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Spontaneous motion of liquid droplets can occur on hydrophobic, micro-structured, solid surfaces comprising a structural gradient. In this study, we examine such motion experimentally and explain our observations by invoking variable droplet–surface interactions (both actuation and resistance forces) arising from the structural gradient. The oscillatory motion of the droplet constitutes an integral aspect of the behaviour and this is incorporated into the overall modelling. The theoretical model features a truncated spheroid for the drop shape (flattened in the region of solid contact) coupled with the oscillatory and alternate leading and trailing motion of the contact line. Results from the model and experiments provide good qualitative and quantitative agreement. The component of the vertical oscillation is found to help overcome wetting hysteresis and actuate the motion, this being a key element for the completeness of the model.

International audience; Liquid droplets move readily under the influence of surface tension gradients on their substrates. Substrates decorated with parallel microgrooves, or striations, presenting the advantage of homogeneous chemical properties yet varying the topological characteristics on either side of a straight-line boundary, are considered in this study. The basic type of geometry consists of hydrophobic micro-striations/rails perpendicular to the boundary, with the systematic variation of the width to spacing ratio, thus changing the solid–liquid contact fraction and inducing a well-defined wettability contrast across the boundary. Droplets in the Cassie–Baxter state, straddling the boundary, move along the wettability contrast in order to reduce the overall surface free energy. The results show the importance of the average solid fraction and contrasting fraction in a wide range of given geometries across the boundary on droplet motion. A unified criterion for contrasting striated surfaces, which describes the displacement and the velocity of the droplets, is suggested, providing guidelines for droplet manipulation on micro-striated/railed surfaces.The authors would like to acknowledge the support of the European Space Agency through ESA Contract No. 4000129506/20/NL/PG and the support received from the Engineering and Physical Sciences Research Council (EPSRC) through Grant No. EP/P005705/1. The authors also acknowledge the EC-RISE-ThermaSMART project, which received funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement No. 778104.

Wetting of liquids involving phase change is a fundamental physical phenomenon, at the heart of numerous natural, biological and technological areas. Exploiting the evaporation of wetting liquids for some exciting recent applications demonstrates the wide range of opportunities offered by this phenomenon. Our fundamental understanding of this, apparently simple, phenomenon, is however far from being complete. Many challenging questions about this process remain unanswered. In this paper we highlight some potential opportunities, which may benefit from the mastery of the process of wetting and evaporation of liquids. We also point to some of the challenges that scientists and researchers need to address to advance our understanding of this important topic.

Surfactants are commonly employed in numerous industrial applications and everyday products due to their enhancing wetting abilities. Trisiloxane surfactants in particular have proved to be matchless in reducing the surface tension of the liquid and consequently in enhanced wetting on even highly hydrophobic surfaces. In many practical applications trisiloxanes are frequently used as water-based solution. For convenience and reduced cost they are often pre-mixed with water and left for long periods of time in aqueous environment. We provide evidence that ageing of trisiloxane surfactants in water solutions may lead to loss of their interfacial activity. A mechanism for the observed behaviour is suggested and the phenomenon quantified. Along with a detailed explanation of the problem, we shed light on the dynamics of the process and provide some limitations of pre-mixed solutions.

The excellent wetting and spreading behavior of superspreader solutions has been known and extensively studied over recent years. However, explanations for wetting dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the time exponents, but none of the models so far has been able to describe successfully the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated partial wetting of Silwet L-77 superspreader solutions of high concentrations (well above cmc) on polymer-coated substrates of varying hydrophobicity. Results obtained can be explained in terms of diffusion at the “subsurface” as the key factor for trisiloxane enhanced wetting. A theory, involving concentration and surface tension gradients governing the dynamic wetting process, has been developed in order to explain the evolution of drop radius and consequent decrease in contact angle. The proposed model, which may explai...

The excellent spreading and wetting behaviour of superspreader solutions has been known and extensively studied over recent years. However, explanations for spreading dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the spreading exponents, but none of the models so far was able successfully to describe the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated the partial wetting of Silwet L-77® superspreader solutions of high concentrations (well above CMC) on polymer coated substrates of varying hydrophobicity. Results obtained can be explained in terms of the Marangoni effect as the major driving force for trisiloxane enhanced spreading. A simple theory, which involves surface tension gradients governing the spreading process, was developed in order to explain the specific evolution of the drop radius and consequent decrease in the contact angle. The proposed model was found to be in excellent agreement with the experimental results. Determined equation coefficients were shown to be dependent on both surfactant concentration and the hydrophobicity of the substrate.

Wetting and spreading processes which involve surfactant solutions are widely used in numerous industrial and practical applications nowadays. The performance of different non-ionic surfactants may vary significantly and so far superspreader solutions show the most promising spreading ability. The addition of trisiloxane surfactants to water was proven to enhance wetting, even on hydrophobic surfaces, on which conventional surfactants seem to have little or no effect. Although these extraordinary surfactants have been extensively studied over recent years, complete understanding of their underlying mechanisms and a suitable mathematical model are still lacking. Here we present a possible explanation for the impressive performance of trisiloxane, which is compared to wetting enhancement of a conventional surfactant. Additionally, we will explain why the hydrophobicity of the surface is a crucial factor for the spreading phenomenon. Light will be also shed on the effect of the pH of the solution to which surfactants are added. Finally, we will investigate long-term effects of the water environment on trisiloxane wetting ability and discuss if ageing may significantly affect their performance.

An experimental investigation into the evaporation of sessile nanofluid droplets is reported in this paper. The effect of nano-particle addition on the evaporative behaviour is studied using ethanol and Titanium Oxide nano-particles. The results show that a distinct ‘stick-slip’ pinning behaviour is observed when nano-particles are added to the base liquid. Increasing the nano-particle concentration was found to enhance the ‘stick-slip’ behaviour. This behaviour is attributed to the effects of evaporatively driven particle accumulation near the contact line. This in turn leads to an increase in localised viscosity, and an enhancement of contact line pinning.

We describe an experimental investigation of the concomitant evaporation and (de)wetting behavior of sessile drops of ethanol, either pure, or containing small amounts of titanium oxide nanoparticles. Pure ethanol behaved in a more or less "ideal" manner, with constantly decreasing contact radius, at essentially constant contact angle. However, distinct "stick-slip" pinning behavior of the triple line occurred when nanoparticles were added to the base liquid. Increased nanoparticle concentration enhanced the "stick-slip" behavior. The observed behavior is attributed to the effects of particle accumulation near the contact line, caused by the now-established advective flow during evaporation. "Slip" behavior can be explained by hysteretic energy barriers, somewhat akin to line tension. The "stick" behavior was not complete: some triple line drift occurred ("pseudo-pinning"). It is postulated that this may be due to small-scale pinning of the triple line by deposited particles, or to increased effective viscosity due to high, local nanoparticle concentrations.

Surfactants are invaluable in a number of agricultural applications in products such as pesticides and herbicides. In these products, surfactants are very often used in conjunction with acidifiers in order to improve their half-life. In this paper, we investigate how the change in pH affects surfactant wetting and spreading. We compare the performance of a conventional surfactant, Triton® X-100, with that of a trisiloxane superspreader, Silwet® L-77, on a number of polymer coated surfaces exhibiting various degrees of hydrophobicity. Silwet® L-77 in water based solutions showed very good wetting capability on all surfaces. However, its wetting ability was drastically reduced with the addition of acetic acid. On the other hand, Triton® X-100 was not affected by the addition of acid and exhibited the same spreading behaviour as in water-based solutions.

Experimental results on the wetting behavior of water, methanol, and binary mixture sessile drops on a smooth, polymer-coated substrate are reported. The wetting behavior of evaporating water/methanol drops was also studied in a water-saturated environment. Drop parameters (contact angle, shape, and volume) were monitored in time. The effects of the initial relative concentrations on subsequent evaporation and wetting dynamics were investigated. Physical mechanisms responsible for the various types of wetting behavior during different stages are proposed and discussed. Competition between evaporation and hydrodynamic flow are evoked. Using an environment saturated with water vapor allowed further exploration of the controlling mechanisms and underlying processes. Wetting stages attributed to differential evaporation of methanol were identified. Methanol, the more volatile component, evaporates predominantly in the initial stage. The data, however, suggest that a small proportion of methanol remained in the drop after the first stage of evaporation. This residual methanol within the drop seems to influence subsequent wetting behavior strongly.

The wetting and evaporation behaviour of methanol-water droplets deposited on a smooth silicon substrate were investigated experimentally. Contact angle and droplet shape kinetics were studied using an optical technique. Drops were deposited onto a silicon substrate and enclosed in a cell with nitrogen as the ambient gas. Besides the case of pure water and pure methanol, three different volume fractions of methanol in water were investigated: 10%, 50% and 80%. Using a Kruss DSA100 contact angle analyser, the behaviour of the contact angle, droplet volume, and base width was determined as a function of time. Results show that evaporation of the droplet takes place in successive stages for mixtures. The more volatile component seems to evaporate principally in the first stage, during which the contact angle of the binary drop is closer to that of pure methanol. Because the wetting behaviour is partly dictated by the surface tension of the liquid-vapour interface, methanol is believed to be concentrated at the interface during this first stage. After complete evaporation of the methanol, the wetting behaviour of the droplet tends towards that of pure water. The mechanisms that dictate the evaporation and wetting behaviour of such binary droplets include many effects: diffusion of methanol in water in the liquid phase; accumulation of one of the component near the interface and preferential evaporation followed by diffusion of one component in another in the vapour phase. In order to model the phenomenon, the above effects must be taken into account. Solutal Marangoni stress as well as interfacial instabilities may also play an important role in the behaviour of theses systems.

Electrochemical treatment of poly(tetrafluoroethylene) (PTFE) surfaces has been effected in localized areas of typical dimension of the order of 200 μm, leading to small regions of increased surface free energy. These have been studied by considering the wetting triple lines of tricresyl phosphate in the vicinity. Good correlation between theoretical predictions and observations of the shapes of the wetting lines was obtained, not only in the outer regions and on the untreated material, but also in the inner, treated zones. A predicted transition from concavity to convexity of the wetting line shape was corroborated by observations. The "excess wettability" of the treated zones of PTFE decreased following intense light exposure. Excess wettability, as evaluated from experimental data, exceeded predictions, possibly due to nonrespect of Antonow's rule at the local scale.

When a liquid spreads spontaneously on a rigid solid, a dynamic equilibrium is set up in which the non-equilibrated Young force causes triple line motion. Most solids may be considered as rigid when submitted to capillary forces. With soft solids, however, a ‘wetting ridge’ is formed as the wetting front advances or recedes. This ridge accompanies the motion leading to energy dissipation due to the hysteretic nature of the soft substrate and the strain cycle undergone. It has been established that the wetting ridge (of the order of tens of nanometers in height on silicone rubbers) can be so important in its consequences that viscous dissipation within the liquid becomes negligible. We propose to review the impact of solid deformation in the kinetics of spreading of liquids on soft materials, in wetting and dewetting modes, and we will consider also liquid drop motion due to gravity. In this last case, liquid drops running down a rubber plane move at a speed largely determined by the shear modulus of the polymer. The possibility of tailoring wetting, dewetting or draining kinetics on soft materials to a specific need is clearly evidenced.

The aim of this paper is to investigate the mesoscale behavior and structure of an adhesive near the fracture front of an asymmetric joint consisting of carbon fiber/epoxy resin composites bonded with a relatively soft, epoxy adhesive. A single cantilever beam fracture test at constant separation rate gave steady-state crack propagation, details of which were followed by digital image correlation (DIC). A deformed, triple line region was found between the adhesive, air, and the composite, somewhat resembling a “wetting ridge,” as found with a liquid meniscus in contact with a soft solid. Importantly, the partially separated bondline layer took part in (non-unidirectional) load transfer between adherends (and thus energy dissipation), contrary to common assumptions where the separated bondline is assumed no longer to play a structural role. A simple model, based on the Flamant contact mechanics approach, is proposed and compared with both a finite element solution and experimental data extracted from image correlation. The model points out the importance of two length scales: process zone extent and adhesive thickness, both being known to affect global properties of bonded structures.

Although the macroscopic, bulk behavior of liquids wetting (or dewetting) can be adequately explained by viscous flow, the intricate details near the solid/liquid/fluid triple line (TL) are still not totally appreciated.These two aspects correspond to the bases of the hydrodynamic approach and of the molecular kinetics approach, respectively, to explain spreading. We have suggested a complementary model to that of molecular kinetics, based on the modified curvature of a dynamic, wetting meniscus near the TL, which leads to local condensation of vapor from the surrounding atmosphere, further contributing to motion. An earlier paper dealt with liquids whose thin film behavior (i.e., near the TL) is dominated by long-range van der Waals forces. Here, we turn our attention to the spreading of water. Basic equations are modified to allow for non-negligible meniscus slope and then applied to a model for water, consisting of two different, thickness dependent, disjoining pressure isotherms.

Wetting and dewetting phenomena involve as yet incompletely understood kinetic behavior occurring near the solid/liquid/fluid triple line. Two major classes of theory exist to explain local motion: the molecular kinetics approach and the hydrodynamic model. We suggest another possibility here which is complementary to the former. Increased meniscus curvature near the triple line of an advancing wetting front leads to an effective slight supersaturation of the liquid vapor, and local condensation ensues. This mass transfer can be in addition to other effects, including hydrodynamic dissipation. Depending on wetting conditions, either effect may dominate, in general agreement with observations reported in the literature.

Wetting or dewetting on soft elastomeric surfaces is largely controlled by the dissipative properties (viscoelasticity) of the substrate. The component of liquid surface tension acting perpendicularly to the solid causes a deformation, or “wetting ridge”, which must be displaced with the triple line. Previously, triple line motion due to capillary imbalance has been studied, whereas in the present work, we consider liquid motion due to gravity. Liquid drops running down an elastomeric plane move at a speed largely determined by the properties of the polymer. Results obtained agree well with the theory and demonstrate how motion depends on the state of the polymer, the angle of inclination of the plane, and drop volume.

Industrial wetting situations often involve the presence of a third component near an interface, potentially modifying the local effective capillary balance. We here consider a simplified situation in which a drop is placed on a substrate supporting a thin layer of "contamination." As spreading of the drop to equilibrium occurs, the contaminant diffuses into the drop, modifying the effective substrate/drop interfacial free energy (increasing it, it is assumed). Thus the kinetics of spreading are altered. The essential effect when the final equilibrium contact angle is zero is that of accelerating the process. However, when the final value of contact angle is finite, "overshoot" may occur. The drop spreads beyond its equilibrium state and then retracts to attain its equilibrium state asymptotically at long times. Copyright 2000 Academic Press.

Most wetting or spreading problems are treated with surface and interfacial free energies taken as constants for the system in question. We consider here the case where a two-dimensional drop (ribbon) is deposited on a solid substrate bearing a very fine layer of a contaminant which modifies the effective drop/solid interfacial free energy. As the contaminant diffuses into the drop, the interfacial free energy, and thus the capillary balance, evolve. This has strange effects on wetting behaviour, including causing a wetting `overshoot'. Possible applications are briefly discussed.

Practical and mathematical problems related to the use of Young’s and Laplace’s equations are discussed and it is pointed out how difficulties are often encountered when heterogeneous solids are being studied. The shape of a wetting triple line on an otherwise homogeneous solid, yet perturbed locally by the proximity of a heterogeneity, has been considered in the past by several workers including the present author. It has been shown that the wetting front is deformed to a shape which is closely approximated by a logarithmically decreasing function, sufficiently far from the heterogeneity itself. In this article, one of the earlier (Fourier series) solutions, issue from the analysis of a sessile drop, is simplified and applied to the limiting case of a straight (in its unperturbed form) triple line. The logarithmic behaviour at long distances is corroborated and, more importantly, fine structure of the triple line within the heterogeneous zone is assessed. A change from concave to convex curvature is clearly shown.

The kinetics of spreading of a liquid drop is usually controlled by conversion of capillary potential energy into viscous dissipation within the liquid when the solid is rigid. However, if the solid is soft, a “wetting ridge” near the solid/liquid/vapour triple line can also be a dissipative sink as the wetting front moves. As a consequence, the kinetics of wetting of rubber may be controlled essentially by viscoelastic losses in the polymer rather than by viscous losses in the liquid drops. Therefore, a direct analogy between the kinetics of wetting and adhesion, respectively, for a liquid and a solid on an elastomeric substrate has been recently proposed. In this paper, the superposition of viscoelastic braking and moderate rubber swelling in the drop spreading phenomenon is considered.

Classic theory predicts a unique value of equilibrium contact angle, θo, for a given solid/liquid/fluid system. However, wetting phenomena are often very complicated in practice, with contact angle hysteresis being a major source of problems in interpretation. Contact angle variability is related to several factors, but we consider two which are particularly relevant to polymeric substrates - effects of orientation of molecular chains near the surface and local solid strain at the wetting front. A model is proposed to explain “stick-slip” motion of the triple line.

It has been known for many years that a spreading liquid droplet can be appreciably slowed on a soft, viscoelastic substrate by the appearance of a "wetting ridge" or protuberance of the solid near the triple phase contact line because of capillary forces. Viscoelastic dissipation in the solid surface can outweigh that of liquid viscosity and, therefore, dominate wetting dynamics. In this paper, we show that a short, rapid spreading stage exists after initial contact. The requisite balance determining the speed of motion is between capillary forces and inertial effects. As spreading proceeds, however, inertia lessens and the lower spreading speed allow for viscoelastic effects in the solid to increase. The transition between early inertial and viscoelastic regimes is studied with high-speed photography and explained by a simple theory.

When a liquid drop is placed on a smooth, rigid, solid substrate, it spreads until the final thermodgnamic equilibrium is attained. The kinetics of spreading of the drop are controlled by conversion of capillary potential energy into viscous dissipation within the liquid. However, if the solid is sufficiently soft, a local deformation, or »wetting ridge«, may form near the wetting front and the motion of the latter may lead to viscoelastic dissipation. We describe cases in which viscoelastic dissipation dominates and thus where spreading speed depends on bulk properties of the solid, rather than on liquid viscosity. This behavior in wetting can be considered to be analogous to dissipation phenomena in the adhesion of elastomers. Therefore, a comparison is made between wetting and adhesion dynamics. It appears clearly that a parallel can be drawn between the wetting of rubber and rubber adhesion to glass when both phenomena involve the same viscoelastic material. Kinetics of formation and breaking of corresponding interfaces is controlled by the damping properties of the soft solid substrate. In this paper, the viscoelastic properties of elastomers are described by two parameters, n and U o ; n is the usual speed power factor (n0.5-0.6) and U o a characteristic speed below which a fraction of the elastic strain energy in the wetting ridge is dissipated

The evolution of contact angles of water on three polymer surfaces has been studied both with and without an environment saturated in water vapour. It was observed that in a saturated atmosphere, evolution was slow and involved a slight reduction in drop height and contact angle at constant contact diameter. This effect is attributed to a slight diffusion of water into the polymer. In a dry atmosphere, evolution was far more rapid and involved three stages. Initially, the contact diameter remained constant whilst both drop height and contact angle decreased. When a small value of contact angle had been reached, both the drop height and diameter decreased concomitantly, thus maintaining an essentially constant value of contact angle. This stage did not occur on a rough substrate. Finally, height, diameter and contact angle all decreased somewhat sporadically as the drop volume tended to zero. This behaviour was probably related to anchoring effects of the triple line on heterogeneities. A simple model explaining the essentially linear decrease in drop height with time during the first stage is proposed.

Wetting hysteresis due to isolated surface heterogeneities is now fairly well understood but when the solid presents a population of defects, complex cooperative effects between neighbours may exist. One such effect is that of 'shadowing', in which a proportion of the flaws near the triple line, and which would otherwise contribute to hysteresis, are masked by already existing deformations to the wetting front caused by neighbouring heterogeneities. This renders them inactive and, as a result, the hysteretic wetting force is only expected to be a linear function of density for sparse populations. Theoretical predictions are compared with experimental results obtained with model heterogeneous surfaces consisting of overhead projector transparencies bestrewn with circular ink spots - the defects. Agreement is found to be satisfactory when intrinsic angles on both the homogeneous solid and the flaws are finite, whereas the concordance is less satisfactory when the contact angle of the liquid on the homogeneo...

The dynamics of the three-phase contact line for water and ethanol is experimentally investigated using substrates of various hydrophobicities. Different evolutions of the droplet profile (contact line, R, and contact angle, θ) are found to be dependent on the hydrophobicity of the substrate. A simple theoretical approach based on the unbalanced Young force is used to explain the depinning of the contact line on hydrophilic surfaces or the monotonic slip on hydrophobic substrates. The second part of the article involves the addition of different quantities of titanium oxide nanoparticles to water, and a comparison of the evaporative behavior of these novel fluids with the base liquid (water) on substrates varying in hydrophobicity (i.e., silicon, Cytop, and PTFE) is presented. The observed stick-slip behavior is found to be dependent on the nanoparticle concentration. The evaporation rate is closely related to the dynamics of the contact line. These findings may have an important impact when considering the evaporation of droplets on different substrates and/or those containing nanoparticles.

Trisiloxane surfactants, known as 'superspreaders', are commonly employed in numerous applications where enhanced wetting is of the utmost importance. The underlying mechanisms of superspreader wetting have been a focus of scientific interest for ca. 2 decades, and a number of mechanisms have been proposed to explain the unique trisiloxane dynamics. We have studied trisiloxane behaviour in thin capillaries to get further insight into their interfacial activity. Additionally, our knowledge of the capillary rise of superspreaders is surprisingly limited, and the effect of this extraordinary group of surfactants on capillary phenomena has been largely overlooked. Diffusion was confirmed to be the limiting factor of trisiloxane behaviour. A tentative theoretical explanation for the phenomenon studied and an appropriate mathematical model are presented. It is concluded that the enhancement of wetting due to surfactant addition is also a function of geometry: the effect is clear for a sessile drop, but more complex and less beneficial in a capillary.

Spreading of liquids can often successfully be described by either hydrodynamic or molecular kinetics theories. The natural wetting of small, sessile drops of glycerol trioleate on smooth, flat, solid surfaces may be expected to comply with the former model, yet nonnegligible differences are manifest, which apparently cannot be accounted for by the addition of a molecular kinetics term. We postulate that modifications to the spreading rate are due to an effect of time-dependent molecular orientation occurring after liquid molecules have contacted the solid substrate. Copyright 2001 Academic Press.

THE spreading of liquids on solid substrates is important in many natural and industrial processes, including eye irrigation, adhesive bonding, tertiary oil recovery and the application of insecticides, paints and inks. A liquid drop placed on a flat, smooth, horizontal solid spreads as excess capillary potential energy is dissipated during the motion. If the solid is rigid, energy is dissipated entirely by viscous flow within the liquid1. For a non-rigid solid, however, a local microscopic deformation, or Vetting ridge'2,3, forms near the wetting front, and its motion also induces viscoelastic dissipation4–6. Here we report direct evidence of the wetting ridge obtained using scanning interferometric microscopy. Our experiments demonstrate, for liquids and solids with properties representative of practically important substances, that the mesoscopic surface deformation can significantly affect macroscopic phenomena. For instance, the formation of a nano-scale wetting ridge can increase the spreading time of a drop by considerably more than an order of magnitude, with potentially important consequences in technological and natural settings.

The kinetics of a spreading drop is usually controlled by conversion of capillary potential energy into viscous dissipation within the liquid when the solid is rigid. However, if the solid is sufficiently sot, a «wetting ridge» near the solid/liquid/vapor triple line can also be a dissipative sink as the wetting front moves. In this paper we provide evidence to show that when the substrate is soft, this local deformation near the triple line may lead to an energy dissipation that may outweigh effects due to viscosity over a large range of speeds and therefore spreading kinetics becomes viscosity independent

Acid/base theory has, over the last decade or so, been developed to describe interfacial free energies, or tensions, in wetting theory. An approach put forward by van Oss and co-workers, involving van der Waals/Lifshitz and Lewis electron acceptor/donor contributions to surface/interfacial free energies, has often been employed. The present study considers use of this theory for evaluating surface data for various polymeric surfaces employing known, characterized liquid probes for obtaining contact angle data. Results are analyzed using extended matrix analyses, originally proposed for treating the dispersive/polar interpretation of wetting results, and good agreement with literature values is obtained. By "inverting" the system, i.e., by treating the known solids as probes and rederiving surface data for liquids, inconsistencies are found to arise. Results for wetting of the same polymers and mica, using a two-liquid system (n-octane/water), are exploited to attempt to rederive the surface characteristics of water. Again, serious incoherence is manifest. Despite the conceptual interest of acid/base theory, clearly the mathematical formulation is presently inadequate. Copyright 1999 Academic Press.

It has been shown over the last few years how the wetting (or dewetting) of a soft, elastomeric substrate can be markedly affected by local deformation of the solid near the triple line, due to the component of liquid surface tension acting perpendicularly to the (undeformed) solid surface, i.e., the “wetting ridge.” Since the degree of cross-linking of an elastomer affects its mechanical properties, we have undertaken a study of the influence of cross-linking on (de)wetting behavior. Using a silicone rubber in four states of cross-linking, we have observed that triple-line motion of tricresyl phosphate increases in speed with degree of cross-linking. Two principal factors influence this behavior, both being directly linked to average intercross-link molecular weight,Mc. A decrease inMcincreases elastic modulus and therefore decreases the importance of the wetting ridge, whilst also lowering the dissipative properties of the polymer. The combined effects lead, in the case studied, to a linear relationship between dewetting speed and elastomeric elastic modulus to the power 10/3.

We present the results of an experimental study investigating electrowetting effects in nano-suspension drops. Wetting of sessile drops of titanium oxide (TiO2) nano-particles dispersed in deionised water was studied under a DC voltage potential. The presence of nano-particles is found to further enhance spreading of drops when a DC potential is applied. Results are explained based on the change in interfacial energy induced by nano-particle adsorption at the solid-liquid interface. An amendment of the Young-Lippmann equation accounting for the presence of nano-particles is proposed. The electrowetting expression proposed is validated against the experimental data for substrates with various dielectric thicknesses.

A theoretical analysis has been carried out on the system consisting of an axisymmetric sessile drop resting on a thin elastic solid in the presence of gravity. The solid is treated in one case as a thin plate and in the other case as a membrane. The consequences of the variational treatment employed are equations relating to contact angle equilibrium, drop and solid profiles. It is shown that contact angles are not intrinsic surface properties of the phases involved but invoke equally such characteristics as bulk properties of the solid and physical dimensions when the solid in question is deformable.

It has previously been shown that, under high stress and consequently at short times to failure, a major factor governing the environmental stress cracking (ESC) of polyethylene is the ability of a liquid environment to penetrate a growing fissure at a sufficiently high speed to maintain contact with the fracture front. In this earlier study, viscosity was shown to play a significant role in this kinetic effect. The purpose of the present paper is to demonstrate that another property of the solid–liquid system influencing ESC under these high stress conditions is the spreading coefficient of the liquid on the polymer—the parameter defining the tendency of the liquid to wet the polyethylene. It has been shown that the spreading coefficient can be considered as a force and this force in conjunction with atmospheric pressure constitutes the force necessary to drive the liquid into the growing crack.

A new technique has been developed to study the surface energy characteristics of small diameter fibres. This method is tensiometric and based on wetting properties in a two phase liquid system. Two types of carbon fibre have been investigated under both static and dynamic conditions in immersion and emersion. The high strength fibre, which was found to have a polymeric coating, gave reproducible results consistent with typical values for a polymer. However, the high modulus fibre could not be characterized due to considerable scatter in the experimental results. It is suggested that the source of this phenomenon is the existence of surface energy gradients which leads to hysteresis effects.

The liquid-fluid tension at the edge of a liquid drop may provoke a wetting ridge on a sufficiently soft solid substrate. Many soft solids such as polymers, and in particular elastomers, are viscoelastic and display hysteretic characteristics in a strain cycle. Such a strain cycle can be present near the advancing wetting ridge of a spreading drop. By considering that the excess capillary free energy is consumed during spreading by both viscous dissipation in the liquid and by mechanical, viscoelastic hysteresis in the solid, it is shown how spreading properties may be modified. An important degree of wetting hysteresis may result. In addition absolute spreading rates are reduced.

At a triple wetting line, the liquid-fluid tension may produce a wetting ridge in sufficiently soft solids. By employing variational methods, it has been shown that the solid strain combined with the Laplace pressure may modify contact angle equilibrium on a mesoscopic scale, situated between that of the molecular structure of the triple line and the macroscopic behavior observed in quotidian contact angle assessment. Young's equation remains perfectly valid in the latter regime. Some possible consequences are discussed.

Recent work on the surface energy characterization of solids has employed a two phase liquid system in which the liquids are immiscible. The validity of this approach depends on the ability of one liquid to displace the other liquid on the solid surface. The present work develops theoretical criteria governing this displacement, based on free energy considerations. Some experimental work supports the theoretical conclusions.

Following an earlier paper, the equilibrium of an axisymmetric sessile drop resting on a thin, elastic plate has been considered using the minimum free energy hypothesis. A consequence is that although Young's equation is still obeyed at the triple line when considering the true contact angle, the apparent contact angle (i.e. with respect to the horizontal) invokes parameters other than simply the interfacial free energies, such as the drop volume and elastic properties of the solid. A quantitative estimate of apparent contact angle dependence on plate thickness has been made in the case of small drops. Finally, consideration of stability, again using the minimum free energy hypothesis, suggests that the axisymmetric configuration may not always be the most likely. This, it is conjectured, may have consequences in cell biology.

The final equilibrium state and the spreading dynamics approaching equilibrium have been studied for a sessile drop posed on a thin solid. For the case of equilibrium, both axisymmetric and cylindrically symmetrical geometries have been considered. In both situations, but particularly the latter, liquid surface tension can markedly curve the initially planar solid. The dynamics of spreading can be speeded up by the presence of a thin substrate. Released elastic energy contributes to capillary effects in overcoming viscous resistance to flow.