strong negative linear correlation between the emulsification index and surface tension (rs= - 0.987) and a strong negative linear correlation between the oil spreading technique and surface tension (rs= - 0.971) and a weaker negative correlation between the drop collapse method and surface tension (rs= - 0.807), suggesting that strains highly active in any one of these methods were active in other three methods. As every method has its advantages and disadvantages, a combination of different methods is appropriate for a successful screening.   Key words: Biosurfactant, drop collapse, emulsification stability, extremely halophilic bacteria, oil spreading, surface tension.

Dispensing of rheologically complex fluids at the dripping regime

Dispensing characteristics of rheologically complex fluids are investigated experimentally at the dripping regime. Two mixtures of ethylene glycol and glycerin (50:50 and 31:69) were used as base Newtonian fluids. As the polymer solution, 1000ppm of polyacrylamide of 5–6Mg/mol was added to the 50:50 base Newtonian fluid. 2.5, 5 or 7.5vol% of polystyrene particles of 2μm in diameter were added to the liquids to prepare particle suspensions. The liquids were dispensed to the air by using a nozzle with the inner and outer diameters of 0.61 and 0.91mm, respectively. It was found that the drop size is primarily determined by the balance of drop weight and surface tension force holding the drop, and hence drop size does not depend on volume flow rate. The drop diameter is approximately 10% smaller for elastic liquids due to the severe extension of the filament before the detachment from the nozzle. The result also shows that the dripping characteristics of the fluids without the polymer are mainly determined by the balance of the gravity force, surface tension force holding the liquid volume and pressure force. The addition of less than approximately 10% of 2μm spherical particles hardly changes the dispensing characteristics of Newtonian fluids except at the final stage of detachment. For polymeric liquids, the transient extensional property drastically changes the dripping dynamics. The force balance shows that the strain hardening of the polyacrylamide solution under the extensional flow causes severe retardation of the filament breakup. The filament breakup of the polymeric suspension is further decelerated by the increase in zero shear viscosity as a result of the particle addition.

Mosquitoes actively remove drops deposited by fog and dew.

We report mosquito behaviors for removing accumulated drops of water which would otherwise increase the energy expended during takeoff and free flight. These techniques take advantage of the insect's small size and great structural strength. To dry their wings before takeoff, mosquitoes employ a flutter stroke, at double the wingbeat frequency of normal flight, generating nearly 2500 gravities of acceleration. Mosquitoes may also remove drops by the respective accelerations associated with takeoff and collision with the ground. We correlate the accelerations and size of drops ejected using a simple model involving the drop's inertial force and surface tension. We note mosquitoes may use similar techniques to remove synthetic drops, making our observations applicable for understanding the resistance of insects to insecticides.

Comparison of CMT with other arc modes for laser-arc hybrid welding of steel

In this study, three different arc modes in laser-arc hybrid welding with a gas metal arc were studied, i.e. the standard, pulsed and cold metal transfer (CMT) modes. The pulsed mode is more controlled than the standard mode and offers reduced heat input to the workpiece, which enables welding of thinner materials. The CMT mode utilizes surface tension drop transfer with controlled wire feeding, and therefore, involves less heat input than the other arc modes, and it is also considered to generate less undercut and spatter than the other modes. This study compares hybrid welds made by the three arc modes with a close-to-production setup for low and medium wire deposition rates, within the limits of the CMT process. The welds were studied by scanning and high speed imaging. The study shows that the differences between the drop transfer modes are reduced due to the presence of a laser keyhole. The dominating influence on the solidification and melt flow is the arc and especially the gouge created ahead of the keyhole. The main pros and cons of the different arc modes are discussed.

Surface tension of spherical drops from surface of tension.

The determination of surface tension of curved interfaces is a topic that raised many controversies during the last century. Explicit liquid-vapor interface modelling (ELVI) was unable up to now to reproduce interfacial behaviors in drops due to ambiguities in the mechanical definition of the surface tension. In this work, we propose a thermodynamic approach based on the location of surface of tension and its use in the Laplace equation to extract the surface tension of spherical interfaces from ELVI modelling.

Shape of Water–Air Interface beneath a Drop on a Superhydrophobic Surface Revealed: Constant Curvature That Approaches Zero

The 3D interface between a sessile water drop that is found on a superhydrophobic microtextured surface has been directly imaged using a confocal microscope with an immersion lens. The local mean curvature of the water–air interface was derived and was shown to be constant and close to zero not only for the case of pure water, but also for cases of variable drop density and surface tension. Although the mean curvature is constant and close to zero, the standard deviation on the mean curvature increases with increasing drop density and lower surface tension. The resulting 3D image of the water–air interface at the bottom of a water drop demonstrates the possibility of investigating practical interfaces of water on given textures and confirms that no matter what the superhydrophobic surface characteristics are, the interface remains with a constant curvature of close to zero.

EFFECTS OF DROP AND FILM VISCOSITY ON DROP IMPACTS ONTO THIN FILMS

While drop-film impacts have been studied extensively in the past, little thought has been given to separating the effects of the drop fluid properties and those of the film. Notably in the field of pool fire suppression, sprays impact pools consisting of different fluids, and both the spray properties and the pool properties will govern the resulting behavior [1, 2]. Distinguishing between the film and droplet’s effects will also provide insight into behaviors observed on more common same-liquid drop-film impacts. In this study, the central parameter examined is the fluid viscosity. Using various mixtures of water and glycerol, as well as FC-72, a range of kinematic viscosity covering 3 orders of magnitude (3.8E-7 to 1.1E-4 m 2 /s) is examined, while the surface tensions cover a smaller range, from 0.01 to 0.07 N∙m. A microliter valve creates mm-scale drops that freefall into the target film at velocities of 0.5 to 3.5 m/s, where a high speed video camera records the impact. The final parameter considered in this study is the ratio of pool depth to droplet diameter, which is examined in three regimes: 0.5 (shallow film), 1.0, and 3.0 (deep pool). This configuration covers Weber numbers from 20 – 3000 and Reynolds from 20 – 14000. The impact outcomes characterized are coalescence, crown formation, jetting, and splashing of the crown and the jet. These regimes have been defined by past work, and for the same film-droplet liquids they have been mapped to ranges of dimensionless numbers, primarily the Weber number. These dimensionless ranges are examined for differing liquid impacts. To correct for differing drop and film properties, alternative dimensionless criteria are suggested. These criteria are proposed in light of substrate effects in the shallow film regime as well as in the deep pool regime where bottom effects are lessened. Crown formation and subsequent crown splashing are shown to be governed by the viscosity and surface tension of the film, with minimal dependence on drop fluid properties. With a dimensionless pool depth (H* = pool depth/drop diameter) of 1, for every fluid examined, the critical Weber number for crown formation is on the order of 100 when pool density and surface tension are used. Applying the drop density and surface tension gives no apparent correlation between common dimensionless groups and observed behavior. In contrast, when characterizing the appearance of a post-impact Worthington jet, the drop’s properties play a much larger role, with a critical minimum Weber number utilizing drop surface tension and density between 50 and 200 resulting in the appearance of a jet. The impact outcomes in other pool depth regimes are explored, as well.

Evaluation of Screening Methods for the Isolation of Biosurfactant Producing Marine Bacteria

Hemolytic assay, bacterial adhesion to hydrocarbons (BATH) assay, drop collapse assay, oil spreading assay, emulsification assay and surface tension measurement were compared for their reliability and ease of use to screen biosurfactant producing bacteria. 105 bacterial strains were screened for their biosurfactant production. Results from screening methods revealed that hemolytic and BATH assays are not reliable methods to check biosurfactant production because hemolytic assay relies on lysis of blood cells, which can be caused by compounds produced by microbes other than the biosurfactants. In that case hemolytic assay may include strains that do not produce biosurfactant. BATH assay relies on hydrophobicity or adherence of cells with hydrophobic compounds. Results obtained in BATH assay for three bacterial isolates, Branhamella catarrhalis, Citrobacter intermedius and Klebsiella ozaenae were positive, but when the same isolates screened for drop collapse, oil spreading, and surface tension measurement the results were negative. This indicates that these bacterial cells are either acting as biosurfactant themselves (cell surface) or they don’t have the ability to produce biosurfactants. In conclusion, drop collapse and oil spreading assays are reliable methods to screen large number of samples. However, for confirmation surface tension measurement can be used at the end. Another interesting finding in this study was the use of crude oil as hydrophobic substrate for the isolation of biosurfactant producing bacterial strains; this approach may be useful during the initial isolation of biosurfactant producing bacteria to reduce the number of strains to be screened.

Effects of Surface Tension on Drop Impact on a Horizontal Rotating Disk

The impact processes of water and ethanol drops on a rotating horizontal aluminum disk were recorded and analyzed using a high-speed digital camera together with an image analysis program. The angular velocities of the disk were altered to study the effect of surface tension of drops on drop impact processes. The experimental results show that a lower surface tension will result in a higher tangential spread factor and a lower receding rate during the receding stage, for the drop impinging and depositing on a rotating disk. In addition, a lower surface tension of the drop tends to promote the occurrence of splash. The experimental results further verify a proposed correlation of splash-deposition boundary for drops impinging on a rotating disk. Both drops, though they have a quite different surface tension, experience four stages, with two new stages different from those of drops impinging on stationary surfaces. Their tangential spreading factors both increase obviously with the tangential velocity at the impact point, while their radial spreading factors vary a little.