Positive Spreading Coefficient Research Articles

Fluorine-Free Extinguishing of a Hydrocarbon Pool Fire with a Suspension of Glass Bubbles Grafted with Nanoscale Polymer Layers.

The current most efficient solution to extinguish liquid hydrocarbon (class B) pool fires involves fire-fighting foams containing fluorinated surfactants. However, fluorocarbon surfactants are unsafe due to their environmental persistence and negative toxicological/bioaccumulative impact. To this end, we show that fluorine-free aqueous suspensions of Glass Bubbles (GB) modified with hydrophilic polymer grafted layers can efficiently extinguish hydrocarbon pool fires. Namely, GB grafted with poly(oligo (ethylene glycol) methyl ether methacrylate) (POEGMA), GB-G was fabricated employing "grafting-through" and "grafting-from" methods and used to obtain the suspensions. It was found that the GB suspension, with a grafted layer of higher molecular weight and lower grafting density (GB-GL), proved superior to the more densely grafted GB-GH and nongrafted GB-0 system. The GB-GL suspensions displayed less negative spreading coefficients and viscosities lower than those of GB-GH/GB-0 compositions. When siloxane-polyoxyethylene surfactant was added to all GB suspensions, the interfacial properties were dominated by the surfactant, with all suspensions having the same positive spreading coefficient. However, the GB-GL-surfactant composition had the lowest viscosity among the suspensions studied in this work. Specifically, the viscosity of GB-GH and GB-0 suspensions at a shear rate of 77 s-1 was ∼110% and 70% higher than that of GB-GL. Due to the lower viscosity, the GB-GL suspension demonstrated the most efficient spreading over model hydrocarbon solid (polyethylene) and liquid (hexadecane) surfaces when the surfactant was added. The suspension also showed the best performance in the retardation of hexane evaporation when placed over the heated hexane pool. After 50 min, the amount of hexane that evaporated through GB-GH and GB-0 suspensions was ∼8 and 11 times higher, respectively, compared to the GB-GL suspension. We found that the GB-GL-surfactant system was the most efficient GB suspension in extinguishing the fire due to its superior spreading and sealing ability. It was within 10% of fluorine-containing foam's fire extinguishment performance. The GB suspensions are much safer in terms of burnback resistance as a torch applied directly to the suspension after extinguishment could not reignite the fire. The GB material is recyclable, since it can be collected and reused after application to a fire.

Cassie's Law Reformulated: Composite Surfaces from Superspreading to Superhydrophobic.

In 1948, Cassie provided an equation describing the wetting of a smooth, heterogeneous surface. He proposed that the cosine of the contact angle, θc, for a droplet on a composite surface could be predicted from a weighted average using the fractional surface areas, fi, of the cosines of contact angles of a droplet on the individual component surfaces, i.e., cos θc = f1 cos θ1 + f2 cos θ2. This was a generalization of an earlier equation for porous materials, which has recently proven fundamental to underpinning the theoretical description of wetting of superhydrophobic and superoleophobic surfaces. However, there has been little attention paid to what happens when a liquid exhibits complete wetting on one of the surface components. Here, we show that Cassie's equation can be reformulated using spreading coefficients. This reformulated equation is capable of describing composite surfaces where the individual surface components have negative (droplet state/partial wetting) or positive (film-forming/complete wetting) spreading coefficients. The original Cassie equation is then a special case when the combination of interfacial tensions results in a droplet state on the composite surface for which a contact angle can be defined. In the case of a composite surface created from a partial wetting (droplet state) surface and a complete wetting (film-forming) surface, there is a threshold surface area fraction at which a liquid on the composite surface transitions from a droplet to a film state. The applicability of this equation is demonstrated from literature data including data on mixed self-assembled monolayers on copper, silver, and gold surfaces that was regarded as definitive in establishing the validity of the Cassie equation. Finally, we discuss the implications of these ideas for super-liquid repellent surfaces.

Adhesion energy, spreading coefficient and interfacial tension as an efficient tool for assessing biocide performance

AbstractIt is estimated that microbiologically influenced corrosion (MIC) contributes to 20%–50% of the total cost of corrosion. Biocides are often used to counter MIC. Normally, the choice of a biocide is based on planktonic kill studies as well as cost of the treatment chemicals. Sessile kill studies have significant advantages over planktonic studies but usually take 8 weeks to complete. Furthermore, both sessile kill and planktonic studies only provide phenomenological results and therefore do not explain the mechanisms. To bridge this gap, we have calculated spreading coefficient and work of adhesion from contact angle measurements as a tool for assessing the performance of a biocide. This novel method provides insights into how biocides interact with a biofilm on a surface and is quicker than sessile kill study. Based on surface energy calculations, we have identified parameters that will aide in the prediction of biofilm removal and prevention. For example, surfactants with a positive spreading coefficient on a substrate and having a stronger affinity for the substrate than the biofilm (i.e., a lower interfacial tension) are effective in biofilm prevention. On the other hand, surfactants with a low interfacial tension with a substrate are effective in biofilm removal. Given the diversity of surfaces to which bacteria attach (carbon steel, near wellbore, membrane, etc.), this rapid technique is a good prediction tool for the effectiveness of a biocide application. This paper will provide an overview of the technique and discuss some examples of surfactants that can be used for biofilm removal and prevention.

Accessing Thin Film Wetting Regimes during Polymer Growth by Initiated Chemical Vapor Deposition.

We investigate the growth of a fluorinated polymer via initiated chemical vapor deposition onto a suite of isotropic and mesogenic liquids with a range of refractive indices. The polymer morphology at fluid interfaces was found to deviate from conformal films predicted by the positive spreading coefficient, and the resulting morphology is attributed to long-range van der Waals interactions during the deposition process. Experiments systematically vary the deposition conditions and compare the liquid phase (isotropic or nematic) to evaluate the effect of kinetic factors and the liquid substrate phase on the interfacial polymer morphology and spatial organization.

Spreading behavior of firefighting foam solutions on typical liquid fuel surfaces

AbstractA series of experiments was performed to investigate the spreading behavior of firefighting foam solutions on liquid fuel surfaces. The spreading coefficients of six kinds of aqueous film‐forming foam solutions and one fluorine‐free foam solution on the surface of four liquid fuels, namely, cyclohexane, diesel, n‐heptane, and ethanol, were calculated on the basis of surface and interfacial tension. Spreading behavior was studied systematically using a high‐speed camera, and then the relationship between spreading behavior and spreading coefficient was analyzed. Furthermore, the spreading area and spreading rate of different foam solution droplets on liquid fuel surfaces were studied in depth. The spreading amount of the foam solution droplets on the liquid fuel surfaces was measured. Four typical spreading phenomena, namely, spreading, suspension, dissolution, and sinking, of AFFF solutions on liquid fuel surfaces were identified. Moreover, a positive spreading coefficient did not necessarily lead to the formation of an aqueous film. The spreading area, spreading rate, and spreading amount were not proportional to the spreading coefficient. During the evaluation of the spreading property of firefighting foam, the spreading coefficient, spreading rate, and spreading amount must be focused on instead of only the spreading coefficient.

Role of surfactant adsorption and surface properties for the efficiency of PDMS-silica antifoams

We study how the composition of various surfactant mixtures affects the efficiency of mixed PDMS–silica antifoam in foamed surfactant solutions. First, systematic experiments are performed to characterize the surface and foam film properties of the studied surfactant solutions. The spreading, bridging and entry coefficients are calculated and the spreading ability of the antifoam is characterized by microscopy observations and by surface tension measurements. Next, the initial antifoam activity and the antifoam durability are characterized in foam tests. The obtained results reveal that the antifoam efficiency in solutions of low-molecular mass surfactants with low surface dilatational modulus depends strongly on the density (area-per-molecule) of the respective adsorption layer. The addition of nonionic surfactants, which increase the mean area-per-molecule in the mixed adsorption layer, enhances significantly the antifoam activity and durability. In contrast, the addition of surfactants, which decrease the mean area-per-molecule, suppresses the antifoam activity. Furthermore, we found that surfactant mixtures which form condensed adsorption layers on the solution surface suppress strongly the antifoam activity. As an extreme, the condensed adsorption layer formed from the natural surfactant Quillaja saponin suppresses the antifoam spreading even at highly positive spreading coefficient which results also in very poor AF efficiency. The obtained results rationalize in a coherent way the observed differences in the AF activity and durability in mixed solutions of various ionic, nonionic and zwitterionic surfactants.

Fuel effects on aqueous film formation and foam degradation and their impact on fire suppression by foams containing fluorosurfactants

SummarySpreading coefficient, foam degradation, fuel transport through a foam layer, foam spread, and fire extinction time were collected by varying fuels for a given fluorosurfactant formulation. Varying fuel structure systematically enabled variation of foam‐fuel interactions, and spreading coefficients, which affected aqueous film formation. A wide range of spreading coefficients were studied using two foam formulations. Surfactant formulations with near‐zero or negative spreading coefficients (−1.2, −0.12 mN/m) suppressed the fires within the same time or faster than those with positive spreading coefficients (1.77, 3.28 mN/m). Both foams extinguished an iso‐octane and methylcyclohexane pool fire despite negative spreading coefficients on iso‐octane and positive spreading coefficients on methylcyclohexane. The fire extinction times collected relate more closely to foam degradation than to film formation. Foam degradation results showed a roughly 20% variation in lifetime between the two foams and the three fuels, consistent with fire extinction times. To understand the relationship between foam degradation and solution properties, a single lamella experiment was designed that monitors lamella thickness and lifetime. This experiment, relating solution to foam properties, will aid in the development of environmentally friendly surfactant replacements in firefighting foams.

Penetration dynamics through nanometer-scale hydrophilic capillaries: Beyond Washburn’s equation and extended menisci

HypothesisRecently, the naoncapillary devices with the channel width about 2–3 water molecules have been fabricated. Water transport through these nanoslits showed unexpectedly fast flow, revealing the failure of Washburn’s equation. ExperimentsLiquid penetration into a nanocapillary made of two parallel walls is explored by many-body dissipative particle dynamics. Both partial wetting and total wetting walls are considered and the no-slip boundary condition is satisfied. FindingsThe wicking velocity generally obeys Washburn’s equation, but the dynamic contact angle (CA) has to be employed. The dynamic CA (θD) relies on the penetration rate and is always larger than the equilibrium CA. The breakdown of Washburn’s equation occurs under two conditions, (i) the channel width close to molecular size and (ii) the positive spreading coefficient is large enough. Both cases come about when the wicking velocity in a nanoslit exceeds the maximum value corresponding to cos(θD) = 1. The failure of Washburn’s equation is attributed to the invalidity of Young-Laplace equation associated with undefined meniscus. The extended meniscus will be developed as a wall of the nanoslit continues to extend outside the exit mouth. The shapes of extended menisci are discussed for both partial wetting and total wetting surfaces.

Surface phenomena in air-assisted solvent extraction

Air-assisted solvent extraction (AASX) which is based on the coating of organic extractants on the air bubbles was developed to recover valuable metals from dilute Cu solutions. The effect of the chemical composition of aqueous and organic phases was investigated on the interfacial tension of aqueous/organic/air phases and related thermodynamic coefficients. Results were used to analyze the behaviour of phases and process efficiency in a continuous-mode AASX pilot plant. Interfacial data indicated that a suitable organic coating occurred in the presence of Flomin-F742, Nasfroth-240, and pine oil frothers with positive spreading coefficients. It was found that organic phase recycling in the AASX column was decreased from 86% to 13% by reducing the bubble size from 659 μm to 366 μm, respectively. By the addition of silicone oil dosage from 0 to 75 mg/L to the organic phase, Cu recovery was enhanced from 44% to 85% mainly as a consequence of decreasing the A/O phase ratio and increasing the coating degree of air bubbles by the organic extractant. An intense drop of organic phase recycling (from 70 to 30%) was observed at the silicone oil dosage beyond the critical micelle concentration of the organic phase, which was accompanied by producing a wide size range of air bubbles. It can be concluded that spreading the organic phase on the air bubbles and having a narrow size range of bubbles to overcome the upward velocity of bubbles to the downward velocity of the aqueous phase are the main issues for achieving a good efficiency in the AASX process. Results indicated that AASX method could widen the application of solvent extraction for the recovery of valuable metals from aqueous solutions.

Spreading and hole formation in natural oil films on aqueous solutions

The dynamics of a natural oil film on a free surface of the water has been studied experimentally. We examine the process of spreading and breakup of an oil film on water as well as on dilute aqueous solutions of Octanoic acid. Oil from Cocos Nucifera (Coconut) is used as the test fluid. A drop of this test fluid is introduced on to the free surface of the substrate fluid. The spatio-temporal evolution of the oil film is imaged and the features are investigated. The oil drop evolves into a film and spreads spontaneously because of the positive spreading coefficient. At a certain critical film thickness, holes, which are voids in the oil film, begin to nucleate. When the hole area fraction approaches the random packing limit, these holes begin to coalesce resulting in a coarser structure of the oil film. Coalescence occurs when the liquid bridge separating a pair of holes is ruptured due to Rayleigh–Plateau instability. This process is completely spontaneous since it is driven by gradients in the interfacial energy landscape. The oil film was observed to evolve through three phases viz., (i) spreading phase, (ii) hole nucleation and growth phase and (iii) hole coalescence phase. Quantitatively, the oil film diameter, number of holes in the oil film, diameter of each hole as well as the total triple line length has been measured. We also investigate the effect of Octanoic acid concentration in the substrate fluid on this dynamics. We show that increasing Octanoic acid concentration in the substrate fluid slows down the dynamics of film break up. This is possibly due to increased hydrophobic/hydrophilic interaction forces between the substrate fluid and the oil film.

Requirements for the Formation and Shape of Microscopic Precursors in Droplet Spreading.

While the mass transport mechanisms and dynamics of molecular precursors, ultrathin films that precede spreading droplets, are well understood, the requirements for their formation and the reasons for the occurrence of different precursor shapes remain unclear. In this work, we study these requirements using molecular dynamics simulations of spreading droplets and extensive free energy computations. For a simple simulation model, we demonstrate that with growing droplet-substrate attraction, spreading passes succesively through regimes with no precursor, a monolayer precursor, and a continuously growing precursor. We show that the onset of layer formation and the changes in the precursor shape correlate with the free energy of layer formation. On the basis of these findings, we show that a positive spreading coefficient is sufficient but not necessary for precursor formation.

Semi-Empirical Modeling of Gas–Liquid Mass Transfer in Gas–Liquid–Liquid Systems in Stirred Tanks

Two semi-empirical models of gas–liquid mass transfer in gas–liquid–liquid systems are presented, one for the case where the dispersed liquid phase has a positive spreading coefficient S > 0, the other for S < 0. The models are based on mathematical expressions inspired by assumed mass transfer mechanisms, which are different depending on the oil spreading characteristics. Model simulations compare well with experimental data (up to oil phase holdup ϕ ∼ 0.1), managing to simulate the shape of the experimental curves of oxygen mass transfer enhancement factor E vs. ϕ for the air–water–heptane (S > 0) and air–water–dodecane (S < 0) systems, better than other models available in the literature.

The Phase-out of Perfluorooctane Sulfonate (PFOS) and the Global Future of Aqueous Film Forming Foam (AFFF), Innovations in Fire Fighting Foam

Aqueous film-forming foams (AFFFs) are among the most popular fire-fighting foams used in liquid fuel fires because of their film forming and fast knock down property. One key ingredient of AFFFs, the fluorocarbon surfactant i.e. perfluorooctane sulfonate (PFOS) which is used to reduce surface tension and positive spreading coefficient, is toxic to aquatic life and is a persistent chemical that accumulates in the blood of humans and other animals. Surfactants are not found naturally in the environment and are man-made. In the year 2000 unexpectedly announcement of phasing out fluorocarbon surfactant’s manufacturing and its storage which effected a number of product lines, including the firefighting foams. Internationally the manufacturing and release of PFOS to the environment will be suspended by 2015. New fluorosurfactants have been introduced into the market with reformulation and used to form aqueous fire-fighting foam concentrates. The toxicity of the new fluorosurfactants and their persistence in the environment are not well established and still are under investigation. Their presence in the future market is unsure. The continuous research and development to find out the substitute for perfluorooctane sulfonate derivative (C8) has brought two choices i.e. Fluorine-free foams or Fluorotelomer (C6)-based Foams. These foams which may fulfill requirement of different international standards of fire fighting but still contain small amounts of fluorochemicals and are thus not truly fluorine-free. There is every possibility that even after 2015 new regulation may come in to effect to restrict the use of these new formulations (C6) of fire fighting foam. Therefore, the fire-fighting industry has an urgent need for new, environmental friendly foaming agents and foam stabilizers to replace fluorosurfactants in aqueous fire-fighting foams with enhanced drain time, low bubble coarsening, and faster knockdown and excellent burn back resistance properties.

Mass-Spring Model of a Self-Pulsating Drop

Self-pulsating sessile drops are a striking example of the richness of far-from-equilibrium liquid/liquid systems. The complex dynamics of such systems is still not fully understood, and simple models are required to grasp the mechanisms at stake. In this article, we present a simple mass-spring mechanical model of the highly regular drop pulsations observed in Pimienta, V.; Brost, M.; Kovalchuk, N.; Bresch, S.; Steinbock, O. Complex shapes and dynamics of dissolving drops of dichloromethane. Angew. Chem., Int. Ed. 2011, 50, 10728-10731. We introduce an effective time-dependent spreading coefficient that sums up all of the forces (due to evaporation, solubilization, surfactant transfer, coffee ring effect, solutal and thermal Marangoni flows, drop elasticity, etc.) that pull or push the edge of a dichloromethane liquid lens, and we show how to account for the periodic rim breakup. The model is examined and compared against experimental observations. The spreading parts of the pulsations are very rapid and cannot be explained by a constant positive spreading coefficient or superspreading.

Dynamic Correlation Function of Monodispersed Colloid

Diffusive behaviors for two-dimensional (2D) monodispersed polystyrene array of droplet spreading by either methanol or ethanol suspension are observed on a rigid surface. Methanol solution can spread speedily on the glass surface because of the positive spreading coefficient. Conversely, the ethanol will not easily proliferate out. Monodisperse polystyrene particles are involved in Brownian motion in a thick layer during alcohol evaporation. A monolayer dimension of the drop has been developed at the final spreading. Lateral capillary forces are produced and caused the aggregation of particles. Evaporation at the edge of the ethanol film did not lead to the film to retract. The formation of 2D array, which preceded the spreading of the microscopic droplet across the surface, is able to indicate the dense structure to the dynamic correlation function.

Superhydrophilic TiO2 surfaces generated by reactive oxygen treatment

The authors show that superhydrophilic TiO2 can be obtained without irradiation of the surface with ultraviolet (UV) light and concomitant excitation of electron-hole pairs. The authors demonstrate that the treatment of TiO2 surfaces with reactive oxygen species generated by air plasma removes the surface organic contaminants, leading to almost 0° contact-angle wetting of the surface. The superhydrophilicity can be explained by the positive spreading coefficient calculated using the effective surface tensions. Our results point toward UV-light irradiation as an indirect cause of the superhydrophilicity of TiO2 and support the hypothesis that this property arises from a self-cleaning effect based on the photo-oxidation and decomposition of organic contaminants at the surface.

On integrable singularities and apparent contact angles within a classical paradigm

Far from claiming any ultimate resolution of the contact line paradoxes, we draw the reader’s attention to the fact that a somewhat controversial paradigm, originally employed by de Gennes and collaborators, actually appears both to be quite reasonable at its foundations and to lead to physically consistent final results in a wide variety of situations. Curiously enough, while containing a singularity in itself, the approach nonetheless renders the classical contact-line singularities — both hydrodynamic and thermal — integrable, in particular as far as several quantities of interest are concerned. It is also readily applicable to quite a few situations: from equilibrium shapes and moving contact lines of a non-volatile liquid, to cases with evaporation into (and even condensation from, although it is not studied here) either a pure-vapor or an inert-gas atmosphere. The paradigm actually consists in an approach involving both the (positive or negative) spreading coefficient and the disjoining pressure in the form of a positive inverse cubic law, a conceptual framework that most notably describes structures with truncated precursor films on a macroscopically bare solid surface. Whether or not the remaining integrable divergences at molecular scale can truly be considered as “benign” has to be discussed on the basis of more involved mesoscopic or microscopic approaches, quite outside the scope of the present study.

Truncated versus Extended Microfilms at a Vapor−Liquid Contact Line on a Heated Substrate

The microstructure of a contact line formed by a liquid and its pure vapor on a perfectly wetted superheated smooth substrate, with the disjoining pressure most often in the form of a positive inverse cubic law (nonpolar case), is routinely considered to end up in a microfilm extended over adjacent "dry" parts of the solid surface. Invoking the spreading coefficient as an additional independent parameter within this framework, we argue however that a regime with a truncated microfilm is chosen instead if the spreading coefficient is decreased below a positive (still perfect wetting) critical value dependent upon the superheat, in which case the extended-microfilm thickness is surpassed by that of the "pancake" introduced by de Gennes and co-workers. Conversely, for a given positive spreading coefficient, there is a critical superheat above which the microfilm gets truncated, whereas for a negative one (partial wetting) the truncated regime should be preferred at any superheat. A parametric study of the apparent contact angle (a nonlinear eigenvalue of the steady microstructure problem) versus the spreading coefficient is carried out. When the latter is negative, Young's law is asymptotically recovered. Microfilm fronts on a bare surface are shown to be advancing or receding in accordance with the selected regime. A slightly more general class of disjoining pressures is also touched upon. The analysis is based in part upon thermodynamic considerations and in part upon a standard one-sided model of an evaporating liquid layer in the lubrication approximation.

PET/EVA/PP ternary blends: An investigation of extended morphological properties

AbstractIn this study, the effects of different parameters on the morphological properties of ternary blends were investigated. Therefore two systems (PET/H‐EVA/PP and PET/L‐EVA/PP, H‐EVA and L‐EVA are high and low viscosity, respectively) were prepared by melt mixing process. In all of the blends, poly (ethylene terephthalate) (PET) as the major phase‐ with poly propylene (PP) and two grades of poly (ethyl‐stat‐vinylacetate) (EVA) with different viscosities and subsequently different interfacial interactions was blended. Theoretical models predicted positive spreading coefficient for two grades of EVA and lower free energy for the samples consisting of EVA and PP as the shell and the core phases respectively. With changing core shell ratio, droplet size of samples containing L‐EVA and H‐EVA increased and decreased, respectively. Subinclusion of shell into the core was observed in some blended samples. In systems containing H‐EVA, by thickening the shell phase; multi core morphology was observed which would be related to the coalescence phenomenon inter the droplets. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The Pursuit of a Scalable Nanofabrication Platform for Use in Material and Life Science Applications

In this Account, we describe the use of perfluoropolyether (PFPE)-based materials that are able to accurately mold and replicate micro- and nanosized features using traditional techniques such as embossing as well as new techniques that we developed to exploit the exceptional surface characteristics of fluorinated substrates. Because of the unique partial wetting and nonwetting characteristics of PFPEs, we were able to go beyond the usual molding and imprint lithography approaches and have created a technique called PRINT (Particle [or Pattern] Replication In Nonwetting Templates). PRINT is a distinctive "top-down" fabrication technique capable of generating isolated particles, arrays of particles, and arrays of patterned features for a plethora of applications in both nanomedicine and materials science. A particular strength of the PRINT technology is the high-resolution molding of well-defined particles with precise control over size, shape, deformability, and surface chemistry. The level of replication obtained showcases some of the unique characteristics of PFPE molding materials. In particular, these materials arise from very low surface energy precursors with positive spreading coefficients, can be photocured at ambient temperature, and are minimally adhesive, nonswelling, and conformable. These distinctive features enable the molding of materials with unique attributes and nanometer resolution that have unprecedented scientific and technological value. For example, in nanomedicine, the use of PFPE materials with the PRINT technique allows us to design particles in which we can tailor key therapeutic parameters such as bioavailability, biodistribution, target-specific cell penetration, and controlled cargo release. Similarly, in materials science, we can fabricate optical films and lens arrays, replicate complex, naturally occurring objects such as adenovirus particles, and create 2D patterned arrays of inorganic oxides.