Contact Angle Hysteresis Measurements Research Articles

Smoothening Perfluoroalkylated Surfaces: Liquid‐Like Despite Molecular Rigidity?

AbstractThe rational design of surfaces at the molecular level is essential toward realizing many engineering applications. However, molecular‐scale defects affect processes such as triboelectrification, scaling, and condensation. These defects are often detectable via contact angle hysteresis (CAH) measurements. Liquid‐like surfaces exhibit extremely low CAH (≤5°) and rely on the use of highly flexible molecular species such as long‐chain alkyls or siloxanes. Their low glass transition temperatures lead to the so‐termed self‐smoothing behavior, reducing sensitivity to defects formed during fabrication. However, utilizing rigid molecular species such as perfluoroalkyl chains often results in higher hysteresis (10° to 60°) as defects are not self‐smoothed after fabrication. Consequently, state‐of‐the‐art perfluoroalkylated surfaces often show sub‐optimal interfacial properties. Here, a customizable chemical vapor deposition process creates molecularly‐thick, low‐defect surfaces from trichloro(1H,1H,2H,2H‐perfluorooctyl)silane. By implementing moisture‐exposure controls, highly homogenous surfaces with root‐mean‐square roughness below 1 nm are fabricated. CAH is achieved down to ≈4° (average: 6°), surpassing the state‐of‐the‐art by ≈5°. Reduction of CAH (26° to 6°) results in condensation suppression, decreasing surface droplet density by one order and surface droplet coverage by 40%. This work guides the synthesis of high‐quality surfaces from tri‐functional perfluoroalkylsilanes with liquid‐like properties despite their molecular rigidity.

Droplet Friction on Superhydrophobic Surfaces Scales With Liquid-Solid Contact Fraction.

It is generally assumed that contact angle hysteresis of superhydrophobic surfaces scales with liquid-solid contact fraction, however, its experimental verification has been problematic due to the limited accuracy of contact angle and sliding angle goniometry. Advances in cantilever-based friction probes enable accurate droplet friction measurements down to the nanonewton regime, thus suiting much better for characterizing the wetting of superhydrophobic surfaces than contact angle hysteresis measurements. This work quantifies the relationship between droplet friction and liquid-solid contact fraction, through theory and experimental validation. Well-defined micropillar and microcone structures are used as model surfaces to provide a wide range of different liquid-solid contact fractions. Micropillars are known to be able to hold the water on top of them, and a theoretical analysis together with confocal laser scanning microscopy shows that despite the spiky nature of the microcones droplets do not sink into the conical structure either, rendering a diminishingly small liquid-solid contact fraction. Droplet friction characterization with a micropipette force sensor technique reveals a strong dependence of the droplet friction on the contact fraction, and the dependency is described with a simple physical equation, despite the nearly three-orders-of-magnitude difference in liquid-solid contact fraction between the sparsest cone surface and the densest pillar surface.

Polyurethane Nanocomposite Coatings Coupled with Titanium-Based Conversion Layers for Enhanced Anticorrosion, Icephobic Properties, and Surface Protection.

This study examines the efficacy of icephobic polyurethane nanocomposite coatings in mitigating corrosion on an aluminum substrate. A titanium-based conversion coating is applied to modify the substrate, and the research focuses on optimizing the dual functionalities of icephobicity and anticorrosion within the polyurethane coatings while ensuring strong substrate adhesion. The coatings are formulated using fluoropolyol, isocyanate, and silica nanoparticles treated with polydimethylsiloxane. Surface properties are analyzed using contact angles, contact angle hysteresis measurements, and atomic force microscopy, and the coatings' icephobicity is evaluated through differential scanning calorimetry, freezing time delay, ice adhesion under impact and non-impact conditions, and ice accretion tests. The corrosion resistance and adhesive strength of the coatings are assessed using electrochemical impedance spectroscopy and cross-cut tests, respectively. Increasing the concentration of silica nanoparticles to 10 wt.% increases contact angles to 167°, although the 4 wt.% coating produces the lowest contact angle hysteresis (3° ± 0.5°) and ice nucleation temperature (-23 °C). The latter coating is then applied to a substrate pretreated with a titanium/cerium-based conversion coating. This prepared surface maintains an ice adhesion of about 15 kPa after 15 icing/de-icing cycles and provides approximately 90 days of surface protection (|Z|lf = 1.6 × 109 Ω·cm2). Notably, the impedance value exceeds that of untreated substrates, underscoring the effectiveness of the titanium/cerium-based conversion coating in enhancing both corrosion resistance and coating adhesion to the substrate.

Roughness and wettability control of soda-lime silica glass surfaces by femtosecond laser texturing and curing environments

The influence of laser texturing parameters and chemical curing environments on the wettability of flat soda-lime silica glass surfaces was studied by contact angle measurements. The surfaces were textured using femtosecond laser pulses with varying fluence, periodicity, and pulse number. By exposing these textured surfaces to different chemical curing environments, a range of hydrophilicity and hydrophobicity was observed. Experimental characterizations of the roughness produced by the texturing, using the optical profilometer, and contact angle hysteresis measurements showed a transition from a Wenzel regime to a Cassie–Baxter regime, induced by the chemical environment.

Image analysis of PDMS/ZnO nanocomposite surfaces for optimized superhydrophobic and self-cleaning surface design

Optimized superhydrophobic and self-cleaning nanocomposite surfaces were obtained by spraying surface modified ZnO nanoparticles (NPs) onto PDMS, using octadecylphosphonic acid and octadecanethiol as hydrophobic modifiers. In this study, it is the first time to our knowledge that surface parameters such as topography, morphology, superhydrophobicity, and self-cleaning are correlated to particle surface distribution and agglomeration parameters obtained by image analysis. The topography, morphology, and wettability of the surfaces were analyzed using atomic force microscopy, scanning electron microscopy, static contact angle (SCA), and contact angle hysteresis measurements. Image analysis was performed using the new enhanced graphical user interface of a previously self-developed Matlab® algorithm. Both hydrophobization methodologies increased the NPs’ surface coverage and the hierarchical rough structure formation on the substrates, resulting in more homogenous superhydrophobic self-cleaning surfaces. A higher coated fraction and lower degree of interconnected uncoated PDMS paths are correlated to an increase in SCA. The combination of a higher agglomerates fraction, lower agglomerate radius, and lower distance between agglomerates obtained for the surfaces with hydrophobized ZnO-NPs rendered self-cleaning surfaces. The observed correlations increase the understanding of the design and modelling of superhydrophobic self-cleaning PDMS/ZnO nanocomposite surfaces for use in high voltage outdoor insulators.

The Contact Angle Hysteresis Puzzle

This short review explains the essence of the theory of contact angle hysteresis. It emphasizes the controversial points and discusses the shortcomings of contact angle hysteresis measurements. The review ends with conclusions regarding the improvements that are required to make these measurements useful for the characterization of surface wettability.

One-step fabrication of superhydrophobic nanocomposite with superior anticorrosion performance

Superhydrophobic coatings having a self-cleaning ability and anticorrosion performance are desired in many industrial contexts; however, high costs and complex fabrication methods limit their application. In this study, thin anticorrosive superhydrophobic coatings were prepared by a scalable, simple, and inexpensive spraying method to protect AA2024-T3 surfaces. The two-component coatings were designed using fluoropolyurethane and varying concentrations of surface modified silica nanoparticles. Surface characterization was performed using contact angle and contact angle hysteresis measurements, field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectrum (EDS), and atomic force microscopy (AFM) techniques. Corrosion protection, water uptake, and adhesion strength of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) and cross-cut tests, respectively. Increased silica nanoparticle concentrations up to 10 wt% led to increased contact angles from 92° to 167°. The lowest contact angle hysteresis was obtained for the coating containing 4 wt% nanoparticles. FE-SEM images showed that high concentrations of nanoparticles (more than 4 wt%) led to cracks in the coating surface. The best performance of a coating was observed when the coating contained 4 wt% nanosilica particles, resulting in a 164° water contact angle, almost 60 days of surface protection, an impedance value of 2.2 × 109 Ω·cm2, 0.9% water uptake, and marked adhesion strength. The impedance value of the coating with 4 wt% nanosilica particles was about 25× that of the blank sample.

Interfacial Instability as Shaping Mechanism for Polystyrene Particles with Tunable Surface Texture

AbstractSurfactant‐driven interfacial instability of emulsion droplets has recently emerged as a means for shaping polymeric particles with controllable surface texture. This paper presents a suspension polymerization‐based method to produce surface‐textured polystyrene particles by inducing an instability at the interface of prepolymerized emulsion droplets followed by rapid cooling. The interaction of two surface‐active components, i.e., arachidic acid and cetyltrimethylammonium bromide (CTAB) at the phase boundary triggers the interfacial deformation at a specific point in time. Rapid cooling freezes the deformed emulsion droplets in a nonequilibrium state. Viscosity is proposed as the key parameter influencing the particle morphology, which can be controlled by easily adjustable factors such as initiator concentration, temperature, time, and cooling rate. Contact angle hysteresis measurements of particle thin layers spread on a flat substrate reveal a strong influence of the particle surface texture on the wetting behavior. The presented particle synthesis method requires no specialized equipment and has a high potential for upscaling, making it promising for various applications including hydrophobic coatings, catalyst supports, separation technology, tissue engineering, or drug delivery.

Reorganization of an Amphiphilic Glassy Polymer Surface in Contact with Water Probed by Contact Angle and Sum Frequency Generation Spectroscopy

We address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hysteresis measurements are also affected by surface roughness and chemical heterogeneity, we have used surface-sensitive sum frequency generation spectroscopy (SFG) in conjunction with water contact angles to answer this question. To increase the magnitude of the surface reorganization, we have designed an amphiphilic polymer, poly(α-hydroxymethyl-n-butyl acrylate) (PHNB), to study the changes in the structure of polar hydroxy groups and nonpolar (methyl and methylene) groups at the interface. The SFG and the water contact angles show that reorganization does occur for PHNB below Tg. However, complete reorganization requires heating the sample above the bulk Tg. These heating experiments were conducted by first heating the sample in the presence of water and then followed by cooling the sample to room temperature in the presence of water to lock the changes in the surface str...

New Electrospun Polystyrene/Al2O3 Nanocomposite Superhydrophobic Coatings; Synthesis, Characterization, and Application

The effect of electrospinning operational parameters on the morphology, surface roughness, and wettability of different compositions of electrospun polystyrene (PS)–aluminum oxide (Al2O3) nanocomposite coatings was investigated using different techniques. For example, a scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) unit, a Fourier transform infrared (FTIR) spectrometer, an atomic force microscope (AFM), and water contact angle (WCA), and contact angle hysteresis (CAH) measurements using the sessile droplet method, were used. The latter used 4 µL of distilled water at room temperature. PS/Al2O3 nanocomposite coatings exhibited different morphologies, such as beaded fibers and microfibers, depending on the concentration ratio between the PS and Al2O3 nanoparticles and the operational parameters of the electrospinning process. The optimum conditions to produce a nanocomposite coating with the highest roughness and superhydrophobic properties (155° ± 1.9° for WCA and 3° ± 4.2° for CAH) are 2.5 and 0.25 wt % of PS and Al2O3, respectively, 25 kV for the applied potential and 1.5 mL·h−1 for the solution flow rate at 35 °C. The corrosion resistance of the as-prepared coatings was investigated using the electrochemical impedance spectroscopy (EIS) technique. The results have revealed that the highly porous superhydrophobic nanocomposite coatings (SHCs) possess a superior corrosion resistance that is higher than the uncoated Al alloy by three orders of magnitude.

Contact Angle and Adhesion Dynamics and Hysteresis on Molecularly Smooth Chemically Homogeneous Surfaces.

Measuring truly equilibrium adhesion energies or contact angles to obtain the thermodynamic values is experimentally difficult because it requires loading/unloading or advancing/receding boundaries to be measured at rates that can be slower than 1 nm/s. We have measured advancing-receding contact angles and loading-unloading adhesion energies for various systems and geometries involving molecularly smooth and chemically homogeneous surfaces moving at different but steady velocities in both directions, ±V, focusing on the thermodynamic limit of ±V → 0. We have used the Bell Theory (1978) to derive expressions for the dynamic (velocity-dependent) adhesion energies and contact angles suitable for both (i) dynamic adhesion measurements using the classic Johnson-Kendall-Roberts (JKR, 1971) theory of "contact mechanics" and (ii) dynamic contact angle hysteresis measurements of both rolling droplets and syringe-controlled (sessile) droplets on various surfaces. We present our results for systems that exhibited both steady and varying velocities from V ≈ 10 mm/s to 1 nm/s, where in all cases but one, the advancing (V > 0) and receding (V < 0) adhesion energies and/or contact angles converged toward the same theoretical (thermodynamic) values as V → 0. Our equations for the dynamic contact angles are similar to the classic equations of Blake & Haynes (1969) and fitted the experimental adhesion data equally well over the range of velocities studied, although with somewhat different fitting parameters for the characteristic molecular length/dimension or area and characteristic bond formation/rupture lifetime or velocity. Our theoretical and experimental methods and results unify previous kinetic theories of adhesion and contact angle hysteresis and offer new experimental methods for testing kinetic models in the thermodynamic, quasi-static, limit. Our analyses are limited to kinetic effects only, and we conclude that hydrodynamic, i.e., viscous, and inertial effects do not play a role at the interfacial velocities of our experiments, i.e., V < (1-10) mm/s (for water and hexadecane, but for viscous polymers it may be different), consistent with previously reported studies.

Photoresponsive hierarchical ZnO-PDMS surfaces with azobenzene-polydopamine coated nanoparticles for reversible wettability tuning

Azobenzene-bearing surfaces are promising for light-controlled wettability switching in microfluidic systems. We investigate the wettability and stability of flat glass, flat polydimethylsiloxane (PDMS) and hierarchical ZnO-PDMS surfaces functionalized with azobenzene bearing polydopamine coated nanoparticles (Azo-PDA-NP). We analyse the stability of the contact-angle switching when using new droplet locations for each measurement and for using the same droplet location with compressed-nitrogen drying between measurements. For flat glass surfaces we observed a wettability change of 15° and 60° for spin-coating and drop-casting deposition of the Azo-PDA-NP, respectively. For the PDMS samples, 60° contact-angle change are obtained, but these surfaces show degradation for repeated switching cycles. In contrast the hierarchical ZnO-PDMS surfaces exhibit a wettability switch of 50° and no degradation for five switching cycles at the same droplet location. We captured water drops during ultra-violet (UV) irradiation on video and obtained a one-sided flowing motion with a speed of 1.9 μm/s. Contact-angle hysteresis measurements show that the minimum criterion for droplet movement is fulfilled for the Azo-PDA-NP functionalized hierarchical ZnO-PDMS surfaces.

A superoleophobic surface with anisotropic flow of hexadecane droplets

In this paper, a grooved mushroom structured surface, with superoleophobicity and anisotropic flow of hexadecane with a low surface tension is reported. The grooved mushroom structure was fabricated using a simple and cost-effective wet etching process on Si wafer. The wetting properties of the fabricated surface were studied through static contact angle (CA), contact angle hysteresis (CAH), and tilted angle measurements in the parallel and orthogonal directions. The maximum static CA of hexadecane droplets was 159° in the parallel direction and 142° in the orthogonal direction, respectively. The results from the CAH measurements indicated that the wetting and flow on the grooved mushroom structure were anisotropic. Furthermore, it was verified experimentally that the ridge height and distance between the top layers were key to determining the oleophobicity and anisotropic flow.

Nanotextured Shrink Wrap Superhydrophobic Surfaces by Argon Plasma Etching.

We present a rapid, simple, and scalable approach to achieve superhydrophobic (SH) substrates directly in commodity shrink wrap film utilizing Argon (Ar) plasma. Ar plasma treatment creates a stiff skin layer on the surface of the shrink film. When the film shrinks, the mismatch in stiffness between the stiff skin layer and bulk shrink film causes the formation of multiscale hierarchical wrinkles with nano-textured features. Scanning electron microscopy (SEM) images confirm the presence of these biomimetic structures. Contact angle (CA) and contact angle hysteresis (CAH) measurements, respectively, defined as values greater than 150° and less than 10°, verified the SH nature of the substrates. Furthermore, we demonstrate the ability to reliably pattern hydrophilic regions onto the SH substrates, allowing precise capture and detection of proteins in urine. Finally, we achieved self-driven microfluidics via patterning contrasting superhydrophilic microchannels on the SH Ar substrates to induce flow for biosensing.

Hydrophobicity and anti-icing performances of nanoimprinted and roughened fluoropolymers films under overcooled temperature

Superhydrophobic (SH) surfaces are promising materials to limit frost, ice and snow formation on cold surfaces submitted to low temperature conditions. Inspired by the hierarchical roughness of lotus leaves, SH surfaces have received great attention in recent years. In this work, FEP (Fluorinated Ethylene Propylene) flexible films are chosen for their intrinsic hydrophobicity and their capability to cover non-flat surfaces with nanostructures. In the context of the use of superhydrophobic surfaces for anti-icing applications, it is essential to confirm that SH surfaces at room temperature retain their properties under overcool condition at −15°C. To this aim hydrophobic performances have been explored by contact angle and contact angle hysteresis measurements in these two conditions of temperature. In order to make these films SH, two methods were investigated. First, FEP has been imprinted by nanoimprint lithography (NIL) with different structures. In another way, plasma treatment etching was used to obtain rough surface structures. Thus, we evaluated the performances of several imprinted and rough surfaces for the use of hydrophobic and SH surfaces in the context of anti-icing applications.

Hydrophobic and ice-phobic properties of self-assembled monolayers (SAMs) coatings on AA6061

The effects of alkyl chain length and immersion time (IT) on hydrophobic and ice-phobic properties were investigated through self-assembled monolayers of alkylsilane molecules, i.e. OT (trichloro-octylsilane) and OD (trichloro-octadecylsilane) on a flat aluminum alloy (AA6061) substrate. By increasing IT, the wetting behavior of the coatings, characterized by contact angle (CA) and contact angle hysteresis (CAH) measurements at room temperature, showed an enhancement of hydrophobic properties. However, the effect of IT on wetting properties was more significant in the case of the sample coated with longer alkyl chains (OD) than that with short alkyl chains (OT). Icing tests showed delayed ice formation and lower adhesion strength on samples with larger CA values and smaller CAH values. The scanning electron microscopy images of the coated surfaces showed the rough structure at micro/-nanometre scale on a polished aluminum surface. X-ray photoelectron spectroscopy (XPS) analysis of the coated samples showed the presence of more alkylsilane molecules when IT was increased from 15 min to 12 h.

Corrosion protection of electrospun PVDF–ZnO superhydrophobic coating

Abstract A one-step electrospinning technique was used to prepare a protective superhydrophobic PVDF–ZnO nanocomposite coating for aluminum against corrosion. The wettability and morphology of the prepared coating surfaces was characterized using water contact angle and contact angle hysteresis measurements, scanning electron microscopy, Fourier transform infrared spectroscopy and atomic force microscopy. In addition, the corrosion resistance for Al with PVDF polymer and the PVDF–ZnO nanocomposite coatings compared to the bare Al was investigated by electrochemical impedance spectroscopy and Tafel polarization techniques. The results showed that, the water contact angle and the contact angle hysteresis of the prepared PVDF–ZnO nanocomposite coating were 155 ± 2 and 4.5 ± 2°, respectively. In addition, compared to the sprayed PVDF–ZnO coating, the concentration of ZnO nanoparticles in the electrospun coating was only one sixth to obtain the same water contact angle, better distribution of ZnO nanoparticles was achieved without using any dispersing agent. The analysis of the corrosion results revealed that, the superhydrophobic PVDF–ZnO coating showed a corrosion protection efficiency for Al that is much higher than that of the PVDF one.

Modeling and Measurement of Contact Angle Hysteresis on Textured High-Contact-Angle Surfaces

A set of surfaces with different Cassie fractions were fabricated. For a given Cassie fraction surfaces had different pillar diameter and spacing combinations. Advancing and receding contact angles of both water and ethylene glycol were measured on the fabricated surfaces. We examined the effects of both surface feature size and Cassie fraction on advancing and receding contact angles and found no relationship between size and contact angle when Cassie fraction was held constant. Also examined was the effect of the Cassie fraction on contact angle hysteresis, which led to the development of a new theoretical framework for understanding contact angle hysteresis on rough surfaces that helps unite previous theory and observations with current observations. The theoretical framework includes empirically determined contact-line pinning forces. From measurements, we found a constant contact-line pinning force for the receding contact angles. For the advancing contact angles, there was also a contact-line pinning force, but one that changed with changing Cassie fraction.

Why Can Organic Liquids Move Easily on Smooth Alkyl-Terminated Surfaces?

The dynamic dewettability of a smooth alkyl-terminated sol-gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.

Interfacial energy of heavy naphthenic bitumen in aqueous medium

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.