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Elastomeric surfaces and oil-infused elastic surfaces reveal low ice adhesion, in part because of their deformability. However, these soft surfaces might jeopardize their mechanical durability. In this work, we analyzed the mechanical durability of elastic polydimethylsiloxane (PDMS) surfaces with different balances between elasticity and deicing performances. The durability was studied in terms of shear/tensile ice adhesion strength before and after different wear tests. These tests consisted of abrasion/erosion cycles using standard procedures aimed to reproduce different environmental wearing agents. The main objective is to evaluate if our PDMS surfaces can become long-lasting solutions for ice removal in real conditions. We found that our elastic surfaces show excellent durability. After the wear tests, the ice adhesion strength values remained low or even unaltered. Although the oil-infused PDMS surface was the softest one, it presented considerable durability and excellent low ice adhesion, being a promising solution.

This research was supported by the project MAT2017-82182-R funded by the State Research Agency (SRA) of Spain and European Regional Development Fund (ERDF). Funding for open access charge: Universidad de Granada / CBUA, Hypothesis: Ice adhesion to rigid materials is reduced with low energy surfaces of high receding contact angles. However, their adhesion strength values are above the threshold value to be considered as icephobic materials. Surface deformability is a promising route to further reduce ice adhesion. Experiments: In this work, we prepared elastomer surfaces with a wide range of elastic moduli and hydrophobicity degree and we measured their ice adhesion strength. Moreover, we also explored the deicing performance of oil-infused elastomeric surfaces. The ice adhesion was characterized by two detachment modes: tensile and shear. Findings: The variety of elastomeric surfaces allowed us to simultaneously analyze the ice adhesion dependence with deformability and contact angle hysteresis. We found that the impact of these properties depends on the detachment mode, being deformability more important in shear mode and hydrophobicity more relevant in tensile mode. In addition, oil infusion further reduces ice adhesion due to the interfacial slippage. From an optimal balance between deformability and hydrophobicity, we were able to identify surfaces with super-low ice adhesion., MAT2017-82182-R State Research Agency (SRA), European Regional Development Fund (ERDF), Universidad de Granada / CBUA

Hypothesis Characterization of contact angle hysteresis on soft surfaces is sensitive to the measurement protocol and might present adventitious time-dependencies. Contact line dynamics on solid surfaces is altered by the surface chemistry, surface roughness and/or surface elasticity. We observed a “slow” spontaneous relaxation of static water sessile drops placed on elastic surfaces. This unexpected drop motion reveals unresolved equilibrium configurations that may affect the observed values of contact angle hysteresis. Drop relaxation on deformable surfaces is partially governed by a viscoelastic dissipation located at the contact line. Experiments In this work, we studied the natural relaxation of water drops formed on several smooth PDMS surfaces with different elastic moduli. We monitored in time the contact angle and contact radius of each drop. For varying the initial contact angle, we used the growing-shrinking drop method. Findings We postulate that the so-called “braking effect”, produced by the surface deformability, affects the contact line velocity and in consequence, the contact angle measurements. We conclude that the wetting properties of elastic surfaces should be properly examined with reliable values of contact angle measured after drop relaxation.

Icing has become a hot topic both in academia and in the industry given its implications in transport, wind turbines, photovoltaics, and telecommunications. Recently proposed de-icing solutions involving the propagation of acoustic waves (AWs) at suitable substrates may open the path for a sustainable alternative to standard de-icing or anti-icing procedures. Herein, the fundamental interactions are unraveled that contribute to the de-icing and/or hinder the icing on AW-activated substrates. The response toward icing of a reliable model system consisting of a piezoelectric plate activated by extended electrodes is characterized at a laboratory scale and in an icing wind tunnel under realistic conditions. Experiments show that surface modification with anti-icing functionalities provides a synergistic response when activated with AWs. A thoughtful analysis of the resonance frequency dependence on experimental variables such as temperature, ice formation, or wind velocity demonstrates the application of AW devices for real-time monitoring of icing processes., ERDF (FEDER) A way of making Europe PID2019-110430GB-C21 PID2019-109603RA-I00 PID2020-112620GB-I00 MCIN/AEI/10.13039/501100011033, European Commission EU H2020 program 899352

In this work, we studied the wettability of hydrophobic surfaces fabricated with three different materials (PTFE, PDMS and paraffin wax) over a wide range of roughness. We estimated the Advancing, Receding and Most-Stable contact angles and we identified the transition from a homogeneous wetting regime (Wenzel) to a hybrid wetting regime (Cassie-Baxter). Using a modified Cassie-Baxter equation which considers the chemical heterogeneity of the samples, we were able to determine the solid area fraction within the contact area. This way, we designed a methodology to identify the superhydrophobic degree of rough-hydrophobic surfaces.

Polymethylmethacrylate/poly-tert-butylmethacrylate Janus nanoparticles were synthesized by the electrohydrodynamic co-jetting method. The Janus character of these nanoparticles was visualized through super-resolution imaging with Structured Illumination Microscopy. The Janus nanoparticles, and the corresponding homogeneous sets, were then morphologically characterized to assess their size, distribution, and concentrations. All nanoparticles presented high interfacial activity as measured by pendant drop tensiometry at water/decane interfaces. At high concentrations and compression states, the Janus nanoparticles exhibited higher interfacial activity than the homogeneous nanoparticles. This is in agreement with theoretical and experimental works in which Janus nanoparticles present higher interfacial activity than homogeneous nanoparticles.

The analysis of wetting properties of superhydrophobic surfaces may be a difficult task due to the restless behavior of drops on this type of surfaces and the limitations of goniometry for high contact angles. A method to validate the performance of superhydrophobic surfaces, rather than standard goniometry, is required. In this work, we used bouncing drop dynamics as a useful tool to predict the water repellency of different superhydrophobic surfaces. From bouncing drop experiments conducted over a wide range of superhydrophobic surfaces, we found that those surfaces with a proper roughness degree and homogeneous chemical composition showed higher water-repellency. We also conducted a drop condensation study at saturating conditions aimed to determine whether there is direct correlation between water repellency and condensation delay. We found that the drop condensation process is strongly related to the surface topography, as well as the intrinsic wettability. The condensation is promoted on rough surfaces but it is delayed on intrinsically hydrophobic surfaces. However, the differences found in condensation delay between the superhydrophobic surfaces explored in this study cannot be justified by their chemical homogeneity nor their efficiency as water repellent surfaces, separately.

One of the main challenges in cookware and baking industry is the fabrication of enhanced nonstick coatings including two main features: highly hydrophobic, resistant and durable. In general, life-long release coatings are prepared with a polymer-filler composite of several microns thickness deposited on food-compatible metals such as aluminum, stainless steel or aluminized steel, as the dimensions and functionality of the product. In this work we present several routes to fabricate water-repellent surfaces on aluminized steel as alternative to current nonstick coatings but with enhanced release properties and high durability. For this purpose, we explored two different strategies to directly create hierarchical texture on aluminized steel surfaces: sandblasting in combination to acid etching and single acid etching. The rough metal surfaces were further hydrophobized by using two different organic coatings: a Teflon film and a silane monolayer. The release properties were quantified through the study of the adhesion of water drops by means of sliding angle and contact angle measurements. We identified the most appropriate texturing process by assessing a balance between the water repellency properties and the lowest damage caused on the metal surface. This damage was quantified through thickness loss measurements of the aluminum layer. The durability of the samples was analyzed with Scotch-peeling test and next, with sliding angle measurements. We analyzed the roughness and morphology of the surface textures by contact profilometry and FE-SEM. The surface chemical composition was also analyzed by EDX. Our results pointed out to that the most durable repellent surfaces were those ones silanized. The combination of sandblasting and acid etching and the hydrophobization process with silane molecules enable to fabricate super-nonstick coatings on aluminized steel of high durability.

Titanium is extensively employed in biomedical devices, in particular as implant. The self-assembly of alkylphosphonates on titanium surfaces enable the specific adsorption of biomolecules to adapt the implant response against external stimuli. In this work, chemically-tailored cpTi surfaces were prepared by self-assembly of alkylphosphonate molecules. By bringing together attributes of two grafting molecules, aqueous mixtures of two alkylphosphonates were used to obtain mixed self-assembled films. Single self-assembled films were also altered by laser abrasion to produce chemically patterned cpTi surfaces. Both mixed and patterned self-assembled films were confirmed by AFM, ESEM and X-ray photoelectron spectroscopy. Water contact angle measurements also revealed the composition of the self-assembly films. Chemical functionalization with two grafting phosphonate molecules and laser surface engineering may be combined to guide the bone-like formation on cpTi, and the future biological response in the host.

Colloidal interactions have been extensively studied due to the wide number of applications where colloids are present. In general, the electric double layer force and the van der Waals interaction dominate the net force acting between two colloids at large separation distances. However, it is well accepted that some other phenomena, especially those acting at short separation distances, might be relevant and induce substantial changes in the force profiles. Within these phenomena, those related to the surface contact angle, the hydration degree of the ions, or the pH, may dominate the force profiles features, not only at short distances. In this paper, we analyzed the effect of the pH and counterion type on the long-range as well as short-range forces between polystyrene colloidal particles by using the colloidal probe technique based on AFM. Our results confirm that the features of the force profiles between polystyrene surfaces are strongly affected by the pH and hydration degree of the counterions in solution. Additionally, we performed a study of the role of the pH on the wettability properties of hydrated and nonhydrated polystyrene sheets to scan the wettability properties of this material with pH. Contact angle measurements confirmed that the polystyrene surface is hydrophobic in aqueous solutions over the entire range of pHs investigated. These results are in good agreement with the features observed in the force profiles at low pH. At high pH, a short-range repulsion similar to the one observed for hydrophilic materials is observed. This repulsion scales with the pH, and it also depends on the hydration degree of the ions in solution. This way, the short-range forces between polystyrene surfaces may be tunable with the pH, and its origin does not seem to be related to the hydrophobicity of the material.

Evaporating salty droplets are ubiquitous in nature, in our home and in the laboratory. Interestingly, the transport processes in such apparently simple systems differ strongly from evaporating "freshwater" droplets since convection is partly inverted due to Marangoni stresses. Such an effect has crucial consequences to the salt crystallization process and to the deposits left behind. In this work we show unprecedented measurements that, not only confirm clearly the patterns of the flow inversion, but also elucidate their impact on the distribution of non-volatile solutes. Contrary to what has been often reported in the literature, such a flow reversal does not prevent the formation of ring-shaped stains: particles accumulate at the contact line driven solely by the interfacial flow. We can therefore conclude that the classical "coffee-stain effect" is not the only mechanism that can generate ring-shaped stains in evaporating droplets.

Fluoropolymer-based coatings are widely used for release applications. However, these hydrophobic surfaces do not reveal a significantly low adhesion. Water repellency incorporated to fluoropolymer coatings might enhance their release performance. In this work, we focused on the surface texturing of a well-known polytetrafluoroethylene (PTFE)-based coating. We explored as texturing routes: sanding, sandblasting and laser ablation. We examined the surface roughness with white light confocal microscopy and the surface morphology with environmental scanning electron microscopy (ESEM). Water-repellent fluoropolymer coatings were reproduced in all cases, although with different degree, parametrized with bounces of water drops (4&ndash, 5 &mu, L). Laser ablation enabled the lowest adhesion of coatings with 24 &plusmn, 2 bounces. This result and the current development of laser patterning for industry assure the incipient use of laser ablation for release coatings.

Aluminum-magnesium (Al-Mg) alloy and aluminum-coated steel (aluminized steel) are typically used for the manufacturing of baking trays and molds. For these applications, these materials must be modified to develop release and hydrophobic properties. With this aim, the bare substrates are typically coated with low-surface energy materials such as fluoropolymers, elastomers, or sol-gel layers. In this work, some alternative strategies to prepare these functional surfaces are presented. We used three-step processes involving (i) micro-texturing, (ii) nano layer deposition through immersion and electrodeposition, and (iii) hydrophobization. The raw substrates were sanded or sandblasted at the micro scale, accordingly. Texturization at the nano scale was achieved with a cerium layer formed by electrodeposition or solution immersion. The cerium layers were hydrophobized with fatty acids. The wetting properties of the samples were studied with tilting-plate and bouncing drop methods. We measured the surface roughness of the samples by contact profiling and analyzed their surface morphology using a field emission scanning electron microscope (FESEM). The elemental chemical composition of the samples was analyzed by energy-dispersive X-ray spectroscopy (EDX). The wettability results indicated that the best performance for the Al-Mg substrates was reached by sandblasting and later immersion in a cerium nitrate solution. For aluminized steel substrates, the best results were obtained with both electrodeposition and immersion methods using a cerium chloride solution., The authors were supported by the University of Cordoba (Spain), which financed this work through the Own Research Plan 2019. This research was partially financed by the State Research Agency (SRA) and European Regional Development Fund (ERDF) through the project MAT2017-82182-R.

Elucidating the predominant factors for the freezing delay of surfaces is still a matter of discussion and controversy. Freezing delay is explained in literature through the classical nucleation theory. It postulates that freezing delay of a surface is enhanced with low surface roughness and sessile drops of high contact angles. However, since surface roughness influences the wetting properties, a better understanding of how each factor affects the freezing delay requires to uncouple both effects systematically. This is indeed the reason why certain contradictions are found in literature. Besides, some works report that further factors, such as the surface-to-drop heat transfer might also be important. In this work, we analyzed independently how drop geometry, surface roughness and thermal conductivity influence the freezing delay of solid surfaces at unsaturated conditions. Our results show that the drop contact angle and surface roughness strongly influences the freezing delay on conductive and insulating materials. Although its importance is minor, we also found that conductive materials delay freezing more efficiently than insulating materials. In conclusion, our results point out that conductive, smooth and hydrophobic surfaces are the most efficient surfaces to delay freezing in unsaturated environments.

Objectives To evaluate the effect of different Self-Assembled Films (SAFs) on the development of calcium-phosphate (Ca/P) biomimetic layers on commercially pure titanium (cpTi) and the final cell adhesion on these coatings. Materials and methods A total of five different surfaces were evaluated. Alkyl phosphonate Self-Assembled Films (SAFs) with three different ending groups (methyl [–CH 3 ], carboxyl [–COOH], and phosphonate [–PO(OH) 2 ]) were used to cover ultra-polished cpTi surfaces. These functionalized surfaces were compared with a hydroxylated ultra-polished cpTi surface. A biomimetic Ca/P layer was deposited on each surface by precipitation. A control cpTi sample with no precipitates was introduced in the study. Topography (AFM), morphology (ESEM), chemistry (XPS, EDX), crystallography (XRD), wettability (dynamic contact angle), layer thickness (WLCM), percolation (fluid retention) and cell response analysis were used for surface characterization. Results XRD showed the formation of hydroxyapatite (HA) in the four biomimetic coatings. The Ca/P ratios obtained by XPS in the four biomimetic coatings showed similar values (≈1.40) corresponding to amorphous hydroxyapatite. The water wettability of the biomimetic coatings was similar but the surface morphology was different. The thickest biomimetic coating with lower percolation threshold and greater cell adhesion was obtained on the carboxyl-terminated SAF. The biomimetic coatings improved the cell adhesion but it was further improved by the presence of underlying SAFs that bond the coating to cpTi surface. Conclusions The use of SAFs chemically stabilizes the biomimetic precipitates and guarantees higher cell adhesion than in absence of SAFs. The apatite nucleation and growth depended on the underlying SAF. Biomimetic coatings on hydroxylated surface increases cell adhesion but on SAFs functionalized surfaces achieve higher cell response. The SAFs with carboxyl ending group promoted the formation of thick and interconnected biomimetic coatings, with low fluid retention and a very significant cell response.

The design of durable superhydrophobic coatings for metal surfaces is a subject of interest and research. Galvanized steel is one of the most used metallic materials for components of automobiles, building structures and roofing. In spite of its wide number of applications, galvanized steel has been scarcely modified to reach superhydrophobicity. The main reason for this is that galvanized steel is essentially a zinc-coated steel surface and most of the strategies to prepare superhydrophobic coatings on metal substrates require partial removal of the surface material. For this reason, providing a non-aggressive strategy to create superhydrophobic galvanized steel (or other metal coated materials) is an important challenge. With this aim, we propose in this paper a methodology based on a two-step texturing process (sandblasting and “soft” acid-etching) followed by a fluoropolymer deposition as a non-invasive strategy to produce water repellent surfaces on this material. The roughness of the samples was analyzed by confocal microscopy and FE-SEM imaging, the surface chemical composition by EDX and the wettability properties by contact angle measurements. Our results show that both texturing methods are necessary to create hierarchically micro/nano-structured surfaces on galvanized steel. This structure induces superhydrophobic properties once the metal is subsequently Teflon-coated. Sandblasting introduces a micro-scale texture, while soft acid-etching incorporates nano-asperities.

An entropic segmentation method is presented and applied to the contour detection of water drops over patterned surfaces. Jensen-Shannon divergence is computed from a double sliding window in the image to get a real number matrix, in which a region growing procedure is performed in a similar way to usual watershed. Then a region merging process is achieved, and the optimal configuration is selected to obtain the complete drop contour. Once the drop contour is detected from top-view images, the contact angle might be readily computed from the area enclosed by the contour and the drop volume.From the results obtained the proposed method outperforms other usual methods, like watershed or Canny.

Hypothesis Wettability of solid surfaces is mostly probed with sessile drops rather than bubbles because this method is readily followed out. This recurrent use may lead to a misleading connection of certain phenomena to the hydrophobicity/hydrophilicity of materials. For instance, the Cassie-Baxter regime and the wicking effect are generally associated only to hydrophobic and hydrophilic surfaces, respectively. However, the same phenomenology should be observed when air bubbles (underwater conditions) in contact with solid surfaces are used instead. In particular, one might expect that rough-hydrophilic surfaces become superaerophobic due to the appearance of a hybrid dewetting regime, like the Cassie-Baxter regime described for rough-hydrophobic surfaces. Otherwise, rough-hydrophobic surfaces might become superaerophilic due to air-wicking. Experiments To elucidate this issue, in this work, we analyzed the wettability of surfaces with very different intrinsic contact angle and roughness degree. The analysis was performed with both Sessile Drop and Captive Bubble methods. Findings Our results with captive bubbles for rough-hydrophilic surfaces revealed phenomena only explained by the occurrence of a transition from the Wenzel regime to an “inverse” Cassie-Baxter regime. In addition, our results with captive bubbles for rough-hydrophobic surfaces showed evidences of air percolation through the interconnected asperities. This effect reminds the wicking effect reproduced on rough-hydrophilic surfaces, responsible for superhydrophilicity.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film. The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

Janus particles are colloids with two different spatial domains. When these domains are synthesized with a contrast in wettability the particle shows an enhanced interfacial activity in comparison with the corresponding homogeneous particles. There are different synthesis routes to fabricate such particles. One of the most common is to start with a homogeneous particle and functionalize just one hemisphere of the particle or form the particle directly with two compartments, the so-called true Janus particles. Other particles which rely on the phase separation during synthesis are rather considered Janus-like particles. The Langmuir balance and the pendant drop tensiometry are useful to characterize the interfacial activity of the Janus and homogeneous particles at liquid interfaces.

Complete understanding of colloidal assembly is still a goal to be reached. In convective assembly deposition, the concentration gradients developed in evaporating drops or reservoirs are usually significant. However, collective diffusion of charge-stabilized particles has been barely explored. The balance between convective and diffusive flows may dictate the particle dynamics inside evaporating colloidal drops. In this work we performed in situ counting of fluorescent particles in the vicinity of the triple line of evaporating sessile drops by using confocal laser scanning microscopy. We used particles of different sizes, with different charge response over the pH scale and we focused on charged and nearly uncharged particles. Two substrates with different receding contact angles were used. Binary colloidal mixtures were used to illustrate simultaneously the accumulation of particles with two different charge states at the triple line. The deposition rate close to the triple line was different depending on the electric state of the particle, regardless of the substrate used.

A successful methodology for obtaining hybrid films which allow thermal triggering and continuous, irreversible, control of their hydrophilicity/hydrophobicity nature was developed. Two types of poly(dimethylsiloxane)-silica (PDMS@SiO2) films were prepared for that purpose: A hydrophilic film in which the thermal treatment causes an irreversible gradual increase of hydrophobicity; and a hydrophobic film that turns more hydrophilic upon thermal treatment. The opposite directionality of the change is dictated by the film substrate, on which the same hybrid is deposited. In both cases the thermal treatment induced a phase separation which caused the change in hydrophobicity. The magnitude of change in hydrophilicity/hydrophobicity is continuously controllable in both types of films by either the temperature or heating time. The films were characterized before and after heating by a variety of methods, including contact angle (CA) measurements with the sessile drop and the tilting plate methods, and by X-ray photoelectron spectroscopy (XPS) analysis. A thorough kinetic study was carried out, following the progress of the changes in the wettability property of the surfaces. The kinetics analyses proved that the changes in the wettability in all cases are due to phase separation processes, the directionality of which is determined by the treatment of the substrate on which the films are deposited. By monitoring the change of wettability (ΔCA) at various temperatures, an Arrhenius plot was obtained from which the activation energy and Arrhenius pre-exponential factor for the phase separation were derived, corroborating the proposed mechanism. To the best of our knowledge, this is the first use of phase separation behavior of a hybrid film in order to apply irreversible, thermally controllable change of surface wettability, tailored to proceed in opposite directions, and the first kinetic study of such a process.

In hot asphalt applications, the adhesion between bitumen and mineral aggregate is usually described in terms of bitumen surface tension and contact angle of the bitumen over the aggregate. However, the quantification of the physico-chemical bond between bitumen and aggregate under realistic conditions is a nontrivial task. In this work, we designed a high-temperature goniometer to measure the contact angle of liquid bitumen on mineral aggregate substrates. The drop deposition was conducted once the thermal equilibrium between liquid bitumen and aggregate was attained. We monitored the spreading of sessile drops of viscous naphthenic bitumen and asphaltic bitumen on polished sheets of calcareous aggregates at high temperature (70–100 °C). A near complete wetting with very low contact angles (13– $$24^{\circ }$$ ) was reproduced regardless of the bitumen origin and temperature. Furthermore, the coating degree of the naphthenic and asphaltic bitumens on the calcareous aggregates at high temperature was apparently similar. We found that the bitumen-aggregate adhesion is adequately described by dynamic spreading rather than by equilibrium wettability. Spreading kinetics was ruled by the particular properties of each bitumen such as viscosity and acid index. We found evidences of acid etching of the naphthenic bitumen on the calcareous aggregates during spreading at high temperature.

The oxidation state of the surfaces of titanium-based biomaterials strongly depends on their previous history. This factor affects the titanium wettability and it probably conditions the success of the implanted biomaterials. However, the separate role of the pollution and oxidation states of metallic titanium surfaces remains still controversial. To elucidate this, it is required to standardize the initial surface state of titanium in terms of roughness and surface chemistry, and then, to monitor its wettability after the corresponding treatment. In this work, we studied finely polished surfaces of commercially pure titanium (cpTi) which were subjected to cleaning surface treatments. X-Photoelectron spectroscopy was used to characterize the surface chemistry and the oxide film thickness. The contact angle hysteresis in underwater conditions was measured with the growing/shrinking captive bubble method, which allowed for mimicking the real conditions of implantable devices. The water wettability of smooth...

The interfacial activity of 3.5 nm homogeneous (HPs) and amphiphilic Janus gold nanoparticles (JPs) was characterized by pendant drop tensiometry for water/air and water/decane interfaces. This technique requires a smaller quantity of nanoparticles than the traditional Langmuir balance technique. The direct deposition at the interface of the nanoparticles dispersed in a spreading solvent also requires smaller quantities of sample than does adsorption from the bulk. From the growing and shrinking of the pendant drops, the interfacial activity of the nanoparticles can be evaluated and compared within a wide range of area per particle. In this work, the JPs exhibited a higher interfacial activity than did the HPs in all cases. A hard disk model fits the piecewise compression isotherm of the HPs, yet this model underestimates the interactions between the JPs adsorbed at the interface.

Surface functionalization based on organophosphonate molecules and Direct Laser Patterning (DLP) technique both allow to design chemically patterned titanium surfaces devoted to biomaterial engineering. Ultrapolished surfaces of commercially pure titanium were modified with 16-phosphonohexadecanoic acid and octadecylphosphonic acid. The DLP technique with a green pulsed laser was applied to selectively desorb the organophosphonate molecules on the functionalized titanium surfaces. Three regimes of laser ablation were found on the bare titanium surfaces as the ratio between the spot diameter and the interspot distance. Finally, the organophosphonate functionalized titanium surfaces subjected to DLP revealed different wettability domains with minimum impact on the roughness, validated by XPS, AFM and ESEM.

Evaluation of superhydrophobic (SH) surfaces based on contact angle measurements is challenging due to the high mobility of drops and the resolution limits of optical goniometry. For this reason, some alternatives to drop-shape methods have been proposed such as the damped-oscillatory motion of ferrofluid sessile drops produced by an external magnetic field. This approach provides information on surface friction (lateral/shear adhesion) from the kinetic energy dissipation of the drop. In this work, we used this method to compare the low adhesion of four commercial SH coatings (Neverwet, WX2100, Ultraever dry, Hydrobead) formed on glass substrates. As ferrofluid, we used a maghemite aqueous suspension (2% v/v) synthesized ad hoc. The rolling magnetic drop is used as a probe to explore shear solid&ndash, liquid adhesion. Additionally, drop energy dissipates due to velocity-dependent viscous stresses developed close to the solid&ndash, liquid interface. By fitting the damped harmonic oscillations, we estimated the decay time on each coating. The SH coatings were statistically different by using the mean damping time. The differences found between SH coatings could be ascribed to surface&ndash, drop adhesion (contact angle hysteresis and apparent contact area). By using this methodology, we were able to grade meaningfully the liquid-repelling properties of superhydrophobic surfaces.

The impact of the interfacial properties of naphthenic bitumen is very relevant for road paving. In this work, we propose an approach for predicting the interfacial energy of bitumen in aqueous media based on contact angle hysteresis measurements. We measured the surface tension of naphthenic bitumen at high temperature with the pendant drop method. We estimated the surface energy of bitumen as the value of surface tension extrapolated to room temperature. The acid–base properties of naphthenic bitumen films were investigated by contact angle titration with buffered water. Due to the occurrence of contact angle hysteresis, we performed advancing–receding contact angle measurements on bitumen films at different values of pH with the low-rate dynamic contact angle technique. We observed that the receding angle was dependent on pH. Instead, advancing contact angle just reflected the nature of bitumen in dry conditions. We chose the average contact angle of advancing and receding contact angles as an estimate of the thermodynamically meaningful contact angle. Finally, the bitumen-water interfacial energy at room temperature was calculated using the Young equation. The low interfacial energy of bitumen at high pH confirmed the existence of native surfactants.

We study the pattern formation induced by drying colloidal suspensions and polymer solutions in a vertical Hele-Shaw cell immersed in a reservoir. The contact line speed is controlled by pumping out the solution from the reservoir. At low capillary number stick–slip motion of the receding contact line can be observed for silica suspensions. Periodic strips spontaneously form during drying. We study the strip periodicity and deposit morphology as a function of the receding velocity and pH of the solution. In the same conditions, dried polymer films are almost flat.

In order to produce well-ordered structures via evaporation, it is essential to control the evaporation flux, solute concentration, interaction between the solute and the substrate, etc. During the drying of particle suspensions, the particle deposition process can be dictated by electrostatic and van der Waals forces. However, the complex physics involved in the drying of colloidal particle suspensions and the erratic contact line dynamics of evaporating sessile drops complicate the analysis of the problem. In this work, we propose a new methodology based on shrinking sessile drops to standardize the contact line dynamics of evaporating drops, but with no observable evaporation (macroscopic scale). We used a microinjector to decrease the drop volume through a small hole drilled in the substrate. Unlike drying drops, with our methodology the particle concentration in the drop bulk remained constant during the entire process and the macroevaporation was negligible. We probed the arrangement of nanoparticles at driven receding contact lines, with low capillary numbers and at time scales shorter than during free evaporation. The electrostatic double layer interactions were modified by diluting the nanoparticles in buffer solutions at different pH values. We also examined the impact of the wettability contrast between the substrate and the particle on the deposit morphology. We found that the ring-like deposits formed at driven contact lines might be suppressed with strongly interacting particles.

Owing to contact angle hysteresis, the contact angle that a sessile drop adopts on a surface can be quite different depending on the process of drop formation. This is particularly noticeable for receding contact angle because it is most susceptible to the details of the measuring method. We performed low-rate dynamic contact angle experiments with steadily moving contact lines and millimeter-size drops in the very low-capillary number regime. We employed a quadratic volumetric flow rate ( V ∝ t 3 ) to achieve the steady motion of contact lines such as happens in the Wilhelmy balance. The values of receding contact angle provided with the quadratic flow rate were stable and time-independent over a larger area of the polymer surfaces studied. Next, we monitored receding sessile drops with equal initial volume but different static contact angle on the same surface. This procedure allowed us to scan the contact angle hysteresis range up to the minimum observable value of contact angle. We probed with distilled water the different response of surfaces of polymer (PMMA, PC and PTFE) and metal oxide (titanium). We found two behaviors according to the substrate employed: a constant value of receding contact angle regardless of the static contact angle and a decrease of receding contact angle as the static contact angle decreased.

Currently, there is no conclusive evidence regarding the global equilibrium condition of vibrated drops. However, it is well-known that vibration of sessile drops effectively reduces the contact angle hysteresis. In this work, applying a recent methodology for evaluating the most-stable contact angle, we examined the impact of the type of excitation signal (random signal versus periodical signal) on the values of the most-stable contact angle for polymer surfaces. Using harmonic signals, the oscillation frequency affected the postvibration contact angle. Instead, the white noise signal enabled sessile drops to relax regardless of their initial configuration. In spite of that, the values of most-stable contact angle obtained with different signals mostly agreed. We concluded that not only the amount of relaxation can be important for relaxing a sessile drop but also the rate of relaxation. Together with receding contact angle, most-stable contact angle, measured with the proposed methodology, was able to capture the thermodynamic changes of "wetted" polymer surfaces.

After the impact of the great review of M. Strobel and C. S. Lyons on contact angle measurements, we discuss some claims of the authors. The Wilhelmy method is not generally "the best technique for measuring the contact angle hysteresis" as the authors claimed. Otherwise, we think that, even though equilibrium contact angle is an "unattainable" angle, the most-stable contact angle obtained from the system relaxation is experimentally accessible. The most-stable contact angle is energetically significant for evaluating quantitatively the surface energy value of rough, chemically homogeneous surfaces from the Wenzel equation, and the average surface energy of smooth, chemically heterogeneous surfaces from the Cassie equation. The most-stable contact angle, the advancing contact angle and the receding contact angles enable the thermodynamic description of the range of contact angle hysteresis and the distribution of metastable system configurations.

We propose a new method for the direct measurement of the most-stable contact angle, using the mechanical vibration of sessile drops from different metastable states. We relaxed sessile drops of identical volume but with different stable contact angles between advancing and receding configurations. Before the vibration, we were able to scan the range of experimentally-accessible drop configurations. In this manner, the most-stable contact angle was experimentally recognized as the observable contact angle unchanged after vibration independent of the previous history (initial state) of the system. We applied this novel strategy to paraffin wax surfaces with a wide range of roughness degree.

In order to characterize a solid surface, the commonly used approach is to measure the advancing and receding contact angles, i.e., the contact angle hysteresis. However, often an estimate of the average wettability of the solid–liquid system is required, which involves both the dry and wetted states of the surface. In this work, we measured advancing and receding contact angles on six polymer surfaces (polystyrene, poly(ethylene terephthalate), poly(methyl methacrylate), polycarbonate, unplasticized poly(vinyl chloride), and poly(tetrafluoroethylene)) with water, ethylene glycol and formamide using the sessile drop and captive bubble methods. We observed a general disagreement between these two methods in the advancing and receding contact angles values and the average contact angle determined separately by each method, although the contact angle hysteresis range mostly agreed. Surface mobility, swelling or liquid penetration might explain this behaviour. However, we found that the 'cross' averages of th...

The capillary rise/fall of a liquid within a thin capillary tube is described by the well-established Jurin's law. The liquid reaches an equilibrium height/depth as the capillary pressure is balanced by the hydrostatic pressure. When the adhesion force at the three-phase contact line is counteracted by the liquid weight, the liquid column also stabilizes. Both descriptions evidently coincide although they are based on different physical quantities. Most undergraduate textbooks do not mention why these two routes draw two identical results; besides, the proofs become conceptually incorrect. We review the underlying ideas in Jurin's law and the two possible derivations.

Texturization of surfaces is usually advantageous in biomaterial engineering. However, the details of the textured surfaces can be more determining on cell adhesion and proliferation, rather than their roughness degree. Titanium is extensively used as a dental implant material in the human body. In this paper, the effect of four surface treatments on commercially pure titanium has been evaluated. These treatments were polishing (pTi); hydrofluoric acid (HF) etching (eTi); Al2O3 blasting (bTi); Al2O3 blasting + HF etching (beTi). Roughness and fractal dimensions were obtained from atomic force microscopy. Wettability was measured using water sessile drops. Morphology and surface chemical composition were analyzed with scanning electron microscopy and energy dispersive X-ray (EDX). MG-63 cell cultures were performed at different times (180 min, 24 h, 48 h, 72 h). Lowest roughness was found in pTi samples followed by eTi, bTi and beTi samples. Etching generated surfaces with the highest fractal dimension and negative skewness. Young contact angles were similar except for pTi and bTi surfaces. Silicon and aluminum traces were found in pTi and bTi samples, respectively. Cell adhesion (≤24 h) was greater on bTi and beTi surfaces. After 48 h, cell proliferation, mediated by specific morphologies, was improved in eTi samples followed by beTi surfaces. For the same surface chemistry, cell growth was driven by topography features.

Beyond the linear model of Schulze et al. [ J. Adhes. Sci. Technol. 3 (1) (1989) 39–48], a more comprehensive model for predicting the Young contact angle of rough and chemically homogeneous surfaces from contact angle hysteresis measurements is proposed. This new model incorporates the barrier effect, described by Shuttleworth and Bailey, and the Wenzel model as well. Both the linear and nonlinear models were evaluated using a thermoplastic polyether-based elastomer and PDMS and two liquids. The elastomer surfaces were textured on purpose at differing degrees of roughness. Both models provided similar values of Young angle, although the nonlinear model also provided information about the distribution of practical metastable configurations around the most stable equilibrium configuration. Despite the linear model showed slightly better goodness-of-fit and minor uncertainties in its parameters, the nonlinear model is more reliable and theoretically consistent.

The proportionality constant in Ohm's law is the electrical resistance. The resistance of conducting objects depends on their geometrical features such as cross-sectional area and length. Since the cross-sectional area of resistors usually varies as its length, the calculation of resistances is a non-trivial task. In this letter, a closed-form integral expression for resistance is just derived for resistors with parallel curved terminals. The derivation proposed here, equally applicable to the calculation of capacitances, uses the properties of an electric field and the mathematical concept of parallel surfaces. This new teaching strategy can be useful for first undergraduate courses of science and engineering studies.

Objectives To evaluate dentin wettability and bonding of self-etching and total-etch adhesives on smear layer-covered and smear layer-free dentin. Methods Three self-etching adhesives (Clearfil SE Bond, AdheSE and Xeno III) and one total-etch adhesive (SingleBond) were evaluated. The substrates were mid coronal smear layer-covered and smear layer-free dentin. Dentin wettability by resins was studied from contact angle measurement using sessile drop method and Axisymmetric Drop Shape Analysis (ADSA). Shear-bond strength was evaluated using a push-out technique (ad hoc design). Data were analysed with two-way ANOVA and Tukey's test. Results Similar values of dentin wettability were obtained for all adhesives tested regardless the presence of smear layer. Even though, Xeno III and AdheSE exhibited slightly lower wettability (higher contact angles values) on smear layer-free dentin. Likewise, the presence of smear layer did not affect the shear-bond strength. Total-etch adhesive obtained higher shear-bond strength than self-etching adhesives, which obtained similar values. Significance Wettability is similar between self-etching and total-etch adhesives. The smear layer affects slightly the wettability of self-etching adhesives. Shear-bond strength is not sensitive to the smear layer presence. Total-etch adhesion is stronger than self-etching adhesion. There is no clear relationship between wettability and bond strength.

Roughness factor is widely used for topography characterization of surfaces. The measurement of meaningful values of roughness factor depends on the instrument settings, e.g. spatial resolution and scan-size, the instrument characteristics (voxel dimensions), the post-treatment of discrete data array (tessellation algorithm), and finally the nature of the surface texture (e.g. fractal). To analyze the influence of all these parameters on the value of roughness factor and evaluate the influence of each parameter, different synthetic (mathematically defined) surfaces and acid-etched/passivated titanium surfaces were used. The titanium surface topographies were studied using two different microscopes: white light confocal microscopy (WLCM) and atomic force microscopy (AFM). In decreasing order of influence, roughness factor values are sensitive to the specific surface nature (fractal or non-fractal), the spatial resolution, the scan-size and the tessellation algorithm, whereas the instrumentation does not seem to be an important parameter in this study. The effect of the variability in the roughness factor depending on the above parameters in interpretation of the Young's contact angle and solid–vapor interfacial energy was studied based on the apparent contact angle observed and the Wenzel's equation. It was found that depending on how roughness factor is measured variations up to 20% in the Young's contact angle or solid–vapor interfacial energy may be found.

The purpose of this article is to present some important advances in the imaging techniques currently used in the characterization of bitumen and bituminous emulsions. Bitumen exhibits some properties, such as a black colour and a reflecting surface at rest, which permit the use of optical techniques to study the macroscopic behaviour of asphalt mixes in the cold mix technology based on emulsion use. Imaging techniques allow monitoring in situ the bitumen thermal sensitivity as well as the complex phenomenon of emulsion breaking. Evaporation-driven breaking was evaluated from the shape of evaporating emulsion drops deposited onto non-porous and hydrophobic substrates. To describe the breaking kinetics, top-view images of a drying emulsion drop placed on an aggregate sheet were acquired and processed properly. We can conclude that computer-aided image analysis in road pavement engineering can elucidate the mechanism of breaking and curing of bituminous emulsion.

Paraffin emulsions are commonly used in the manufacture of chipboard panels to provide resistance to water and humidity. The quality and performance of chipboards are improved with the use of paraffin emulsions stabilized by mixed surfactant systems, although little is known about the basic colloidal chemistry of such systems and their implications in the manufacturing and processing of the chipboard panels. In the present work, the stability and the phase behavior of paraffin emulsions stabilized by a mixture of anionic and nonionic surfactants, are described. Stability is studied by applying thermal and ultracentrifugation cycles, and also by rheology (steady state and dynamic determinations). A significant increase of stability is observed at high {anionic surfactant/(anionic surfactant+nonionic surfactant)} ratios. Phase behavior studies have demonstrated the presence of hexagonal liquid crystalline structures at high ionic surfactant/nonionic surfactant ratios and the presence of lamellar structures ...

The motion of electrically charged particles under crowding conditions and subjected to evaporation-driven capillary flow might be ruled by collective diffusion. The concentration gradient developed inside an evaporating drop of colloidal suspension may reduce by diffusion the number of particles transported toward the contact line by convection. Unlike self-diffusion coefficient, the cooperative diffusion coefficient of interacting particles becomes more pronounced in crowded environments. In this work, we examined experimentally the role of the collective diffusion of charge-stabilized nanoparticles in colloidal patterning. To decouple the sustained evaporation from the contact line motion, we conducted evaporating menisci experiments with driven receding contact lines at low capillary number. This allowed us to explore convective assembly at fixed and low bulk concentration, which enabled to develop high concentration gradients. At fixed velocity of receding contact line, we explored a variety of substrate-particle systems where the particle-particle electrostatic interaction was changed (via p H) as well as the substrate receding contact angle and the relative humidity. We found that the particle deposition directed by receding contact lines may be controlled by the interplay between evaporative convection and collective diffusion, particularly at low particle concentration.

Segregation of particles during capillary/convective self-assembly is interesting for self-stratification in colloidal deposits. In evaporating drops containing colloidal particles, the wettability properties of substrate and the sedimentation of particles can affect their accumulation at contact lines. In this work we studied the size segregation and discrimination of charged particles with different densities. We performed in-plane particle counting at evaporating triple lines by using fluorescence confocal microscopy. We studied separately substrates with very different wettability properties and particles with different charge-mass ratios at low ionic strength. We used binary colloidal suspensions to compare simultaneously the deposition of two different particles. The particle deposition rate strongly depends on the receding contact angle of the substrate. We further observed a singular behavior of charged polystyrene particles in binary mixtures under "salt-free" conditions explained by the "colloidal Brazil nut" effect.

Since de Gennes coined in 1992 the term Janus particle (JP), there has been a continued effort to develop this field. The purpose of this review is to present the most relevant theoretical and experimental results obtained so far on the surface activity of amphiphilic JPs at fluid interfaces. The surface activity of JPs at fluid-fluid interfaces can be experimentally determined using two different methods: the classical Langmuir balance or the pendant drop tensiometry. The second method requires much less amount of sample than the first one, but it has also some experimental limitations. In all cases collected here the JPs exhibited a higher surface or interfacial activity than the corresponding homogeneous particles. This reveals the significant advantage of JPs for the stabilization of emulsions and foams.

For decades, paraffin emulsion has been used in chipboard panel manufacturing for the purpose of making it both water and damp proof. A most important variable when assessing the effectiveness of the emulsion as a water‐resistant agent in the panel is the oil content of the paraffin. Although this aspect has been dealt with from the manufacturing (Carll, 1996) approach, there is scarcely any literature that relates the oil content in paraffin to chipboard panel waterproofing or to the stability of the emulsion. In this article the influence that oil content exerts on waterproofing capacity is determined using the ADSA technique by studying the water‐vapor‐solid interface in a wood substrate coated in paraffin with a drop of water placed on top. Thus, the water‐resistant capacity of a chipboard panel is related to the paraffin oil content in the emulsion. Furthermore, in this article, paraffin oil content is also related to the emulsion stability through its rheological behavior in both the fluid and dynam...