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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.

A novel test configuration is proposed to characterize the adhesion between an elastomeric matrix and an included steel cord. The circular cylindrical, rubber envelope is bonded to the cord along the axis of symmetry. A crack is forced to propagate along the interface by inflating the rubber envelope using a pressurised liquid in between the two media. The constant pressure recorded during the test is used to evaluate the interfacial critical strain energy release rate (Critical SERR). This is achieved using an analytical model, taking into account both the incompressible, hyperelastic nature of the rubber and geometrical nonlinearity due to large deformations. An energy balance is proposed to evaluate the interfacial SERR.

We examine the effects of nanoparticle addition at low concentration on the evaporation kinetics of droplets in the constant radius mode. The evaporative behaviour of deionized water and Al2O3 nanoparticle laden water on an aluminium substrate was observed at atmospheric and at different sub-atmospheric pressures. The two fluids exhibit the same evaporative behaviour, independent of the droplet volume or the sub-atmospheric pressure. Moreover, the linear relationship between evaporation rate and droplet radius, initially proposed by Picknett and Bexon nearly four decades ago for droplets evaporating in the constant radius mode, is satisfied for both liquids. In addition we have established a unified correlation solely function of fluid properties that extends this relationship to any sub-atmospheric pressure and fluid tested. We conclude that the addition of a small quantity of nanoparticles to the base fluid does not modify the kinetics of evaporation for pinned volatile droplets.

Small sessile drops of water containing either long or short strands of DNA ("biodrops") were deposited on silicon substrates and allowed to evaporate. Initially, the triple line (TL) of both types of droplet remained pinned but later receded. The TL recession mode continued at constant speed until almost the end of drop lifetime for the biodrops with Short DNA strands, whereas those containing long DNA strands entered a regime of significantly lower TL recession. We propose a tentative explanation of our observations based on free energy barriers to unpinning and increases in the viscosity of the base liquid due to the presence of DNA molecules. In addition, the structure of DNA deposits after evaporation was investigated by AFM. DNA self-assembly in a series of perpendicular and parallel orientations was observed near the contact line for the long-strand DNA, while, with the short-stranded DNA, smoother ring-stains with some nanostructuring but no striations were evident. At the interior of the deposits, dendritic and faceted crystals were formed from short and long strands, respectively, due to diffusion and nucleation limited processes, respectively. We suggest that the above results related to the biodrop drying and nanostructuring are indicative of the importance of DNA length, i.e., longer DNA chains consisting of linearly bonded shorter, rod-like DNA strands.

A Mixed Mode Bending (MMB) test is used to study the fracture process of an adhesively bonded specimen. Simple Beam analysis is proposed to estimate the Strain Energy Released Rate (SERR) from the evolution of specimen compliance. The effect of interface compliance on the process zone extension and specimen deformation is studied. Additionally, the Backface Strain Monitoring (BSM) technique is used to probe the peel and shear cohesive stresses along the bondline. With a simple analysis, a systematic procedure is proposed to evaluate the cohesive stresses as well as the peel and shear strain evolutions in the vicinity of the crack tip position during crack propagation experiment under mixed mode conditions.

Backface strain monitoring is used for probing the peel and shear cohesive stresses distribution along an adhesive layer during a crack propagation test under mixed mode conditions. An elastic model consisting of two Timoshenko beams attached with an elastic layer is derived to obtain simple analytical estimate of the specimen deformation from which a simple data reduction procedure is proposed. Taking benefit of the specimen symmetry, a proper experimental arrangement allows to separate the mode I and mode II contributions. Such protocol is less affected by the experimental artifacts which are observed on classical force versus displacement measurements from which are based the traditional fracture mechanics data reduction protocols. Additionally, backface strain measurements show superior sensitivity to non-linear interface behavior which should improve the quality of cohesive zone models identification.

We have studied the effect of interface heterogeneity on fracture, at both local and global scales. The single cantilever beam adhesion test was used to investigate interfacial fracture between polycarbonate plates and an elastic/fragile epoxy adhesive. Two surface treatments were applied to a (given) polycarbonate plate giving zones of strong and weak adhesion parallel to the crack direction. Calculated fracture energies differed from those expected from a simple rule-of-mixtures. A perturbation method, proposed by Rice, was used and results compared with crack fronts observed in situ. The technique was applied successfully but the difference in values of stress intensity factor between the zones was found substantially different from the experimental value. In an attempt to explain discrepancies, specimens with discontinuous crack fronts (adhesive and/or plates severed along the strong/weak adhesion frontier) were tested. Good agreement was found with the rule-of-mixtures predictions raising questions a...

Analysis of the three-point bending, End Notched Flexure (ENF), test is presented for exploitation with a new experimental methodology using the backface strain monitoring technique. The model consists of two Timoshenko beams joined with a two parameter elastic foundation, to evaluate stress and strain fields in both adherends and the adhesive layer, together with the specimen compliance and energy release rate. This model is compared to a simplified case, from which a comprehensive experimental method is proposed for precise monitoring of crack propagation and measurement of interface shear compliance during the ENF experiment. In addition, simple formulae are proposed for evaluating mode II energy release rate, and possible experimental artefacts due to asymmetric loading conditions are evidenced.

Standard tests of adhesively bonded specimens are likely to produce heterogeneous stress distribution along the crack front and its vicinity. High separation rate mode I dominated fracture test is performed. Observation of post mortem fractured surfaces with an optical microscope reveals characteristic features of mixed mode I/III fracture near the sides of the specimen but not in the middle. At first, finite elements calculations are presented to highlight that the adhesive layer is not loaded in pure mode I. We focus on the effect of mode III contribution near the side of the test pieces. To accommodate the anticlastic curvature of the adherends and the stress heterogeneity in the bondline, the crack front is curved during propagation. The anticlastic curvature of the adherend is evidenced with interferometric profilometry. These experimental observations are compared with finite element calculations in case of straight and curved crack front. Mode mixity along the crack front, local strain energy density and global energy release rate are evaluated. Difference between local and global approaches are discussed. It is found that the local strain energy density criterion suffices to allow for predicting the crack front shape.

It may be observed that, when dew drops form, although they may be positioned randomly on flat leaves, they tend to accumulate at the pointed ends of thin, slightly conical growths. We discuss here the basic physics leading to this phenomenon.

This paper describes an experimental arrangement to evaluate stress/strain fields in the process zone of asymmetric adhesively bonded joints. A transparent polycarbonate flexible beam was bonded to an aluminium alloy rigid block with an epoxy adhesive in a Single Cantilever Beam (SCB) configuration. The flexible adherend was loaded in the direction parallel to the initial crack front at constant rate. To monitor strains induced by bending and shear along the beam, electric strain gauges were attached to the upper surface of the flexible adherend. Thus strain distribution was measured above the bonded surface, which could be used to monitor crack propagation and investigate stress redistribution in the process zone. A Timoshenko beam lying on a Pasternak elastic foundation model was used for the analysis of experimental findings. Subsequently, the Digital Image Correlation technique was used to measure the flexible substrate in-plane displacement field in the vicinity of the crack front and to assess the specimen kinematics. We found that strain gauge instrumentation of the fracture mechanics specimen was a very sensitive technique for experimental analysis of crack propagation under complex loading, offering fine investigation of stress distribution in the cohesive zone.

Single Cantilever Beam (SCB) specimen loaded with a transverse force parallel to the crack front is proposed for the analysis of crack propagation phenomena under mixed mode conditions. The stress redistribution in the adhesive layer in the vicinity of the crack front so as the beam deformation are estimated using a Timoshenko beam on elastic foundation model. This model emphasizes the Mode II contribution due to flexural beam rotation but also the cleavage due to the beam torsion induced by the eccentricity of the adhesive layer with respect to the beam neutral axis. Finally, one dimensional representation is assessed by comparing analytical solution with finite elements calculations, proving that this analysis is suitable for the analysis of the SCB test under transverse loading.

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.

Preliminary results on a technique of Electrochemical-Impedance Spectroscopy (EIS) applied to the characterization of diffusion and absorption properties of water in an epoxy resin are reported. Thin films of polymer adhering to aluminum constitute simulated adhesive bonds. Immersion in water and assessment via electro-chemical impedance measurements allowed estimation of diffusion rates and water uptake. In parallel, classic gravimetric experiments were undertaken on thin, free films of the same polymer. Results of the two techniques are acceptably comparable, and thus EIS could be developed as a new tool for in situ assessment of adhesives.

We report on the drying press of sessile droplets of aqueous poly(ethylene oxide) (PEO) solutions studied by contact angle analysis. Liquid samples Were prepared with the same initial concentration of four different molecular weights, M-w, of PEO, Droplets with initial volumes of between: 1 and 5 mu L were left to evaporate while temperature, pressure, and relative humidity were kept constant Residues were formed with,either a disklike puddle Or a distinctive tall conical pillar shape The latter occurred following a four stage deposition process: pinned drying, during which the contact line is stationary, pseudodewetting, where the receding contact line is hid-need by precipitation, bootstrap building; eluting which the liquid droplet is lifted on freshly precipitated solid; and late drying Contact angle analysis allowed us to monitor all stages during drying and consider transitions between stages for different molecular weights. We illustrate the mechanisms taking place during the crucial stages of pinning and depinning revealing the effect of adhesion and contact line friction for high molecular weights and its influence on the final morphology of the dried PEO solute. To this end, we performed PEO solution droplet evaporation on PEO and PTFE films demonstrating the importance of interfacial interaction phenomena We show that the formation of disklike puddles for high molecular weights on glass is associated with continuous droplet contact line pinning. ThiS' results from the strong adhesion due to the interdigitation of the loops and tails of a polymer layer (adsorbed on glass during evaporation) with the polymer gel network inside the droplet that form as water evaporates.

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.

Water diffusion in polymers can often be approximated by a Fickian description, but a 2- phase model was proposed some years ago by Carter and Kibler (C&K), often referred to as “Langmuirtype” diffusion, by analogy with the Langmuir theory of adsorption. The two phases in question correspond to “mobile” and “bound” diffusant molecules. In this study, we have considered water uptake in an epoxy resin (an adhesive), employing gravimetry. A good, overall, empirical agreement with the C&K mathematical description of total mass increase with time has been obtained. In many applications of the C&K theory when used to quantify diffusion of water in polymers, only total water uptake is considered as a datum. However, a simple mathematical treatment of the theory enables the separate mobile and bound contributions to be isolated. These supplementary data have been used to try to get a better understanding of the meaning of the terms “mobile” and “bound” phases. Deuterium NMR analysis has been employed to study the mobility of the absorbed water. Decomposition of spectra has permitted us to assign two signals to the fractions of “mobile” and “bound” water. Analysis of peak evolution and a comparison with gravimetric data lead us to suggest that the “mobile” phase corresponds to diffusing molecules, whereas the “bound” phase corresponds to “clusters”.

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.

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.

Modern developments in polymer technology include the use of multimaterials. Two or more polymers are fashioned together to produce a composite with a desired combination of properties. However, the success of such materials depends on the quality of the adhesion between the various phases. In this study, we considered the adhesion between a grafted polyethylene and an ethylene/vinyl alcohol copolymer with various peel test geometries. The principal, L-peel, test was employed with various peel angles. Using elastoplastic analysis, we found that the effective peel energy, increasing with the peel angle, may be corrected for bulk energy dissipation, leading to a relatively constant local value for intrinsic peel resistance, independent of geometry. Effects of the peel rate and temperature may be combined to give a master curve, apparently a novelty for thermoplastics, although well known for elastomers. At a (corrected) peel rate, a jump in adhesion energy, apparently related to a change to a stick–slip peel regime, was observed. This jump appeared to be absent for some other peel geometries. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2843–2851, 2001

The thermal and thermo-oxidative degradation of a structural epoxy adhesive based on TGDDM and cured with DDA and DDS has been studied using thermogravimetric techniques. Both dynamic, or non-isothermal, and isothermal heating modes have been used and ageing has been undertaken in air at atmospheric pressure, primary vacuum and low pressure nitrogen. Degradation and weight loss of the material occur in two principal stages. In the first stage, weight loss occurs independently of the environment used. In addition, the activation energies calculated by three methods are very close for ageing in the various media. We conclude that this stage corresponds to thermolysis. The second stage exists only in the presence of oxygen, the material remaining relatively stable in nitrogen. This thermo-oxidative process leads to virtually total disappearance of the material with time, leaving only fillers behind.

Torsional adhesive joints were made using a filled DGEBA based epoxy resin cured with dicyandiamide (DDA). Ageing, both of the joints and the bulk adhesive, was effected at 40, 55 and 70°C at ca. 98% relative humidity. Joint strength was monitored after various ageing periods. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to analyze the adhesive and joint fracture surfaces. It was found that the adhesive composition near the adherent (zinc-electrocoated and bare steel) was modified compared with the bulk material. Before ageing there appeared to be an enrichment in hardener (DDA) near the adherent/adhesive interface and apparently adhesive failures were indeed interfacial. During ageing, the formation of ester groups subsequently transformed into carboxylates was observed. After ageing for up to 11 000 h, joints using a zinc electrocoated steel adherent showed fracture near the interface brought about by the modification of the adhesive. For longer ageing periods, failure occurred in a corrosion layer mixed with adhesive. A slightly better performance observed with the zinc electrocoated adherents has been attributed to the migration of Zn or Zn species into the adhesive during ageing.

The spreading of a sessile drop can be explained by a dynamic energy balance in which released excess capillary energy causes triple-line motion while wetting front speed is moderated by viscous dissipation within the liquid. This energy balance leads to a well-known relationship between dynamic and equilibrium contact angles and spreading speed. Experiments conducted with a silicone oil, poly(dimethylsiloxane), confirmed the validity of this equation. Similar experiments of spreading of tricresyl phosphate (TCP) on the same three substrates, glass, Halar (polyethylene−chlorotrifluoroethylene), and polypropylene, led to anomalous behavior: among other observations, we noted that the (apparent) spreading force needed for a given spreading speed was too high. Here, we suggest a tentative theory involving molecular orientation or conformation of the asymmetrical liquid molecules after they have been “laid down” on the solid substrates during spreading. As TCP molecules orientate near the triple line, the li...

Hygrothermal aging at elevated temperatures tends to induce degradation in epoxy resins. To predict the effects of this degradation, a knowledge of absorption and transport behavior of water is needed. In this work, a model material (DGEBA/DDA) has been employed to study the water absorption and absorption/desorption behavior during hygrothermal aging at 90°C, accompanied by degradation. The absorption results show an weight increase during the initial aging period followed by a decrease at later times. Absorption/desorption results show a similar phenomenon but with a net, overall weight loss after a certain period of aging. By assuming that water diffusion is approximately Fickian and that degradation of the resin is mainly caused by hydrolysis reactions, a model has been developed to describe the above-observed phenomena. Results show that the model is in good agreement with experimental data. Moreover, the model proposed can be used to estimate the average molecular weight of the intercrosslink chains after aging. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2659–2670, 1997

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.

Monodisperse nanosuspension droplets, placed on a flat surface, evaporated following the stick-slip motion of the three-phase contact line. Unexpectedly, a disordered region formed at the exterior edge of a closely packed nanocolloidal crystalline structure during the "stick" period. In order to assess the role of particle velocity on particle structuring, we did experiments in a reduced pressure environment which allowed the enhancement of particle velocity. These experiments revealed the promotion of hexagonal packing at the very edge of the crystallite with increasing velocity. Quantification of particle velocity and comparison with measured deposit shape for each case allowed us to provide a tentative description of the underlying mechanisms that govern particle deposition of nanoparticles at the triple line of an evaporating droplet. Behavior is governed by an interplay between the fluid, and hence particle, flow velocity (main ordering parameter) and wedge constraints, and consequently disjoining pressure (main disordering parameter). Furthermore, the formation of a second disordered particle region at the interior edge of the deposit (towards bulk fluid) was found and attributed to the rapid motion of the triple line during the "slip" regime. Additionally, the magnitude of the pinning forces acting on the triple line of the same drops was calculated. These findings provide further insight into the mechanisms of the phenomenon and could facilitate its exploitation in various nanotechnological applications.

The evaporation of sessile drops of water and n-decane placed on various substrates has been studied using a projection method. Drop dimensions and contact angle have been measured as a function of time. The first stage of the experiment corresponds to a saturated atmosphere ; then, when evaporation is allowed to occur, two or three different stages appear (depending on the surface roughness). For the first of these, a model is proposed which allows us to calculate the coefficient of diffusion of the liquid vapor in air.

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

When droplets of dilute suspensions are left to evaporate the final dry residue is typically the familiar coffee-ring stain, with nearly all material deposited at the initial triple line (Deegan et al., Nature, vol. 389, 1997, pp. 827–829). However, aqueous poly(ethylene oxide) (PEO) droplets only form coffee-ring stains for a very narrow range of the experimental parameters molecular weight, concentration and drying rate. Instead, over a wide range of values they form either a flat disk or a very distinctive tall central monolith via a four-stage deposition process which includes a remarkable bootstrap-building step. To predict which deposit will form, we present a quantitative model comparing the effects of advective build-up at the triple line to diffusive flux and use this to calculate a dimensionless number $\chi $. By experimentally varying concentration and flux (using a low-pressure drying chamber), the prediction is tested over nearly two orders of magnitude in both variables and shown to be in good agreement with the boundary between disks and monoliths at $\chi \approx 1. 6$.

It seems reasonable to think that a sessile drop, in an atmosphere saturated in the vapor of the liquid in question, will remain at equilibrium (and therefore not evaporate). However, meniscus curvature is an ingredient in equilibrium vapor pressure, and the former depends on height, albeit very weakly for small drops, due to hydrostatic pressure. Possible consequences in the context of (very slow) evaporation or condensation are discussed.

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.

In this Article, we demonstrate the dependence of the lifetime of a volatile droplet on the hydrophobicity of the substrate. Ethanol droplets placed on the molecularly smooth surfaces of three polymers, applied to substrates by spin-coating, showed distinct types of behavior depending on the hydrophobicity of the latter. High contact angles, θ, lead to fairly regular recession of the triple line during liquid evaporation at essentially constant θ, whereas low contact angle caused pinning, θ decreasing with time. The latter case leads to shorter drop lifetimes.

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

The effects of variability of intrinsic adhesion within a joint have been studied using a single cantilever beam (SCB) test. Fracture energy was found not to be a simple function of relative areas of 2 surface pre-treatments: a ‘weak’ zone decreased strength more than expected from simple, additive considerations. By severing the adhesive along the strong–weak transition, fracture energy increased. The prior antagonist effect appears to be directly linked to supplementary stresses generated near the crossover by continuity of the adhesive layer, existent despite differences in adhesion.

We report on the structure of ring deposits formed following the free evaporation of a sessile droplet containing nanoparticles. The nanostructure of the evaporatively formed deposits was revealed using atomic force microscopy (AFM). The structures were found to form a peak about 5 microns wide and one micron high and to exhibit a gentle slope gradually increasing in height in the direction of the three phase contact line motion. A theory is proposed to explain the formation and shape of these deposits. A comparison between the theory and the experiments shows a very good qualitative and quantitative agreement.

The spontaneous adhesion of vulcanized filled elastomers to steel has been studied using a rolling cylinder test. Owing to the relatively high degree of crosslinking, heavy cylinders are required to ensure good contact with the hard rubber track. This in turn invokes compression within the bulk of the rubber. A consequence is that hysteresis losses are associated not only with the adhesion phenomenon but also with the compression deformation of the system. Theoretical and experimental analyses have led to the conclusion that the two effects may be treated as additive. Both hysteresis losses can be related to the WLF superposition principle.

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.

A mode II instrumented end notched flexure three point bending (ENF) adhesion test is described. The adhesive joint consists of two aluminium alloy (AW7075-T6) plates bonded with a structural epoxy adhesive (Hysol® EA 9395™). Strain gauges are attached to the outer surface (backface) of the substrates in the lengthwise direction to measure local surface strain during crack propagation. Simultaneously, load/displacement measurements are performed. Two cases were investigated. The first was static: the joint was loaded below the crack propagation threshold. In the second, applied load above the threshold led to crack propagation. The former test confirmed the predicted load transfer mechanism between bonded and unbonded parts of the joint. In the second case, the crack front process zone was revealed in situ in mode II, we believe for the first time. These new results permitted validation of simple or refined analytical/numerical models including those of the cohesive zone. In addition, the backface strain gauge monitoring technique exhibited unexpected mode I contributions, quantitatively evaluated. Finally, R-curves are presented, as estimated with various standard models and compared with that postulated, where the process zone is accounted for.

Over the last thirty years or so, the use of contact angle measurement in the study of solid-liquid interactions and problems of adhesion has become very frequent. Methods have been developed based on the concepts 1 2 of authors such as Zisman1 (critical surface tension, γc) and Fowkes2 (polar and apolar interactions), to name but two examples. The essence of the method of contact angle measurement is that, unless considering a very thin substrate3, the triple line where the solid, S, liquid 1 and second, immiscible fluid 2 meet can be described by Young’s equation relating the three interfacial tensions, γij, and the contact angle θ: $${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$ (1) Provided the surface characteristics of two of the phases are known, those of the third may be assessed using the theoretically unique value of θ. This contact angle will be unique provided the three phases are strictly homogeneous and smooth. Unfortunately, in practice, it is very common to obtain experimentally a whole range of contact angles for a given three- phase system. The largest angle obtainable corresponds to that observed just after a drop of liquid has advanced on the solid surface and is therefore known as the advancing angle, θA. Similarly the smallest angle is obtained just after the arrest of a receding liquid drop and the corresponding angle is θR.